Millimeter Wave Technology

transcript

Mrinal K Mandal

mkmandal@ece.iitkgp.ernet.in

Department of E & ECE

I.I.T. Kharagpur. 721302.

www.ecdept.iitkgp.ernet.in

Microwave Devices

Applications

Fighter Jet

Radio astronomy

Ship radar Short range communication 2

Various Types of Microwave Power Devices

Microwave Devices

Solid-state devices Microwave tubes

Transistor Field-

effect

transistor

Transferred

electron

devices

Avalanche

transit-time

devices

•BJT, HBT,

Tunnel

diode etc.

•JFET,

MESFET,

MOSFET,

Memories,

CCD etc.

•Gunn diode,

LSA diode,

InP diode,

CdTe diode

•READ

diode,

IMPATT

diode,

TRAPATT

diode,

BARITT

diode etc.

Linear-

beam tubes

Crossed-

field tubes

•Vacuum

triodes,

pentodes,

Klystrons,

Reflex

Klystron,

Couple-

cavity tubes

•Magnetron,

Forward-

crossed-

field,

backward-

wave cross-

coupled etc.

Tunnel diode Gunn diode Magnetron Reflex clystron 3

Microwave Devices

Reference books:

1. Microwave Devices and Circuits, Samuel Y. Liao, Prentice-Hall of

India.

2. Microwave Engineering, David M. Pozar, John Wiley and sons.

3. Physics of semiconductor devices, S.M. Sze, John Wiley and

Power Handling Capacity

Dominated

Gyrotron

Microwave Power Requirements

Missile

transmitters

Dominated by tubes

Air defences

EW Jammers

Smart weapons

Commercial

telecommunication

EW Phased array MCMs

Commercial

stations

EW Jammers

Radar array

Dominated by solid-state devices

Frequency (GHz)

1 10 100

Microwave Tubes

• Large size, bulky

• Usually, fixed frequency

• Complicated power supply (HV)

• Poor quality of waveform spectrum

• Slow tuning and coupling

• Cost

Disadvantages:

AWACS Satellite transponder Ship RADAR

7 Magnetron - spectrum

Microwave Solid-State Devices (Power Handling)

(2003 data)

Solid-state Devices and Their Applications

Major Applications Substrate

Material

Frequency

Limitation Device

Transmitter Amplifiers Si, GaAs, InP < 300 GHz IMPATT

Local oscillators, Transmitter Amplifiers

GaAs, InP < 140 GHz Gunn

Amplifiers , Oscillators, Switches,

Mixers, and Phase shifters GaAs, InP < 100 GHz FET&HEMT

Switches, Limiters, Phase shifters,

Modulators, and Attenuators Si, GaAs < 100 GHz p-i-n

Multipliers, Tuning, Phase shifters, and

Modulators GaAs < 300 GHz Varactor

Material Selection

•Substrate: GaAs substrate because of its high mobility.

: silicon substrate, low cost and high yield.

: GaN substrate for high power.

Performance Characterization

Out put power Pmax

Pmax a Vmax x Imax

Vmax : Voltage breakdown

Imax: Heat removed, gate width and length.

Power Density PD

PD = Vmax x Current density

Vmax: Voltage breakdown

Current density: limited by bandgap and thermal conductivity.

Frequency f

f max a (Vs/L), where Vs: saturated carrier velocity, L: Gate length

Pmax a 1/f2

Power-added-efficiency, PAE = 100*{[POUT]RF – [PIN]RF} / [PDC]TOTAL

Depends on wave shape, impedance, leakage current and power gain.

Microwave Bipolar Junction Transistors (BJT)

Different forms of microwave transistors

Schematic diagram and symbols of microwave BJTs

• npn - for high frequency operation. 12

BJT Fabrication

Electron microscope photograph of a BJT.

3d view of a BJT.

Standard techniques:

•Diffusion,

•Ion implantation,

•Molecular beam epitaxy.

BJT Carrier Flows

High Frequency Model of a BJT

Low-signal, low-frequency model.

Low-signal, high-frequency model.

• at microwave frequencies, S-parameters are measured and then converted to

equivalent y-parameters. 15

Frequency Limitation

Some important points (Johnson conditions):

•Saturated drift velocity – maximum possible velocity of carriers vs.

•Dielectric break down – a maximum electric field Em.

•Maximum current is limited by the base width.

Voltage-frequency limitation:

VmfT = Emvs/(2π) (2×1011 V/s for Si, 1×1011 V/s for Ge)

fT = 1/(2πτ)

τ = L/v

Vm = EmLmin

where : Transit time cutoff frequency.

: Avg. time with velocity v to traverse the emitter-collector distance.

: Maximum allowable applied voltage. 16

Frequency Limitation

Current-frequency limitation:

(ImXc ) fT = Emvs/(2π)

Xc = 1/(2π fT Cbc)

where : maximum current of the device.

