Chapter 3: Sensing the light: Detectors for the Optical and Infrared

3.1 Basic Properties of Photo-detectors

Modern photon detectors operate by placing a bias voltage across a semiconductor crystal, illuminating

it with light, and measuring the resulting photo-current. These devices dominate in the ultraviolet,

visible, and near- and mid-

infrared. Heritage detectors

that operate on other principles

are discussed elsewhere (e.g.,

Rieke 2003).

3.1.1 Photon Absorption

For the simplest photo-

detectors, absorption occurs in

a semi-conductor where a

photon is absorbed and its

energy elevates an electron

from the valence band into the

conduction band (see Figure 3.1: equivalently, we can describe the process as freeing an electron from

its bonds to the semiconductor crystal so it can move freely through the detector volume). This type of

photoconductivity is termed

intrinsic, because the energy

required is an intrinsic property

of the detector material.

Semiconductors have band gaps

appropriate for intrinsic

detection of visible and near

infrared photons.

For longer wavelengths, doped

semiconductors can be used in

detectors. Dopants are

impurities that do not supply the

right number of bonding

electrons to complete the

semiconductor crystal; p-type

dopants are missing a bond within the crystal lattice, while n-type ones have an extra, see Figure 3.2.

The terminology semiconductor:dopant is used, for example Si:As or Ge:Ga.

It takes relatively little energy, called the excitation energy, to free one of the n-type unbounded

electrons. Similarly, a small amount of energy can cause the bonds to shift in the p-type material to

cause the empty bond to move through the crystal (in which case it is called a hole and treated as a

Figure 3.2. Crystal structure of intrinsic (1), p-type (2), and n-type

(3)semiconductors

Figure 3.1. Band gap diagrams for insulators, semiconductors, and

metals.

positively charged mobile particle). Extra energy levels are added to the band diagram to indicate these

dopants (Figure 3.3: n-type to the

left, p-type to the right). The

resulting photoconductivity is

termed extrinsic, because the

dopants that make it possible are

not a fundamental constituent of

the crystal material.

The efficiency of the detection

process depends on the strength of

the photon absorption in the

detector. The absorption coefficient is a() (in cm-1) and has a characteristic cut off at the band gap

energy (intrinsic material; see Figure 3.4) or the excitation energy (extrinsic). The absorption of a flux S

of photons passing through dl is

With solution at depth l

where S0 is the flux that penetrates the surface to the bulk material. The absorptive quantum efficiency,

ab, is the portion of this flux absorbed in the detector:

where d1 is the thickness of the detector. From Figure 3.4, some semiconductors have high levels of

absorption right down to their

bandgap energies (InSb, GaAs).

These materials allow direct

transitions from the top of valence

band to the bottom of the

conduction one. Detectors made of

them will have high quantum

efficiency up to their cutoff

wavelengths, corresponding to

photons at the bandgap energy.

Other materials, the most

noteworthy of which is silicon, have

low absorption just above their

cutoff energies. For them, the

minimum-energy transition is

Figure 3.4. Absorption coefficients for some intrinsic

photoconductors.

Figure 3.3. Bandgap diagrams for extrinsic photoconductors.

forbidden by quantum mechanical selection rules and absorption at this energy must be achieved

indirectly. Detectors of these materials will have low quantum efficiencies near their cutoff wavelengths.

The net absorption must allow for losses such as reflection and incomplete collection of the signals from

freed electrons, so the realized quantum efficiency, , is generally less than the value for absorption

alone.

3.1.2 Detective Quantum Efficiency

As discussed in Section 1.5, the intrinsic signal to noise in n photons is:

However, in a detector, each absorbed photon generally produces one free charge carrier that is

eventually detected. Therefore, n photons produce n charge carriers, and in the ideal case the detector

can achieve

In general a detector will fall short of (S/N)det – there are

many mechanisms that add noise. A succinct way to

describe the detector performance is the “detective

quantum efficiency”, DQE. Let nin be the actual input

photon signal and nout be an imaginary input signal that

would yield the observed S/N if the detector were perfect.

That is, (S/N)out is the observed signal to noise, and (S/N)in

is the potential signal to noise in the incoming photon

stream, which is determined from the intrinsic photon

statistics (Section 1.5 in Chapter 1). Then,

In addition to the basic noise associated with the input photons, photodetector noise includes a

contribution from the Brownian Motion of free charge carriers, called Johnson or Nyquist Noise.

Minimizing this contribution requires operating the detector with very high resistance and/or at very

low temperature. Excess noise may also result from dark current, the flow of charge carriers when the

detector is shielded from light. Low operating temperatures reduce dark current to some minimum level

but it still may contribute noise. As implied by equation (3.6), such noise mechanisms may also reduce

the DQE.

3.1.3 Linearity/dynamic range As larger and larger fluxes fall on a detector, its output might look like Figure 3.5 (where we have

assumed the input flux grows linearly with time). Where the output is a linear function of the input

signal, it is easy to interpret what the detector is telling us. When the detector is saturated, all

information about the input flux is lost (other than it is too large!). With care, information can be

derived in the nonlinear regime before saturation. The dynamic range is the total range of signal over

which useful information is yielded.

Figure 3.5. Saturation Behavior.

Figure 3. Response of a detector to a

constantly (linearly) growing signal.

