Microwave Engineering/Active Microwave Devices 9-13 September 20061

Semiconductor Microwave 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.

Microwave Engineering/Active Microwave Devices 9-13 September 20062

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

Microwave Engineering/Active Microwave Devices 9-13 September 20063

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

Microwave Engineering/Active Microwave Devices 9-13 September 20064

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.

Microwave Engineering/Active Microwave Devices 9-13 September 20065

Applications:

(1) Voltage controlled Oscillator VCO:

FM systems and frequency agile systems

Instrumentation

Electronic warfare (EW)

Electronic counter measurement (ECM) systems.

Microwave Engineering/Active Microwave Devices 9-13 September 20066

(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

Microwave Engineering/Active Microwave Devices 9-13 September 20067

(3 )Parametric Amplifiers:

Provide very low noise amplification

Pump signal

Input

Output

CirculatorCombiner

and Varactor

Microwave Engineering/Active Microwave Devices 9-13 September 20068

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

Microwave Engineering/Active Microwave Devices 9-13 September 20069

Switches Applications

t

Switch

Bias

Source

Output

(1) Modulators in communication systems

.

.

.

Wideband switch

(2) Switch in wide band system

Microwave Engineering/Active Microwave Devices 9-13 September 200610

(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

Microwave Engineering/Active Microwave Devices 9-13 September 200611

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

Microwave Engineering/Active Microwave Devices 9-13 September 200612

p-i-n diode

p-i-n diode

Zo

ZoBias

Input

Output

3-dB quadrature coupler

Matched attenuator

Microwave Engineering/Active Microwave Devices 9-13 September 200613

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

Microwave Engineering/Active Microwave Devices 9-13 September 200614

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

Microwave Engineering/Active Microwave Devices 9-13 September 200615

Limiter p-i-n Diodes

Used for protection applications

3 dB Coupler

3 dB Coupler

Limiter

Transmitter Receiver

Limiter

Microwave Engineering/Active Microwave Devices 9-13 September 200616

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

Microwave Engineering/Active Microwave Devices 9-13 September 200617

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

Microwave Engineering/Active Microwave Devices 9-13 September 200618

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%.

Microwave Engineering/Active Microwave Devices 9-13 September 200619

Varactor Tuned Gunn Oscillators Circuits

Microwave Engineering/Active Microwave Devices 9-13 September 200620

Microwave Engineering/Active Microwave Devices 9-13 September 200621

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.

Microwave Engineering/Active Microwave Devices 9-13 September 200622

Some IMPATT Circuits