: reactive impedance.

: collector-base capacitance.

Power-frequency limitation:

(PmXc)0.5 fT = Emvs/(2π)

Power gain-frequency limitation:

Vth = kT/e

where : maximum available power gain.

: thermal voltage.

: Boltzmann’s constant.

(GmVthVm)0.5 fT = Emvs/(2π)

Gain-frequency plot

Output characteristics of a BJT.

A typical Si high power BJT characteristics:

Frequency; 2.7-2.9 GHz Output power: 105 W Pulse width: 50 mm Duty cycle: 10% Gain: 6.5 dB (min) Efficiency: 40% (min) Supply voltage: 40V

• Si BJT

– < 5 GHz

– 100-600W at 1 GHz

– > 40% Efficiency

– Low cost

Hetero Junction Bipolar Transistor (HBT)

• Differing semiconductor materials (similar lattice constant) for the emitter-base

and the base-collector junction, creating a heterojunction.

• Injection of holes from the base into the emitter region is limited (potential

barrier in the valence band is higher than in the conduction band).

• This allows a high doping density in the base, reducing the base resistance

while maintaining gain.

• Can be used at very high frequencies (a few hundred GHz).

• Materials used: Si, GaAs, and InP, AlGaAs, wide-bandgap semiconductors

(GaN, InGaN) are especially promising.

InPxSb(1-x) InPxSb(1-x)

n-Ge p-

GaAs n-Ge

Typical Output Characteristics of a HBT

Output characteristics. Output characteristics.

GaN based HBT Sb based HBT

Hetero Junction Bipolar Transistor (HBT)

Junction current:

VT = kT/e

where : device cross-section.

: electronic charge

: thermal voltage.

I = AeDn ppo(eV/VT -1) /Ln

: electron diffusion constant.

: equilibrium electron density in the base.

: hole diffusion length.

Schematic diagram of a HBT.

Substrate Comparison

Microwave Diodes

Non-linear C-V Characteristics Non-linear I-V Characteristics

Frequency multiplication Frequency mixing

Voltage Controlled Oscillator Harmonic generation

Voltage tuned filter Switching

Frequency conversion Modulation

Harmonic generation Limiting

Parametric amplification Detection

Applications

Non-linear I-V characteristics. Non-linear C-V characteristics.

Tunnel Diode

Non-linear I-V characteristics.

• Negative resistance semiconductor device (generates power).

• p+-n+ junction, depletion layer without bias voltage ~ a few Ǻ.

• Classical transit time concept (τ = L/v) does not hold.

• Transit time depends on the quantum transition probability per unit time.

• Particle will tunnel through the barrier from filled energy states.

• Empty state on the other side of the barrier.

• Application in amplifier, oscillator, binary memory.

Circuit symbol.

Characteristics of a Tunnel Diode

Band diagram under zero bias. Tunnel diode I-V characteristics.

Equivalent circuit of a Tunnel diode.

Typical values:

Tunnel Diode

•Input impedance:

•Resistive cutoff frequency:

•Self-resonance frequency:

Tunnel Diode

Tunnel diode amplifier (8-12 GHz)

Tunnel diode amplifier using a circulator.

•Input reflection coefficient:

•Amplification:

(series loading)

• Schottky diode does not have a recovery time (no charge carrier depletion region).

The switching time is ~100 ps (p–n diode ~ a few100 ns).

• Switching speed ~50 GHz.

• Less reverse recovery current (low EMI noise).

• no slow, random recombination.

Reverse recovery time:

• Hot carrier diode.

• Metal-semiconductor (n type) junction.

• Low forward voltage drop (0.1–0.4 V) and a very fast

switching action.

Characteristics:

Schottky Diode

Schottky diodes.

Symbol of a Schottky diodes.

Schottky Diode

Limitations:

• Low reverse voltage ratings, (typically <50 V), high reverse leakage current.

• Reverse leakage current increases with temperature - thermal instability.

• This often limits the useful reverse voltage to well below the actual rating.

• Higher reverse voltages are accompanied by higher forward voltage drops.

Recent developments: SiC – provides low leakage current and a high voltage

rating (~1700 V).

Zero bias. Forward bias.

P-I-N Diode

• P+-I-N+ (+ regions for ohmic contacts).

• Obeys the standard diode equation at low frequencies, an

almost perfect resistor at higher frequencies.

• At higher frequencies, not enough time to remove the

charges, so the diode never turns off (poor reverse recovery

time).

Characteristics:

• The high-frequency resistance is inversely proportional to the DC bias current (acts

as a variable resistor).

• Wide intrinsic region: low capacitance in reverse bias.

• In a PIN diode, the depletion region exists almost completely within the intrinsic

region and almost independent of the reverse bias applied to the diode. This increases

the volume where electron-hole pairs can be generated by an incident photon:

photodetector.