3.1.4 Resolution

The capability of a detector array for spatial resolution over its face is expressed to first order as its

number of pixels, or detector elements. However, if there is cross talk -- signal spilling from one pixel to

its neighbors -- then the pixels are no longer independent and a more sophisticated description of the

imaging capability is required.

A simple measure of the resolution is the number of line pairs per millimeter it can distinguish. A pattern of alternating black and white lines is projected onto the detector array and imaging scale is adjusted until they can just be distinguished (defined at 4% or greater modulation). The resolution is then quoted as this limiting value of line pairs per millimeter. This performance metric may be useful for comparing different detectors of basically similar type, but it is difficult to integrate with the performance of other elements in a detector system. The modulation transfer function, discussed in the previous chapter, is a more general and powerful description of the detector resolution.

3.1.5 Time response

The time response of a detector is set

by phenomena like the time required

for the photo-generated charge

carriers to recombine or be collected,

so the detector returns to its state

before it was exposed to light.

For electronic detectors, the “frequency response” is analogous to the MTF, but is computed in the time

domain. Consider a detector with electrical characteristics that can be represented by a simple R-C

circuit with an exponential time constant RC=RC. Let a sharp voltage impulse vin=v0 (t) be put on the

capacitor. The response, analogous to the point spread function, is

If we analyze the event in terms of frequency instead of time, the signal amplitude is

(see Figure 3.6). The ‘cutoff frequency’ is where the amplitude drops by 2 compared with its value at

very low frequency:

Figure 3.6. Frequency response characteristics of a

detector.

The rise time or fall time are the times for the output to change from 10 to 90% of its final value, which

is 2.2 RC

3.1.6 Spectral response

Over what range of wavelengths does the detector respond to light? For an ideal photon detector, there

is a characteristic answer as illustrated in Figure 3.7. The detector does not absorb at energies below its

excitation energy (= the bandgap energy for an intrinsic semiconductor), corresponding to a cutoff

wavelength of

where Eg is the bandgap

(or excitation) energy. In

an ideal photon detector,

each absorbed photon

creates one free charge

carrier, so the detector

responsivity, S (in amps of

current out per watt of

photons in), rises linearly

with wavelength to c. A

number of effects in real

detectors act to round off

this ideal response curve

(dashed lines).

3.2. Some Photon Detector Types

In this section, we describe the operation of two types of photon detector: Si:As BIB devices; and photodiodes. We discuss readouts and arrays in Section 3.3 and charge coupled devices (CCDs) in Section 3.4. This organization has been adopted because, conceptually, the infrared arrays are simpler than CCDs.

Extrinsic absorption photon detectors (for wavelengths longer than ~ 5 m) must operate under two diametrically opposed requirements. They need to have very high resistance to suppress noise currents, e.g. Johnson noise. At the same time, they need to have a high impurity level for good photon absorption, which drives the resistance down. The solution is to separate the detector regions responsible for the electrical characteristics (high resistance) and photon absorption (heavy doping).

Figure 3.7. Quantum efficiency (left) and responsivity (amps out per watt in)

of an idealized photo-detector.

3.2.1 Blocked Impurity Band (BIB) detectors

Silicon Blocked Impurity Band (BIB) or

Impurity Band Conduction (IBC) – they are the

same thing – are the detectors of choice for

wavelengths between about 5 and 35 m.

They are one approach to separating the

absorbing region from the one responsible for

the detector electrical characteristics. We

consider the most common type, Si:As BIB

detectors, which respond to about 27.5 m.

Consider Figure 3.8. A thin blocking layer of

intrinsic silicon with a transparent contact is

grown over an infrared-active layer, relatively

heavily doped with arsenic. Still heavier

doping is used for the degenerate –

electrically conducting -- contact layer. The

infrared-active layer is so heavily doped that the electrons (as Fermions they cannot share the same

quantum mechanical state) are forced to occupy a band of closely spaced energy levels, the impurity

band. When a bias is placed across the contacts, the free charge carriers are driven out of the infrared

active layer, and it is said to be ‘depleted’. Because it has few free charge carriers, it has a high

resistance and there is a significant electric field across it. When infrared photons are absorbed in this

depletion region, they free electrons that are attracted to the transparent contact where they are

collected to produce the photocurrent that we use to detect the photons.

But how do these free electrons get

through the intrinsic blocking layer? Why

don’t the thermally generated carriers in

the impurity band flood the contact? The

answer is in the solid state ‘trick’ in Figure

3.9. The blocking layer has no impurity

layer, so the carriers in the impurity band

are blocked at that layer (hence ‘BIB’).

However, a photon-generated carrier has

been promoted into the silicon conduction

band, and it can traverse the intrinsic layer

with no problems.

The critical issue with this detector type is

to adjust the parameters so it works!

Arsenic is the dominant impurity (since we

put it there at high concentration); it is an

Figure 3.8. Operation of a Si:As BIB Detector

Figure 3.9. Band Diagram for a Si:As BIB Detector

n-type impurity. There will be a much lower level of minority impurities, of p-type, in the IR-active layer.

The lower this ‘minority’ impurity level can be kept, the better.

