Q3Microwave Theory And Techniques

Construction: a Gunn diode is a bulk semiconductor device (no p-n junction) made from a compound semiconductor exhibiting the two-valley conduction-band structure — most commonly n-type GaAs, structured as a thin, uniformly-doped active layer of n-type material (roughly 10 μm, chosen so the transit time across it matches the desired oscillation frequency) sandwiched between two heavily-doped n+ ohmic contact regions.

RWH two-valley theory: the Ridley-Watkins-Hilsum theory explains the Gunn effect's negative differential resistance in terms of the semiconductor's conduction-band structure, which for GaAs (and similar III-V compounds) has a lower-energy central (Γ) valley with low electron effective mass (hence high mobility, ~8000 cm²/V·s) and a higher-energy satellite (L) valley located only about 0.36 eV above it, with much higher effective mass (hence low mobility, ~180 cm²/V·s). At low applied electric field, nearly all electrons remain in the high-mobility central valley, and current rises normally with field (Ohmic behaviour). As the field increases past a critical threshold (~3.2 kV/cm for GaAs), an increasing fraction of electrons gain enough energy to scatter into the low-mobility satellite valley; since the average electron mobility (and hence drift velocity, and current, at fixed field) falls as more electrons transfer to the satellite valley, the material's current-vs-field curve exhibits a region where current decreases as field increases further — negative differential mobility, and hence negative differential resistance for the bulk device.

Modes of operation: (1) Gunn (transit-time) mode: the classic mode, where a high-field domain nucleates at the cathode, travels through the device at the electron drift velocity, and is collected at the anode, with oscillation frequency set primarily by the transit time across the active region (f≈vd/L); operates over a moderate DC bias range with the device's applied voltage above threshold but the sample not specially tuned. (2) Delayed-domain mode: the applied RF circuit is tuned so that a new domain cannot begin forming until the applied field drops back below threshold following extraction of the previous domain, delaying formation and giving oscillation frequency somewhat different from the pure transit-time value. (3) Quenched-domain mode: the resonant circuit swings the field so far below the sustaining field partway through each cycle that the travelling domain is completely quenched (collapses) before reaching the anode, allowing operating frequencies well above the natural transit-time frequency — useful for higher-frequency operation from a given device length. (4) LSA (Limited Space-charge Accumulation) mode: the bias is a very high frequency AC signal (with the device's transit-time frequency deliberately much lower than the operating frequency) chosen so that high-field domains never have time to fully form before the field swings back below threshold each cycle; the entire bulk of the diode instead switches nearly uniformly between high- and low-mobility states, permitting substantially higher output power than the other domain-based modes since the whole device volume (not just a thin travelling domain) participates in power generation.

Applications: low-to-medium power microwave and millimetre-wave signal sources for local oscillators in receivers, Doppler radar transmitters (police radar, motion sensors), signal generators and test equipment, and as pump sources in parametric amplifiers — valued for their simplicity (a two-terminal bulk device with no junction), solid-state reliability, and ability to operate from a simple DC bias without needing high voltages, though generally at lower output power and efficiency than IMPATT diodes or vacuum-tube sources such as klystrons and magnetrons at comparable frequencies.