• Intrinsic region: inferior rectifier, but it makes suitable for attenuators, fast switches,

photodetectors, and high voltage power electronics applications.

PIN Diode Switch

PIN diode as a switch. PIN diode switch.

•Under zero or reverse bias: low capacitance, high resistance to an RF signal.

•Under a low forward bias (~1 mA): typical RF resistance of about 1 ohm

•Switching speed ~1 microsecond.

Step Recovery Diode (SRD)

•SRD is a semiconductor junction diode, also called charge-storage diode.

•When diodes switch from forward conduction to reverse cut-off, a reverse current flows

briefly as stored charge is removed.

•Application: pulse generator, parametric amplifier.

Stored charge: Qs = If τ If :

where Steady state forward bias current.

Minority carrier life time. I

For sinusoidal input pulse.

For rectangular input pulse.

Measured pulse response for

rectangular input pulse. 32

Transferred Electron Devices (TEDs)

•No p-n junction.

•Same type of material with different grading profile.

• Materials: GaAs, InP, CdTe etc.

•Normal operation at low electric field.

•Negative resistance at high electric field (hot electron). Most popular application – in

oscillators to generate microwaves.

Gunn diode Typical I-V characteristics.

Gunn Effect

J.B. Gunn (1963), IBM Lab.

Drift velocity vs electric field in GaAs

Probe to measure Gunn effect.

GaAs layer •Threshold electric field varies with:

length, type of the material, temperature.

Gunn Effect

Current waveform of n-type GaAs reported by Gunn.

•Input is a voltage pulse, amplitude – 16 V, duration 10-nS.

•Specimen length 25 micro meter, oscillation frequency – 4.5 GHz.

Explanation of Gunn Effect

Ridley-Watkins-Hilsum (RWH) theory:

High field domain. High current filament.

Low field High field Low current High current

Voltage-controlled mode. Current-controlled mode.

•Differential resistance developed in a bulk III-V compound.

RWH Theory

Data for two-valley semiconductors:

E-k diagram of a n-type GaAs

RWH Theory

E < El Ev <E< Eu Eu < E

: applied electric field.

: E-field of the lower valley.

: E-field of the valance band.

: E-field of the upper valley.

Conductivity of the sample:

Where electron densities in the lower and upper valleys are nl and nu. 38

Differential Resistance

Differentiating the conductivity with respect to E

Comparing with Ohm’s law, finally

So, condition for negative differential resistance is

Positive and negative electron mobility. 39

Criteria for Negative Resistance

1. Energy difference between bottom of the lower and upper valleys much larger than

2. The separation energy must be smaller than the band gap. Otherwise the

semiconductor becomes conductor.

3. Electron in the lower valley must have high mobility, small effective mass, and low

density of state (dE/dk larger).

•Si, Ge, InSb, InAs, GaP do not meet the conditions.

•GaAs, InP, CdTe meet the conditions.

Current Modulation

Current vs. Electric field.

•Charge accumulation, formation of high

field domain.

•Additional voltage is absorbed by this

domain.

•Domain travels along the length and

reaches anode and disappears – current

modulation.

•Domain’s length is inversely

proportional to doping.

Consider applied ≈ Eth on a n-GaAs sample,

Dipole formation from noise.

E-field variation inside the sample.

Velocity variation within the sample. 41

Application – Gunn Diode

GaAs-sample

Cathode

Electron microscope picture of

a Gunn diode.

Gunn diode as a microwave

source.

Internal connection of the source. Gunn diode

A typical Gunn diode internal layers. 42

Gunn Diode Oscillation

1. Transit time mode: τ0 = τt

2. Delayed mode: τ0 > τt

Drift velocity vs electric field.

1. τ0 = τt :

vd = vs= fL ≈ 107 cm/S

Current is collected only when the

domain arrives at the cathode.

2. τ0 > τt :

106 cm/S< fL < 107 cm/S.

Domain is collected while E<Eth, a new

domain cannot form until the field rises

above threshold again. 43

3. Quenched mode: τ0 < τt

4. Limited-space –charge (LSA) mode: τ0 < τt (τ0 = 3τd).

Gunn Diode Oscillation

3. τ0 < τt :

fL < 2x107 cm/S.

•If bias field is below Es, domain

collapses before it reaches the anode.

•When the bias field swings back above

Eth, a new domain while E<Eth, a new

domain is nucleated.

4. τ0 < τt :

fL > 2x107 cm/S.

•Domain do not have sufficient time to

•Most of the part maintains negative

conductance region.

•Electric field is uniform.

•Current is proportional to drift velocity.

•Application – LSA diode. 44

Avalanche Transit-Time Devices

•Rely on the effect of voltage breakdown across a reverse biased p-n junction.

•Carrier impact ionization and drift in the high field region of a semiconductor junction

produces a negative resistance at microwave frequencies.