Let’s consider why. The p-type impurities will tend to acquire electrons from the arsenic, leaving them as

negative charge centers. The resulting negative space charge tends to neutralize the electric field in the

As-doped, infrared-absorbing layer (established by putting a positive voltage on the transparent contact

in Figure 3.9). Photo-electrons are only collected where this field has been established, i.e., in the

depleted portion of the layer. The thickness, w, of the depletion region is:

where NA is the density of ionized p-type impurity atoms, tB is the thickness of the blocking layer and Vb

is the bias voltage, q is the electronic charge, 0 is the dielectric constant (11.8 for silicon) , and 0 is the

permittivity of free space. The larger NA, the smaller is w and the lower the quantum efficiency of the

detector. One might try to combat this problem by increasing the bias voltage, but the result is incipient

avalanching that increases the noise.

State of the art semiconductor processing allows control of NA to be less than 1012 cm-3. An acceptable

arsenic concentration is 3 x 1017 cm-3. For arsenic in silicon, the absorption cross section is 2.2 x 10-15

cm2, so the absorption length is about 15 m. Assuming NA = 1012 cm-3 and Vb = 1V, w=32 m from

equation (3.11), so a high quantum efficiency detector can be built. Si IBC detectors have good quantum

efficiency (~ 60%) at the longer wavelengths (10 – 26 m), but the absorption falls toward shorter

wavelengths. Because the photon absorption is not complete, they can have strong fringing – periodic

variations in their wavelength response. They are typically read out with simple source-follower

integrating amplifiers (to be discussed below) and have a modest and readily correctable nonlinearity in

such a circuit. Very high performance versions have been manufactured for infrared space missions

where the thermal backgrounds are very low and the full potential detector performance can be

realized.

However, in use on the ground, the fully optimized arrays are overwhelmed by the high thermal

backgrounds. Very rapid readout of the detector/amplifier unit is required to avoid saturation, and as a

result there are compromises in the sizes of the arrays, the read noise, and possibly other parameters.

3.2.2 Solid State Photomultiplier It is possible to modify the detector architecture described above and optimize the temperature of

operation to enhance the avalanching gain. In this way, it is possible to make a detector that provides a

fast pulse whenever a photon is absorbed. Where rapidly varying signals are to be observed, the solid-

state photomultiplier (SSPM’) can have unique advantages.

3.2.3 Photodiodes

To make a diode, one dopes adjacent regions in a semiconductor with opposite type impurities. At the resulting “junction”,

charges migrate over a narrow

region to fill all open bonds.

This situation defines a

depletion region – no free

charge carriers. The resulting

charge sheets on either side of

the junction create an electric

field. See Figure 3.10.

Electrons in a semiconductor are in a Fermi-Dirac distribution relative to the energy levels:

where f(E) is the probability that an electron will be in a state of energy E and EF is the Fermi level, f(EF) =

0.5. When dopants are added to the semiconductor, they shift the Fermi level – n-type move it toward

the conduction band and p-type toward the valence band.

More or less by definition of the Fermi level, current will flow in a semiconductor to bring it to the same

energy throughout the crystal. The valence and conduction bands must then shift with different doping

levels in the crystal, resulting in built-in electric fields. The field across a diode junction is an example,

leading to a ‘contact potential’, V0.

Figure 3.11 shows how a diode

works, in the formalism of Fermi

levels and contact potentials.

When a semiconductor has two

zones with differing Fermi levels

(due to different doping), bringing

the Fermi levels to the same

energy results in a contact

potential being established across

the region in question. Figure 3.11

shows how this concept illustrates

the creation of the depletion

region in a diode and explains the

voltage that drives free charge

Figure 3.10. Charge structure of a junction.

Figure 3.11. Schematic operation of a diode.

carriers across it.

Of course, diodes cannot be made as implied – by bringing differently doped pieces of semiconductor

into contact. Instead, the different dopants have to be introduced into the crystal in ways that do not do

damage to the crystal lattice.

High impedance is achieved when the diode is modestly ‘back-biased’ – that is, when the external bias

adds to the internal one and increases the size of the depletion region. As the diode is forward biased,

the depletion region shrinks and more current is

conducted. If the back bias is too large, the field

across the depletion region can be so large that

avalanche gain occurs and the diode ‘breaks down’.

Detectors are operated with either zero bias or a

modest back bias.

When a photon is absorbed in a way that the free

charge carrier it produces can reach the depletion

region of the diode, the field maintained by the

contact potential drives the carrier across the

junction, and the resulting current can be sensed to

produce a signal. Photons are generally not

absorbed in significant numbers in the junction

region because it is very thin. To be detected, the

charge carriers they free must diffuse through the

material until they are captured in the contact

potential. Diffusion describes the tendency of free

charge carriers to spread through the material due

to thermal motions. It is characterized by the

diffusion coefficient,

where is the “mobility” of the free charge carriers and T is the temperature. The distance over which

the free electrons can travel is characterized by the diffusion length, L:

The recombination time, , is how long the charge carriers remain free before being captured into

bonds. For the diode to have good quantum efficiency, intuitively we expect that the thickness of the

layer overlying the junction must be less than a diffusion length. This requirement rules out operation

with extrinsic absorption, since the absorption lengths are too long. Photodiodes are made of intrinsic

materials such as:

Figure 3.12. Detection of a Photon in a Diode.

