Q6Microwave Theory And Techniques

The Gunn effect, discovered by J.B. Gunn in 1963, is the spontaneous generation of coherent microwave-frequency oscillations in the current flowing through a uniform, unstructured (no p-n junction needed) bar of certain compound semiconductors — most notably gallium arsenide (GaAs) and indium phosphide (InP) — when a sufficiently strong DC electric field (above a critical threshold, typically a few kV/cm) is applied across it. The effect arises because these particular semiconductors' conduction-band structure has two (or more) distinct energy valleys: a lower-energy valley where electrons have high mobility, and a higher-energy satellite valley where electrons have much lower mobility. At low applied field, essentially all conduction electrons remain in the high-mobility valley; but above the threshold field, electrons gain enough energy to be scattered into the low-mobility satellite valley, and since this electron transfer reduces the average electron drift velocity as the field increases further, the material exhibits a region of negative differential resistance (current decreasing as voltage increases) — precisely the condition needed to sustain self-generated oscillations, exploited in Gunn-diode microwave oscillators discussed further in Part C, Q.3 of this paper.