1. Impact ionization avalanche transit-time operation (IMPATT): efficiency 5-10%.

2. Trapped plasma avalanche triggered transit operation (TRAPATT): efficiency 20-60%.

3. Barrier injected transit-time operation (BARITT): depends on many factors.

Mode of operation:

Abrupt pn-junction Linearly graded pn-junction P-I-N diode

Three types of IMPATT diode.

Abrupt pn-junction Linearly graded pn-junction P-I-N diode

IMPATT Diode Operation

•The space between n+ -p junction and the i –p+ junction: space charge region.

•The diode is reverse biased and mounted in a microwave cavity. The impedance of the

cavity – inductive, diode impedance – capacitive, together form a resonant circuit.

•Can produce a negative ac resistance – oscillation.

•Free electron with sufficient energy strikes a atom - breaks the covalent bond.

•Liberated electron gains energy - chain reaction.

• This phenomenon is called impact avalanche.

IMPATT housing. Internal structure. 46

Diode terminal resistance: Diode terminal resistance:

Eqv. Circuit of the IMPATT housing.

Negative resistance vs transit angle.

Operating frequency range:

Transit angle:

IMPATT diode. 48

Field Effect Transistors

•Uses an electric field to control the conductivity of a channel.

•Unipolar transistors - involve single-carrier-type operation.

•Usually follow square law.

•Field effect transistor (FET) type:

JFET, MESFET, HEMT, MOSFET, Memories, CCD etc.

Dc characteristics.

Junction-Field Effect Transistors

GaAs FET dc characteristics. Square law.

•Square law:

•Mutual conductance:

Operation of a JFET.

Self bias method. Dual source bias method.

Change in voltage across Rs due a change

in source voltage:

Internal structure of a JFET.

A power JFET.

Stability factor:

Metal-Semiconductor Field Effect Transistor

• similar construction as JFET. But uses a Schottky(metal-semiconductor)

junction.

• Compound semiconductor - GaAs, InP, or SiC.

•Faster but more expensive than si-based JFETs or MOSFETs.

•Uses – microwave communications and radar, not good for digital integrated

circuits.

Drain current: Mutual conductance:

Internal structure of a MESFET.

Metal–Oxide–Semiconductor FETs (MOSFET)

•A four-terminal device with source (S), gate (G), drain (D), and body (B) terminals,

the body may be internally connected to the source terminal.

•Enhancement mode and depletion mode.

• Channel length has been shrunk to ~ hundred nm.

Historical development of RF MOSFETs. Schematic cross section of a

MOSFET. 54

MOSFET Operation

Cross section diagram of a MOSFET.

Output characteristics. Transfer characteristics. 55

Linear operating region.

Saturation mode at pinch -off. Saturation mode.

MOSFET Operation

MOSFET Drain Current

•Drain current:

•In linear region:

•In saturation region:

MOSFET High Frequency Operation

Theoretical limitation on gate length. Sources of capacitances.

Circuit Model of a MOSFET

Hi frequency circuit model of a MOSFET.

High-Electron-Mobility Transistor (HEMT)

•High-electron-mobility

transistor (HEMT) -heterostructure FET.

•Incorporate a junction between two

materials with different band

gaps (heterojunction) as the channel.

• Commonly used material -

GaAs with AlGaAs. Indium - better high-

frequency performance, GaN - high-

power.

• Thin highly-doped n-channel donates

mobile electrons (n-AlGaN wide-

bandgap). They are transferred to non-

doped narrow-bandgap channel layer

(GaN), free to move without collision

with impurities – low resistivity, high

mobility.

•2d electron gas (thickness ~ 100Ǻ).

•Uses: mm-wave products such ascell

phones, satellite television receivers,

and radar equipment.

Band diagram.

HEMT dc Characteristics

Measured transfer and input

characteristics.

Measured output characteristics.

HEMT dc Characteristics

Measured transconductance. Measured gate capacitance variation.

Electron microscope photograph of a GaN based HEMT.

Performance comparison:

Comparison of Solid-State Devices

Microwave Linear-Beam Tubes (O-Type)

O-types

Slow-wave structure Resonant cavity

Forward-wave

structure

Backward-

wave structure

Klystron

Reflex Klystron

Twystron Helix TWT,

Coupled-

cavity TWT

BWA, BWO

•Electron receives PE from the dc voltage before arriving in the m-wave interaction

region. PE converted to KE.

•Acceleration-deacceleration – bunching effect.

•Electron’s KE converted to m-wave energy.

•A magnetic field whose axis coincides with the electron beam is used to hold the

•Suitable for amplification. 65

mkmandal@ieee.org

Ph. – +91-3222-283550 (o)

Department of E. & E.C.E.

I.I.T. Kharagpur, 721302.

Thank you

Millimeter Wave Technology

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