Material Cutoff wavelength ( m)

(transition type)

Si 1.1 (indirect)

Ge 1.8 (indirect)

InAs 3.4 (direct)

InSb 6.8 (direct)

HgCdTe ~1.2 - ~ 15 (direct)

GaInAs 1.65 (direct)

AlGaAsSb 0.75 – 1.7 (direct)

For materials with a range of c, the bandgap can be controlled by changes in composition, such as by

increasing the relative amount of Hg over Cd in HgCdTe to reduce the bandgap.

The design of a photodiode must take into account the temperature dependence of the diffusion length.

In the regime of doping and temperature of interest D is proportional to T. The recombination time goes

as roughly T1/2. Hence, L goes as T3/4. Thus, the thickness of the layer overlying the junction must be

made very thin for photodiodes operating at low temperature, as is required for the low backgrounds

encountered in astronomy.

Photodiodes can be front

illuminated, in which case

they are exposed directly to

the incoming photons.

However, to build arrays,

they must be attached to

amplifiers that would block

the light if they were front-

illuminated. Instead, the light

is brought in through the

substrate carrying the

diodes, in an arrangement

called back illumination. To

allow the charge carriers to

diffuse to the junction, it is

necessary that this back

Figure 3.13. Diode array construction.

substrate layer be made very thin.

Two general approaches to making large arrays of very thin back illuminated diodes are illustrated in

Figure 3.13 – to grow the diodes on the back of a transparent carrier, or to thin them after they have

been attached to a strong substrate, in this case the silicon wafer carrying the readouts. The diode

thickness in some cases must be less than 10 m! Fortunately, the absorption is high enough in the

intrinsic materials with direct absorption transitions that very high quantum efficiencies are possible

right up to the bandgap.

The current conducted by a diode can be expressed to first order by the ‘diode equation’:

where I0 is the “saturation current” and Vb is

the bias voltage on the junction. I0 depends on

the junction area, the diffusion coefficients and

lengths, and on the concentration of minority

impurities on either side of the junction. When

a photodiode is exposed to light, the I – V curve

is modified from that in equation (3.15):

where is the quantum efficiency, q is the

electronic charge, P is the power falling on the

detector, h is Planck’s constant, and is the

photon frequency. The diode curve shifts with

increasing illumination (1)(2)(3) as in

Figure 3.14.

With its two charge sheets separated by a thin layer of dielectric at the junction, a diode looks like a

classic parallel plate capacitor. The plate separation is just the width of the depletion region, so the

capacitance is

where w is the width of the depletion region and A is the area of the detector. The width decreases with

increased doping and increases with increased back bias. Small capacitance is desirable to minimize the

noise in reading out the diode.

Photodiodes have the following operating characteristics. Their absorption of photons is very efficient

because it occurs intrinsically. However, for low background performance, they must be operated cold,

and hence the charges can only be collected over short distances. Difficulties in achieving exactly the

Figure 3.14. Electrical response to illumination.

right thickness and uniformity resulted in substantial response non-uniformities in early devices, but

these issues are now resolved and near-infrared photodiode arrays show excellent quantum efficiency

(~ 90%). Because of the very high absorption efficiency, spectral fringing is much less than with Si IBC

devices. Diodes are usually read out by simple source-follower amplifiers, as described in the next

section. A characteristic of the source-follower circuit is that the detector is de-biased as signal is

accumulated. As this process occurs, the width of the depletion region in the photodiode decreases and

an integrating amplifier/detector system becomes less responsive because of the increased capacitance

(equation (3.17)). However, this form of nonlinearity is highly repeatable and can be compensated

readily in data processing. The depletion region allows extremely high impedances to be achieved, and

correspondingly tiny dark currents, for diodes of materials allowing response to about 6-8 m. These

attributes are available in very high performance detector arrays up to 2k X 2k format (and larger

formats in development) operating from 0.6 to 5 m with detectors made in either InSb or HgCdTe.

These devices are the detectors of choice for the near infrared. At wavelengths longer than 9 m,

however, control of the material properties to minimize dark current becomes very difficult and

photodiodes are no longer competitive with Si:As BIB detectors.

3.2.4 Diode variants

So far, we have described photodiodes based on a simple junction between oppositely doped regions of

semiconductor. Some variants are:

PIN diodes Some of the limitations discussed above can be removed with a thicker depletion region – it gives

lower capacitance, and if the photon absorption occurs mostly in the depletion region, the

limitations due to charge diffusion into the junction are removed. The thicker absorption region can

improve the quantum efficiency just short of the bandgap for indirect transition absorbers. All of

these gains can be achieved by growing the diode with an intrinsic region between to p- and n-type

doped ones, hence “P-I-N” or “PIN” diode.

Avalanche diodes If the back bias across a PIN diode is increased sufficiently, the field in the intrinsic material becomes

so large that the charge carriers gain enough energy to break more bonds, freeing more charge

carriers and leading to a large gain in the device. This process adds noise to the signal (as we already

mentioned for Si BIB detectors, which can operate in a similar but less extreme fashion), but it can

be useful if one needs a fast detector. The same effect can be used to make diodes that pulse count

on single photons (analogous to, but invented much earlier than, the SSPM). Where a very fast

detector is needed, pulse counting has great advantages over measuring a detector current because

the read noise on the signal is eliminated. Of course, the inverse problem is that where the photon

rate is high, or one wants to read out many detectors in an array, pulse counting is far more complex

to implement than measuring photocurrents.

