Microwave Engineering/Active Microwave Devices 9-13 September Semiconductor Microwave Devices Major Applications Substrate Material Frequency Limitation Device Transmitters AmplifiersSi, GaAs, InP< 300 GHzIMPATT Local oscillators, Amplifiers Transmitters GaAs, InP< 140 GHzGunn Amplifiers, Oscillators, Switches, Mixers, and Phase shifters GaAs, InP< 100 GHzFET&HEMT Switches, Limiters, Phase shifters, Modulators, and Attenuators Si, GaAs< 100 GHzp-i-n Multipliers, Tuning, Phase shifters, and Modulators GaAs< 300 GHzVaractor 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 Microwave Diodes Non-linear C-V CharacteristicsNon-linear I-V Characteristics Frequency multiplication Frequency mixing Voltage Controlled Oscillator Harmonic generation Voltage tuned filter Switching Frequency conversion Modulation Harmonic generation Limiting Parametric amplificationDetection 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 Non-Linear Characteristics of p-n and Schottky diodes V I IsIs VBVB Non-linear I-V Characteristics of a diode V C VBVB V bi

Microwave Engineering/Active Microwave Devices 9-13 September 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 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 (2) Multiplier and harmonic generation Feasible alternative for the generation of high frequency signal ZoZo ZoZo LPF and matching BPF and matching C j (V) RsRs Varactor

Microwave Engineering/Active Microwave Devices 9-13 September (3) Parametric Amplifiers: Provide very low noise amplification Pump signal Input Output Circulator Combiner and Varactor

Microwave Engineering/Active Microwave Devices 9-13 September p-i-n Diodes Similar to the pn diode with smaller junction capacitance Very useful for a diode used a microwave switch P+P+ n+n+ i Weakly doped f.b. r.b. R j (V) C j (V) RpRp LsLs RsRs 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 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 (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 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 ZoZo ZoZo A = 20log (1 + Z o /2R s ) Reflective type

Microwave Engineering/Active Microwave Devices 9-13 September p-i-n diode ZoZo ZoZo Bias Input Output 3-dB quadrature coupler Matched attenuator

Microwave Engineering/Active Microwave Devices 9-13 September p-i-n Phase Shifters 3-dB, 90 o Hybrid B2B2 B1B1 B2B2 B1B1 Diode /4 ZoZo 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 Input Output Input Output

Microwave Engineering/Active Microwave Devices 9-13 September Switched line phase shifter L1L1 L2L2 Bias 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 Limiter p-i-n Diodes Used for protection applications 3 dB Coupler Limiter Transmitter Receiver Limiter

Microwave Engineering/Active Microwave Devices 9-13 September P in P out Insertion loss Maximum Isolation P in P out 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 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 P in P out Schottky diode p-i-n diode P in P out Control pulse p-i-n diode

Microwave Engineering/Active Microwave Devices 9-13 September 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 Varactor Tuned Gunn Oscillators Circuits

Microwave Engineering/Active Microwave Devices 9-13 September

Microwave Engineering/Active Microwave Devices 9-13 September 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 Some IMPATT Circuits