Solid-state has eaten the low-power microwave market and is encroaching on the high-power one. Below 30 GHz, GaN HEMTs are pushing out TWTs in many applications. Above 30 GHz, solid-state still dominates the receive side (LNAs) while tubes hold on for high-power transmit.
9.1 Gunn diodes (transferred-electron devices)
In gallium arsenide (GaAs) and indium phosphide (InP), the conduction band has a low-energy "central valley" with light effective mass and a higher-energy "satellite valley" with heavier effective mass. At low electric field, electrons sit in the light-mass valley and have high mobility. Above a threshold field (~3.2 kV/cm in GaAs), electrons gain enough energy to scatter into the heavy-mass valley, where their mobility is much lower. As more electrons transfer, the bulk current actually decreases despite increased voltage.
This is negative differential resistance (NDR): in part of the I-V curve. Stick a chunk of GaAs with this property in a microwave cavity tuned to the right frequency, bias it into NDR, and the cavity oscillates. The Gunn diode is born.
Output power: 10 mW to a few W in CW; up to tens of W pulsed. Frequency: a few GHz to 100+ GHz. Efficiency: a few percent.
Applications: police and traffic radar (10 GHz X-band, 24 GHz K-band, 35 GHz Ka-band), automatic-door radar sensors, satellite-LNB local oscillators (older units), low-power microwave test sources.
9.2 IMPATT and TRAPATT diodes
IMPATT (IMPact Avalanche and Transit Time) uses a different mechanism for NDR. A reverse-biased pn junction is driven into avalanche breakdown. The avalanche carriers, having generated, must transit the drift region before reaching the contact. The combination of impact-ionization delay (avalanche multiplication takes finite time) and transit-time delay (carriers move at saturation velocity through a fixed distance) creates a phase lag between voltage and current that, in the right cavity, looks like NDR.
Output power: hundreds of mW to a few W CW; up to 50 W pulsed at X-band. Frequency: up to 100 GHz. Higher noise than Gunn diodes but more output power.
TRAPATT (TRApped Plasma Avalanche Triggered Transit) is a high-efficiency relative of IMPATT operating at lower frequencies but higher peak power. BARITT (BARrier Injection Transit Time) is a related lower-noise variant.
Applications: radar transmitters (especially small phased arrays), military comms, EW.
9.3 Tunnel diodes
A heavily-doped pn junction with NDR coming from quantum-mechanical tunneling. Forward bias the junction; at low voltage, electrons tunnel from p-side to n-side states; raise voltage, and the alignment of bands shifts so tunneling decreases; raise more, and normal forward-current diode behavior takes over. The intermediate region has .
Tunnel diodes are very fast (ps switching times) and very low noise. They were popular in 1960s microwave amplifiers but largely displaced by FETs. They survive in some niche cryogenic and very-high-frequency applications.
9.4 Varactor diodes
A varactor (variable-capacitance diode) is a reverse-biased pn junction whose depletion-layer capacitance varies with bias. Used as a voltage-controlled capacitor. Two main applications:
- Tuning. Drop a varactor across an LC tank and you can sweep the resonant frequency by varying its bias. Every voltage-controlled oscillator (VCO) in a phase-locked loop uses varactors.
- Frequency multiplication. A varactor's nonlinear capacitance generates harmonics of the input signal. Useful for generating millimeter-wave from a lower-frequency clean source.
9.5 PIN diodes
A PIN diode has an intrinsic (very lightly doped) layer between p and n regions. Under forward bias, the I-region is flooded with carriers and conducts; under reverse bias, the I-region is depleted and acts like an insulator with low capacitance. The transition is governed by carrier lifetime, which makes PIN diodes act as resistors at microwave frequencies (not as the rectifying diodes they are at DC).
A PIN diode is therefore a DC-controlled microwave switch. With forward bias, it shorts (a few ohms); with reverse bias, it opens (high impedance, low capacitance). Used everywhere:
- T/R switches. Toggle between transmitter and receiver in a transceiver.
- Phase shifters. PIN-switched line sections or loaded lines provide digital phase shifts in phased arrays.
- Variable attenuators. PIN current sets the resistance.
- Limiters. A PIN diode in shunt automatically clamps incoming power above a threshold, protecting downstream LNAs from accidental high-power input.
9.6 HEMTs and pHEMTs
The high-electron-mobility transistor is a heterojunction FET where the channel sits in an undoped layer with carriers donated from a separate doped layer above. The result is a 2D electron gas with mobility much higher than any silicon FET, allowing above 100 GHz. pHEMT (pseudomorphic HEMT) is a variant with strain-engineered layers for even higher performance.
Standard pHEMT material is GaAs-based. Used in:
- Low-noise amplifiers (LNAs). Below 1 dB noise figure at 10 GHz is routine.
- Power amplifiers at moderate power.
- Mixers.
Every modern satellite-TV LNB has a pHEMT LNA. Every Wi-Fi access point's front end is a HEMT-based LNA. The 8.4 GHz LNA on the Cassini spacecraft was a HEMT.
9.7 GaN HEMTs
Switch the substrate from GaAs to gallium nitride and you get the wide-bandgap version. GaN HEMTs handle ten to a hundred times the power density of GaAs HEMTs, with comparable speed. The wide bandgap supports higher breakdown voltages and the wide-bandgap material handles the resulting heat better.
GaN has revolutionized microwave power amps in the last 15 years:
- 5G base stations at sub-6 GHz and at 28 GHz mm-wave.
- Modern radar PAs (active electronically scanned arrays use thousands of GaN modules instead of one big tube).
- Satellite uplink amplifiers.
- Electronic-warfare jammers.
A single GaN MMIC the size of a thumbnail can deliver tens of watts at 10 GHz. The same task in 1990 required a TWT the size of a loaf of bread.