Schottky diode

A junction between a metal and a semiconductor produces an asymmetric potential barrier that acts

as a diode. These devices can be used as infrared detectors, although they have quite low quantum

efficiencies.

3.3. Readouts for Infrared Arrays

Conceptually, the “easiest”

way to make an array of Si

IBC detectors or infrared

photodiodes is to make the

detectors and amplifiers

separately and then join

them together. In practice,

this isn’t easy at all – it

requires making more than

a million solder connections

for a 1024x1024 array. The

best arrays are made as

direct hybrids – make

detectors in an optimized

material with each one

placed in a grid,

evaporate indium

bumps on contacts to

each of these

detectors, make

amplifiers in a

matching grid on a

silicon wafer,

evaporate bumps on

them, and squeeze the

two grids together.

The process is

described as ‘bump

bonding,’ or ‘flip chip.’ The device, illustrated in Figure 3.15, is a ‘direct hybrid array.’

Each pixel is given its own complete amplifier, built from a small number of metal-oxide-field-effect-

transistors (MOSFETs). The amplifier functions require a minimum of four MOSFETS per amplifier;

squeezing them into the pixel shadow currently limits pixel sizes to > 18 m. The amplifiers’ outputs are

‘multiplexed,’ meaning they are switched successively to the array output.

One type of readout amplifier is shown in Figure 3.16. With the switch open as shown, the current

through the detector causes charge to collect on the integrating capacitor, CS, which is the combination

Figure 3.15. Direct Hybrid Infrared Array.

Figure 3.16. Source follower integrating amplifier.

of the detector capacitance and the input capacitance of the FET. As charge collects, it changes the gate

voltage Vg = q/CS, which in turn modulates the signal through the channel of the FET and causes a

change in Vout. Once sufficient charge has accumulated to produce a useful output signal, the amplifier is

reset by closing the switch to get rid of the charge on the capacitor. It can be shown through a series

expansion of equation (3.7) that the output is linear so long as the integration time,

with Rd the effective resistance of the detector. The output waveforms of an integrating amplifier show

a linear ramp as charge accumulates, until the reset switch is closed (Figure 3.17). The ramp can be

sampled in a number of ways. For example, the voltage can be read just before and after reset, with the

signal given as V(tbefore) – V(tafter). This strategy has the advantage that it is not affected by 1/f noise in

the amplifier, since the signal is extracted over a short time interval. However, it is subject to kTC or

reset noise, which in units of electrons is

This type of noise is fundamental, since it involved the exchange of potential energy (stored on the

capacitance) and kinetic energy (Brownian motion of the charge carriers). For example, with a MOSFET

with an input

capacitance of 0.1 pF

at T = 150K, the noise

is nearly 100

electrons. Therefore,

it is more common to

sample the signal at

the beginning and

end of the integration

ramp. This strategy

can avoid kTC noise

because the time

constant for changes

in the charge on CS is

S = RdCS, and if t2 – t1

<< S, then the noise electrons are “frozen” on the integrating capacitor during the integration. Although

the starting voltage for an integration ramp will vary due to kTC noise from integration to integration,

the effect is automatically subtracted out. The condition for freezing kTC noise on CS is the same as that

in equation (3.18) for linearity of the circuit.

In addition to drift and low frequency noise components, the high frequency amplifier noise can also degrade the results, causing the read at a specific time to deviate from the true average at that time. This noise can be reduced by making multiple reads of the output and averaging them. Figure 3.17 illustrates two ways to implement multiple sampling to reduce the high-frequency noise. In the first ramp, a number of samples are taken at the beginning and end, but otherwise the pattern is identical to

Figure 3.17. Two ways of reading out an integrating amplifier.

Vout

time

our previous discussion. This pattern is sometimes called “Fowler Sampling.” In the second ramp, sampling is continued at a constant rate while the ramp accumulates – hence “sample up the ramp”. The slope can then be fitted by least squares. Fowler sampling has the advantage of delivering the lowest noise, at least in principle. Sampling up the ramp allows recovery of most of the signal if the integration is disturbed, for example by a cosmic ray hitting the detector; it also allows extracting a valid measurement from the first few samples on a source so bright it saturates in the full integration.

A typical source follower circuit

diagram with all the switching to

operate a detector array is shown

in Figure 3.18. The signal is

integrated on the gate of T1.

When integrating, the voltage on

C1 is set to pinch off T2 and T3, so

T1 is turned off. The voltage on R1

is also set to pinch off T4. To read

out the result, the voltages on C1

and R1 are set to turn on T2, T3,

and T4 and the output of the pixel

at address 1,1 appears on the multiplexed output. If desired, T5 can be turned on after the reading and

the amplifier will be reset, or if it is just desired to read the signal, T5 is left pinched off and T2, T3, and T4

are pinched off after the signal has been measured to turn off T1 and continue the integration. This

readout permits access to any pixel and does not necessarily reset the accumulated charge when

reading it out. It is called a random access, nondestructive readout. Although accessing the pixels

randomly does not sound like a good thing, the design easily allows isolation of a sub-area of the array

for more rapid operation than can be achieved with the entire device.

