Semiconductor optoelectronics: light-emitters and photodetectors

Light Source

Light Detector

Solid state Lighting

Solid state Lighting

Solid-state Lighting at Olympic Games 2008 Opening Ceremony

Other applications: Solar cells

Solar power plant in Tabernas desert, Andalusia, Spain. (from www.britannica.com)

Other applications: Optical radars (ladars)

Volvo introduced a new safety system aimed at avoiding low-speed collisions. The "City Safety" technology uses an optical radar system that can automatically brake the vehicle to avoid a rear-ender (from blog.wired.com/cars/2006/12/)

Other applications: IR-remote

Infrared (IR) Remote Door Locksets.

Other applications: Laser surgery

p-n junction photo-diodes

• Reverse bias applied to the p-n junction creates a depletion region with high electric field.

• Photons absorbed in the depletion regions create electron-hole pairs, which are separated by the electric field and contribute to the photocurrent.

The potential barrier BLOCKS the electrons in n-type material from diffusing into p-region; electrons in p-material generate the reverse current

The potential barrier BLOCKS the holes in p-type material from diffusing into n-region; holes in n-material generate the reverse current

Equilibrium conditions(diode in the dark, zero bias)– the diode current is zero

Light is generating the e-h pairs

--+

Junction under illumination

The light creates the carriers that move in the same directions as the minority carriers in the

reverse biased junction.Therefore, under illumination, there is a

photocurrent, which direction corresponds to the reverse current of the junction.

The origin of this photo-current is the DRIFT of photo-generated carriers.

Photodiode photocurrent

Assuming that every photon generates an electron-hole pair:Iph = q (NPH/t )= q* Pinc/(EPH).

To get the current in A, we need the power in W and the photon energy in J: EPH = hν [eV]∗q;

from this:

Iph[A] = Pinc[W]/ hν [eV];

Optical beam consists of photons with energy EPH.If there are (NPH/t) photons per second in the beam, the optical power: Pinc = EPH * NPH/t

The quantum efficiency of the photodiode:

The responsivity of the photodiode:

)/(24.1

)( WAmh

qPI

R extext

inc

ph μλην

η===

( )Ph ext incI q P h/η υ=

ηext is the photodiode efficiency.

In real photodiodes,

Photodiode modes of operation

V

I 1. “Dark” I-V: – in the absence of illumination phodiode operates as regular p-n diode

2. I-V under illumination

3. Biased photodetectormode: fast response due to strong electric field in the depletion region

4. Photovoltaic photodetectormode - no external bias: minimal noise

5. Photovoltaic source (solar cell) mode: photodiode acts as a power source converting optical energy into electrical energy.

Photodiode response time

1) Photo-carrier transit timeIf the electric field in the depletion region is strong enough, both

electrons and holes move with the saturation velocity, S ≈ 107 cm/s.

Transit time ttr ≅ W/vS, where W is the depletion region width

Note that W depends on the applied voltage:

20da 2

1)( ,N NFor WNqVV dε=−>>

The diode response time has two components: 1) transit time2) RC-limited time constant

2) The RC component of the photodiode speed of response:

WAC 0εε

=

An intrinsic RC – time constant of the photoresponse:

When the photodetector is connected to the external load (e.g., the amplifier),

τRC = C (RS + RL), where RL is the load resistance

AWdRS

)( −= ρ

RCi sC Rτ = ×

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

d

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

d

(A is the diode area)

Photodiode overall response time

RC – time constant of the photoresponse: RC C×Rτ =

Transit time ttr ≅ W/vS

Total response time: tT ≅ ttr + τRC

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

d

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

p+ region

W1 @ |V| =V1

W2 @ |V2| > |V1|

n- region

d

C(V)

ttr

Vbi - V

τRC

tT

Minimum tT achieved at optimal bias voltage, when ttr = τRC

Light-emitting diodesIntense photon emission requires high recombination rate, which, in

turn requires high concentration of both electrons and holes.

The spontaneous recombination rate: Rsp ~ n x p.

EF EF EF

Intrinsic semiconductor:

n = p = ni;

n x p = ni2

Donor doped semiconductor (n-type):

n >> p

n x p = ni2

Acceptor doped semiconductor (p-type):

p>> n

n x p = ni2

Recombination rate in i-, n-, and p- materials is the same (very low)

Forward biased p-n junction and light emission

Excess electrons

Excess holes

0

0

0 02

qV kTn n

nqV kT

n nqV kT

n n i

p p en n

p n p n e

p n n e

/

/

/

=

=

= ⋅

= ⋅

0

0

0 02

qV kTp p

pqV kT

n nqV kT

n n i

n n e

p p

p n p n e

p n n e

/

/

/

=

=

= ⋅

= ⋅

Power efficiency and responsivity of LED

The wall-plug power efficiency, ηwp = POpt/Pelectr;.

ηwp = POpt/(V*I) = (PO/I)/V = R / V

ηwp ≈ 5%...25 %. For the above example, and V = 5 V,

Where η0 = LED quantum efficiency:

R = 0.39 W/A

ηwp = 0.078 = 7.8%

The responsivity of an ideal LED:

Example: For λ = 0.63 μm and η0 = 20 %, find R

1 24OPTP hR W AI q m

.[ / ][ ]

νλ μ

= = =

For real LED: 01 24R W A

m.[ / ]

[ ]η

λ μ=

LED access resistances

nsn n

dR

Aρ=

where the resistivity of the n-layer is ρn, the thickness is dn and the contact areas is A

The p-layer series access resistance psp p

dR

Aρ=

n-layerseries access resistance

p-layerseries access resistance

The n-layer series access resistance

Semitransparent metal(optional)

dn

dp

The photon energy,h ν ≈ Єg;

(for band-to-band recombination).

LED and photodiode operating wavelength