1 microwave semiconductor devices major applications substrate material frequency limitation device...
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Microwave Semiconductor Devices
DeviceFrequency Limitation
Substrate Material
Major Applications
IMPATT< 300 GHzSi, GaAs, InPTransmitters Amplifiers
Gunn< 140 GHzGaAs, InPLocal oscillators, Amplifiers
Transmitters
FET&HEMT< 100 GHzGaAs, InPAmplifiers , Oscillators, Switches,
Mixers, and Phase shifters
p-i-n< 100 GHzSi, GaAsSwitches, Limiters, Phase shifters,
Modulators, and Attenuators
Varactor< 300 GHzGaAsMultipliers, Tuning, Phase shifters,
and Modulators
Most microwave devices are fabricated on a GaAs substrate because of its high mobility. A silicon substrate, on the other hand, has the advantages of low cost and high yield. The following table summarizes the various
microwave solid-state devices and their applications.
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Microwave Diodes
Non-linear I-V CharacteristicsNon-linear C-V Characteristics
Frequency mixingFrequency multiplication
Harmonic generationVoltage Controlled Oscillator
SwitchingVoltage tuned filter
ModulationFrequency conversion
LimitingHarmonic generation
DetectionParametric amplification
A microwave diode is much more than just a two-element device which has limited capabilities. It is a complex device which an integral part of many sophisticated microwave systems. Many devices have been developed using the non-linear I-V and C-V characteristics of the p-n or
Schottky-barrier junction. Various applications are summarized below
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Non-Linear Characteristics of p-n and Schottky diodes
V
I
IsVB
Non-linear I-V Characteristics of a
diode
V
C
VB
Non-linear I-V Characteristics of a diode
Vbi
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Varactor Devices and Circuits
Semiconductor p-n junction, or Schottky-barrier
n-type semiconductors with p-type diffusion
Important parameters:
Q factor
Cutoff frequency
Breakdown voltage
Sensitivity.
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Applications:
(1) Voltage controlled Oscillator VCO:
FM systems and frequency agile systems
Instrumentation
Electronic warfare (EW)
Electronic counter measurement (ECM) systems.
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(2) Multiplier and harmonic generation
Feasible alternative for the generation of high frequency signal
ZoZo
LPF and matching
BPF and matching
Cj(V)
Rs Varactor
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(3 )Parametric Amplifiers:
Provide very low noise amplification
Pump signal
Input
Output
CirculatorCombiner
and Varactor
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p-i-n Diodes
Similar to the pn diode with smaller junction capacitance
Very useful for a diode used a microwave switch
P+ n+i
Weakly doped f.b.
r.b.
Rj(V) Cj(V) Rp
Ls
Rs
P-i-n structure
Equivalent circuit of p-i-n
Parasitics Ls~ 0.1 nH Cp~ 0.3 pF
Rs~ 0.3
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Switches Applications
t
Switch
Bias
Source
Output
(1) Modulators in communication systems
.
.
.
Wideband switch
(2) Switch in wide band system
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(4) Channel selection in wideband system
(5) Signal path control in measurement systems
As a switch the main important p-i-n diode parameters are Isolation and Insertion loss
(3) To protect receiver from the transmitter (such as in radar system)
Rx
Tx
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p-i-n Diode Attenuator
p-i-n diode attenuator circuits are used extensively in automatic gain control (AGC) and RF leveling applications as well as in electronically controlled attenuators and modulators
Zo
Zo
A = 20log (1 + Zo/2Rs)
Reflective type
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p-i-n diode
p-i-n diode
Zo
ZoBias
Input
Output
3-dB quadrature coupler
Matched attenuator
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p-i-n Phase Shifters
3-dB, 90o
Hybrid
B2B1 B2B1
DiodeDiode
/4
Zo
Hybrid coupler phase shifter. Uses the fewest diodes. Any phase shift increment can be obtained with proper design of the terminating circuit.
The loaded line phase shifter
InputOutput
Input Output
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Switched line phase shifter
L1
L2Bias
Switching action is used to obtain insertion phase by providing alternative transmission paths, the difference in electrical length being the desired phase shift
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Limiter p-i-n Diodes
Used for protection applications
3 dB Coupler
3 dB Coupler
Limiter
Transmitter Receiver
Limiter
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Pin
Pout
Insertion loss
Maximum Isolation
Pin Pout
p-i-n diode
Passive Limitation. No exterior control is needed and the incident microwave power is responsible for switching from the high impedance state to low impedance state of the diode
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Controlled limitations. A small part of the incident signal is sampled and detected by Schottky diode whose the rectified current biases the diode in the forward state. The losses at low level are slightly higher, adjustments are very difficult
Controlled limitations. This method gives lower losses, better isolation, but require a delicate control circuit. Any loose of control affect receiver protection
Pin Pout
Schottky diode p-i-n diode
Pin Pout
Control pulse
p-i-n diode
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Gunn Diodes
Single piece of GaAs or Inp and contains no junctions
Exhibits negative differential resistance
Applications:low-noise local oscillators for mixers (2 to 140 GHz). Low-power transmitters and wide band tunable sources
Continuous-wave (CW) power levels of up to several hundred mill watts can be obtained in the X-, Ku-, and Ka-bands. A power output of 30 mW can be achieved from commercially available devices at 94 GHz.
Higher power can be achieved by combining several devices in a power combiner.
Gunn oscillators exhibit very low dc-to-RF efficiency of 1 to 4%.
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Varactor Tuned Gunn Oscillators Circuits
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IMPATT Devices and Circuits
IMPact Ionization Transit Time
IMPATT devices can be used for oscillator and amplifier applications
They can be fabricated with Si, GaAs, and InP
Can be used up 400 GHz.
Noisy oscillator
In general, IMPATTs have 10 dB higher AM noise than that of Gunn diodes
IMPATT diode is not suitable for use as a local oscillator in a receiver.
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Some IMPATT Circuits