Remarkably, all the switching of transistors has virtually no effect on the final signal, which emerges

accurate to a few electrons in the best arrays. Even more remarkable, the DC stability of these circuits is

so good that they can integrate for many minutes without drifting so far as to compromise the accuracy

of the signal measurement. That is, the strategy of freezing the charge between amplifier reads can be

implemented even for time intervals of thousands of seconds between reads.

3.4 Charge Coupled Devices (CCDs)

3.4.1 Basics

Figure 3.18. Simple multiplexed array readout.

Charge coupled devices were an elegant solution to constructing detector arrays before integrated

circuitry allowed dedicating an amplifier to each detector. They are still popular because they have a

number of advantages compared with the approach that must be used for infrared arrays, including

simpler fabrication

Consider a wafer of silicon with a thick oxide insulator layer and an electrode deposited on the oxide. In

Figure 3.19, the silicon is doped p-type to reduce the concentration of free electrons. A positive voltage

has been put on the electrode, or “gate”. The voltage forms a depletion region in the silicon and attracts

any free electrons into the potential well

against the oxide. If light is allowed to

penetrate the silicon, then the collected

charges are a measure of the level of

illumination. The illumination can be

supplied from the right in the picture,

through and around the electrodes, in which

case the CCD is described as front

illuminated. If the light comes from the left

and avoids the electrodes, it is a back

illuminated device. The detector is a fancy

form of intrinsic photoconductor with

integral charge collection.

The unique aspect of CCDs is the manner in

which the collected charge is read out. It is

conventional to draw the potential wells as

depressions with ‘water’ representing the

sea of electrons. By manipulating the

voltages on the gates, the ‘water’ can be

passed from one to another without allowing

that from one gate to get mixed with that

from another. Not only do the collected electrons not mix, but the presence of a depletion region

between them means that the rest of the array is electrically isolated from each charge packet. Thus,

the capacitance associated with the packet is just that of its gate, not the relatively huge capacitance of

all the gates in the array together. Without this isolation, there would be no way to get low read noise.

Figure 3.19. CCD charge collection under an

electrode.

There are a number of ways

to implement charge

transfer besides the three-

phase version in Figure 3.20.

Whichever is used, the

transfer brings the collected

charge packets to an output

amplifier. Doped regions

along the transfer direction

– called channel stops --

prevent the charge from

spreading orthogonally to

the transfer direction. A

performance liability of

CCDs is the tendency of

strong signals to spill into

adjacent wells, producing

“blooming” – images that

are extended along the

direction of charge transfer

(Figure 3.21). A form of

beefed up channel stop

called an anti blooming gate can intercept the extra

charge and conduct it away before it spills into the

adjacent well, but at a price in fill factor, well depth,

and effective quantum efficiency.

The three approaches in Figure 3.22 represent three

ways of dealing with the continued creation of free

charge carriers as the array is exposed to light:

(a) In line transfer devices, the problem is ignored.

Either they need to be used with a shutter and read

with it closed, or at very low illumination so few

additional charge carriers are generated during the

time to read them out.

Figure 3.20. Three-phase charge transfer. At time step t1, the charge

is collected under a single electrode with by the positive voltage.

When the voltage on the neighboring electrode is set to the same

voltage, the collection well expands and the electrons migrate

accordingly. When the first electrode is set to a negative voltage,

this migration is completed. These steps are repeated to pass the

charge along a column of the array.

Figure 3.21. Blooming.

(b) In interline transfer devices, the charge packets are moved at the end of an exposure to a

neighboring set of gates that are shielded from light. These gates can be read out as desired. These

devices generally have low net efficiency because of the real estate occupied by the shielded gates.

(c) In frame transfer devices, the whole image is transferred to a shielded region of the array where it is

read out. The efficiency of the light sensitive region is not compromised.

In all cases, the line of gates that feeds the output amp is called the output register.

For good

performance in

a CCD, the

charge transfer

efficiency (CTE

= 1 - , where

is the fraction

of charge lost in

a transfer) must

be very high.

Poor CTE leads

to cross talk

between pixels

and to excess

noise. To

consider the

noise issue, if

N0 charges are

transferred then on average N0 are left behind on each transfer, and the uncertainty in the number left

behind is N0. In general, there is also the possibility of N0 + N0 charge carriers from the preceding

packet joining the packet. Thus, if there are n transfers to get to the output amp, the charge transfer

noise is:

Consider a 20482 2-phase CCD with CTE = 0.9999 ( = 0.0001), and an average signal level of N0 = 1000.

The number of transfers to the output amplifier is about 8192, and NCTE = 40 electrons (it is larger than

the n noise from the signal!).

Poor charge transfer can result if the device is read out too quickly to allow all the electrons to migrate

from one electrode to the next. A number of mechanisms drive this migration: 1.) electrostatic

repulsion of among the electrons in the well; 2.) fringing fields from neighboring electrodes; and 3.)

diffusion. A simple figure of merit can be based on the slowest of these, diffusion. We can adapt our

discussion of diffusion by replacing the diffusion length in equation (3.14) with the electrode spacing,

Le,and the recombination time with the transfer time between electrodes, e. We then estimate that the

exponential time constant for electron transfer by diffusion is

Figure 3.21. Using charge transfer to bring the signals to the output amplifier: (a)

line address; (b) interline transfer; (c) frame transfer.

