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Mrinal K Mandal [email protected] Department of E & ECE I.I.T. Kharagpur. 721302. www.ecdept.iitkgp.ernet.in Microwave Devices 1
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Mrinal K Mandal
Department of E & ECE
I.I.T. Kharagpur. 721302.
www.ecdept.iitkgp.ernet.in
Microwave Devices
1
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,
HEMT,
MOSFET,
Memories,
CCD etc.
•Gunn diode,
LSA diode,
InP diode,
CdTe diode
etc.
•READ
diode,
IMPATT
diode,
TRAPATT
diode,
BARITT
diode etc.
Linear-
beam tubes
(O)
Crossed-
field tubes
(M)
•Vacuum
triodes,
pentodes,
Klystrons,
Reflex
Klystron,
Couple-
cavity tubes
etc.
•Magnetron,
Forward-
wave
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
sons.
4
Microwave Power Requirements
Missile
transmitters
Dominated by tubes
Air defences
EW Jammers
Smart weapons
Commercial
telecommunication
EW Phased array MCMs
Commercial
base
stations
EW Jammers
Radar array
Dominated by solid-state devices
Frequency (GHz)
Maxim
um
po
wer
(Watt
)
100k
10k
1k
100
10
1
0.1
1 10 100
6
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
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
9
Material Selection
•Substrate: GaAs substrate because of its high mobility.
: silicon substrate, low cost and high yield.
: GaN substrate for high power.
10
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.
11
Microwave Bipolar Junction Transistors (BJT)
E B C
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.
13
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)
Im
Xc = 1/(2π fT Cbc)
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:
Gm
Vth = kT/e
k
where : maximum available power gain.
: thermal voltage.
: Boltzmann’s constant.
(GmVthVm)0.5 fT = Emvs/(2π)
Gain-frequency plot
17
BJT
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
18
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.
CB
VB
E F
InPxSb(1-x) InPxSb(1-x)
InAs
CB
VB
E F
n-Ge p-
GaAs n-Ge
19
Typical Output Characteristics of a HBT
Output characteristics. Output characteristics.
GaN based HBT Sb based HBT
20
Hetero Junction Bipolar Transistor (HBT)
Junction current:
A
e
VT = kT/e
where : device cross-section.
: electronic charge
: thermal voltage.
I = AeDn ppo(eV/VT -1) /Ln
Dn
ppo
Ln
: electron diffusion constant.
: equilibrium electron density in the base.
: hole diffusion length.
Schematic diagram of a HBT.
21
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
V
I
Is VB
V
C
VB
Vbi
Non-linear I-V characteristics. Non-linear C-V characteristics.
23
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.
24
Characteristics of a Tunnel Diode
Band diagram under zero bias. Tunnel diode I-V characteristics.
25
•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)
27
• 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.
28
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.
29
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.
30
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.
31
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 :
τ:
where Steady state forward bias current.
Minority carrier life time. I
t
I
t
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.
33
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.
34
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.
35
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.
I I
•Differential resistance developed in a bulk III-V compound.
36
RWH Theory
E < El Ev <E< Eu Eu < E
E
El
Ev
Eu
: 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
Vth.
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.
40
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
form.
•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
45
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
IMPATT Diode Operation
Diode terminal resistance: Diode terminal resistance:
Eqv. Circuit of the IMPATT housing.
47
Negative resistance vs transit angle.
Operating frequency range:
Transit angle:
IMPATT Diode Operation
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.
49
Junction-Field Effect Transistors
GaAs FET dc characteristics. Square law.
•Square law:
•Mutual conductance:
Operation of a JFET.
50
Self bias method. Dual source bias method.
Change in voltage across Rs due a change
in source voltage:
Junction-Field Effect Transistors
51
Junction-Field Effect Transistors
Internal structure of a JFET.
A power JFET.
Stability factor:
52
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.
53
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
56
MOSFET High Frequency Operation
Theoretical limitation on gate length. Sources of capacitances.
58
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.
60
HEMT dc Characteristics
Measured transfer and input
characteristics.
Measured output characteristics.
61
HEMT dc Characteristics
Measured transconductance. Measured gate capacitance variation.
62
HEMT
Electron microscope photograph of a GaN based HEMT.
Performance comparison:
63
Comparison of Solid-State Devices
64
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
beam.
•Suitable for amplification. 65
Ph. – +91-3222-283550 (o)
Department of E. & E.C.E.
I.I.T. Kharagpur, 721302.
Thank you
?
66
