Short note: Gunn diode

A Gunn diode is a two-terminal microwave source made of a single piece of n-type GaAs (or InP). It has no p-n junction. It works on the transferred electron effect (Ridley–Watkins–Hilsum theory), which gives the bulk material a negative differential resistance.

Principle

• The GaAs conduction band has two valleys. In the lower valley electrons are light (m* ≈ 0.068 m₀) and fast (μ ≈ 8000 cm²/V·s). The upper valley is about 0.36 eV higher, and there electrons are heavy (m* ≈ 1.2 m₀) and slow (μ ≈ 180 cm²/V·s).
• At low fields, nearly all electrons are in the lower valley, so current rises with field.
• Above a threshold field of about 3.2–3.4 kV/cm, electrons gain energy and move to the upper valley. Average drift velocity then falls as the field rises, so dI/dV < 0.
• In this negative-resistance region a high-field domain forms near the cathode, drifts to the anode at about 10⁷ cm/s and disappears. Then a new domain forms. Each domain gives one current pulse, so the frequency is about f = v_d / L.

 I |     peak
   |    /\
   |   /  \___  NDR region
   |  /
   | /
   +-------------- E
       E_th ~3.2 kV/cm

Modes of operation: transit-time (Gunn) mode, delayed-domain mode, quenched-domain mode and LSA (limited space-charge accumulation) mode.

Applications: local oscillators in receivers, police and automotive radar, microwave links, and sweep generators. Gunn diodes give mW up to about 1 W in the 1–100 GHz range, with low noise and a low bias voltage.