Taking Le = 15 m, and calculating D from equation (3.13) with 0 = 4.5 for SiO and T = 300K, we find a

transfer time of about 0.026 s. Since a 2Kx2K pixel three-phase CCD needs to make 6000 transfers in its

output register to read out, and each transfer needs to wait a number of e-folding times to approach

completion, the required time is a

significant fraction of a second.

Another source of poor CTE and noise is

traps at the open crystal bonds at the

silicon-silicon oxide interface. Charge

carriers get “caught” at the interface and

rejoin charge packets that come by later.

To circumvent this problem, low noise

CCDs are made with buried channels, in

which a weak junction is used to move the

potential well away from the oxide

interface and the charge packets can be

moved from gate to gate entirely within

the silicon crystal. Figure 3.22 shows the

concept, with panel (b) demonstrating

how the rule that the Fermi level must

remain constant through the material

explains the bending of the conduction

band that produces a “buried” well to

collect the photo-electrons.

Maintaining the buried channel also puts

constraints on the gate voltage, and

reduces well capacity since, if the wells are overfilled the charge carriers will contact the oxide and the

device becomes surface channel. Also, if the CCD is operated below 70-100K, the buried channel

“freezes out,” that is the charge carriers no longer have enough energy to detach themselves from

bonds and carry currents. As a result, the device becomes surface channel, with the resulting problems

with charge transfer and noise. This is one reason why CCD readouts are not used with infrared

detectors, given their low operating temperatures.

Figure 3.22. Buried channel CCD. Panel (a) shows the

doping pattern, while panel (b) is a band diagram.

The charge transfer structure allows an elegant solution to the kTC noise issue. The CCD electrodes can

be used to pass the charge packets over a floating gate, which couples them capacitively to the gate of

the output amplifier (see Figure 3.23). The charge on the gate can be removed by closing the reset

switch and then opening it. A reading is taken of the amplifier output. Then, the gate voltages are

manipulated to pass a charge packet over the floating gate, which transfers a charge into the amplifier

while maintaining high impedance to the rest of the world for the FET gate capacitance. Thus, the

conditions for freezing the thermal charge on the capacitor are satisfied.

Lower noise can be achieved (but

with even longer read out times)

by reading the signal a number of

times, for example by using CCD

structures to pass the charge into

a series of amplifiers, or back and

forth between two amplifiers.

Through a combination of slow

readout, buried channels, high

quality material, and multiple

read strategies, CCDs can achieve

read noises of only a couple of

electrons.

3.4.2 Other aspects of CCD

performance

1.) UV performance

The absorption coefficient of silicon is so high in the blue and UV that the photons are absorbed right at

the back surface of the device, away from the field created by the electrodes. There, they can fall into

surface traps at the silicon-oxide layer (all silicon grows an oxide layer upon exposure to air). To instead

drive them across the device into the wells, a variety of steps are taken:

a.) physically thinning the CCD to ~ 20 m

Thinning also reduces cross talk, since the photoelectrons have less chance to diffuse into the ‘wrong’

well. However, a thinned CCD has very little path to absorb red photons, so the quantum efficiency is

very low in that spectral range (see Figure 3.23).

Figure 3.23. CCD readout amplifier.

b.) back surface charging: Special coatings have been developed for the back surface that can repel

photoelectrons from the oxide layer.

2.) Although the CCD is

basically a linear transfer

device, a clever design

developed at Lincoln

Laboratory allows two

dimensional charge shifting

in a 4-phase device.

3.) CCD clocking can be

modified to combine

charge packets, a process

called pixel binning (see

Figure 3.24). To allow

binning without

overflowing the well, many

CCDs have a larger-capacity

“output summing well” for

the last transfer.

4.) Time-Delay-Integration

Figure 3.23. Quantum efficiency vs. absorbing thickness (from Mike Lesser). The curves, in order

of increasing absorption, are for 5, 10, 15, 30, and 50 m.

Figure 3.24. Pixel binning

(TDI): The CCD charge transfer process lends itself naturally to clocking charge in one direction at a set rate. This capability can be useful in applications where images drift across the detector array at a constant (relatively slow) rate – the charge generated by a source can be moved across the CCD to match the motion of the source. As a result, the CCD can integrate efficiently on the moving scene of sources without physically moving anything to track their motion. 5.) Deep Depletion or Fully Depleted: Silicon has low absorption efficiency in the near

infrared (~ 0.9 m; see Figure 3.4). The brute force way to get good absorption in this spectral range with a CCD would be just to make the absorbing region thicker, but there are bad side-effects like reduced charge collection and increased cross talk. These problems can be mitigated by establishing a voltage across the absorbing region that drives the photoelectrons towards the electrodes and their potential wells where charge is collected. Because the added field allows for a much deeper depletion layer, these detectors are called “deep depletion or “fully depleted” CCDs (Figure 3.24). They can have absorbing layers a few hundreds of microns thick and achieve

quantum efficiencies of ~ 90% at 0.9 m, with

usable response beyond 1 m. The price is higher dark current, more susceptibility to cosmic rays, and generally greater difficulty in fabrication (need for high purity silicon, issues in fabricating the backside contact). 6) L3 technology – E2V, Inc. supplies CCDs that take a high bias voltage on the output register, close to the point of avalanching. The result is a small gain per transfer, 1 – 2%; after many transfers, the gain is significant. This style of operation can be useful when clocking the CCD fast, since the larger outputs allow faster settling in the output. 7) CCDs can be operated in a subarray mode by clocking the output register very quickly, not worrying about CTE, until the pixels of interest are about to be read out. Because the columns clock much more slowly than the output register, good CTE can be preserved for these pixels, and the output register can be slowed to provide low noise for them. 3.4.3 Some alternative optical detectors

