Gunn Diode Fundamentals

Gunn diodes are used as transferred electron oscillators (TEO) by using the negative resistance property of bulk Gallium Arsenide. Greater than about an electric field of 3.2 KV/cm, the electrons in N type GaAs move from a high-mobility, low-energy valley to another valley where the mobility is lower. Consequently, the net electron velocity is lower. This negative resistance is used for generation of microwave power.

Oscillator Design Considerations
Stable TEO oscillators are most easily designed by mounting a Gunn diode in a coaxial cavity or in a waveguide cavity as shown in the accompanying schematics. Microstrip oscillators also may be used. However, particular attention must be paid to heatsink the diode adequately.

Normally, TE modes are used for excitation in the waveguide cavities. The series equivalent circuit of a Gunn diode is a capacitance in series with a negative resistance. This capacitance may be resonated with the inductance of a shorted waveguide section of suitable length located behind the diode. The coupling to the external load may be controlled by an iris of appropriate dimensions. The distance between the diode and the iris is roughly ?g/2 at the desired frequency.

The Gunn diode may be mounted on a post to provide adequate heatsinking. The coupling to the waveguide is through the post. The DC bias is conveniently applied through a dumbbell filter choke combination. For the coaxial cavity design, the diode is conveniently mounted at the end of the line to provide adequate heatsinking. The diode may be coupled to the load by an inductive loop located near the diode. Such a coupling also isolates the bias supply from the load. A sliding short on the inner conductor may be used to adjust the frequency.