Direct hybrid PIN silicon diodes High performance arrays can be manufactured in the same fashion as hybrid infrared arrays, but using

PIN silicon diodes for the detectors. These devices have excellent red sensitivity, but they have liabilities

because of the complex processing required for any bump bonded array (e.g., generating the bumps,

and the process of aligning and bonding the detector and amplifier wafers), resulting in high cost and

size limitations.

Figure 3.25. Deep depleted CCD, as supplied by

LBL (commercial versions are significantly

thinner).

CMOS imagers Another spinoff from infrared arrays, CMOS imagers fabricate a silicon PN diode along with the

amplifiers in a read out similar (or even identical!) to those hybridized onto infrared detectors. CMOS

imagers can be produced in standard integrated circuit foundries and hence are relatively cheap. They

are being vigorously promoted as cheap replacements for CCDs in many applications. Since they are

more-or-less conventional integrated circuits, it is easy to add circuitry to them that carries out various

signal processing functions. In addition, they do not require charge transfer with its attendant issues and

they are radiation tolerant.

However, for use at low light levels, CMOS imagers have their own list of problems:

1.) Poor fill factor

The amplifiers compete for space on the wafer with the diodes, so fill factors range from ~ 70%

downwards, depending on the pixel size and the complexity of the amplifier. In devices not designed to

be cooled, the fill factor can be improved with an array of tiny lenses, one over each sensor (e.g., Canon

7D camera). It may be possible to improve the fill factor with a more robust design that uses charged

implants in the device to steer the photo-electrons toward that active regions. Back-illuminated devices

are being explored as a solution, but they take the devices out of the integrated circuit mainstream and

thus lose one of the advantages of these detectors.

2.) Higher noise, local peaks in dark current, poorer pixel-to-pixel uniformity

3.5. Image Intensifiers

Image intensifiers have largely been supplanted by CCDs in the near ultraviolet out to 0.9 m. However, they are very competitive farther into the ultraviolet when a large field imaging detectors is desired, such as for the GALEX mission. The basic principle of

operation of these

devices is the same

physically as for

photodiodes (Figure

3.26). When a photon

interacts in the

photocathode, the

resulting free charge

carrier must diffuse

to the edge of the

depletion region.

There it must escape

(photoelectric effect),

Figure 3.26. A vacuum photodiode.

which is more difficult than just falling

into the field as in a semiconductor

device. The escape probability < 1

contributes to the lower quantum

efficiency. Once the electron gets into

the depletion region, it is driven across

by the field and interacts (perhaps

focused by electron optics) with an

output device – a phosphor, array of

amplifiers, or microchannel plate, for

example.

A popular version that can be quite

compact and allows efficient readout

takes the output to a microchannel

plate. Microchannels are thin tubes of

lead-oxide glass with inner diameters of

5 – 45 m and length to diameter ratios of about 40 (see Figure 3.27). Their inside surfaces are coated

with a layer of PbO, which acts as an electron multiplier – when a high energy electron impacts onto it,

the PbO tends to release a number of electrons.

The full intensifier is shown in Figure 3.28. A high voltage is established from the photocathode to the

entrance of the microchannel array and then from one end of the microchannel tubes to the other;

when the photocathode releases an electron, it enters one of the array of microchannels, called a

microchannel plate (or MCP). The electron is accelerated into a wall of the microchannel, produces more

electrons that are accelerated into the wall again, and so forth. Thus, the MCP amplifies the single

electron to produce a pulse of electrons that emerges from the other end. Orthogonal electronic delay

lines are placed at the output of the MCP and any emerging pulse produces twin signals that travel in

both directions along the delay line; see Figure 3.28. By measuring the time interval between the

emergence of these two signals at the opposite ends of the delay line, it is possible to locate where the

signal originated and thus where the original photon hit the photocathode.

Figure 3.31. Operation of a Microchannel.

Recommended Reading:

Csorbe, I. P. 1985, “Image Tubes,” Indianapolis, IN: Howard Sims

Howell, S. B. 2000, “Handbook of CCD Astronomy,” Cambridge, New York: Cambridge University Press

Janesick, J. R. 2001, “Scientific Charge-Coupled Devices,” Bellingham, WA: SPIE

Joseph, C. L. 1995, “UV Image Sensors and Associated Technologies,” Exp. Ast., 6, 97

Rieke, G. H. 2003, “Detection of Light from the Ultraviolet to the Submillimeter,” Cambridge University

Press

Rieke, G. H. 2007, “Infrared Detector Arrays for Astronomy,” ARAA, 45, 77

Figure 3.28. The GALEX image intensifier with microchannel readout.