Amplification in Two‐Valley Semiconductors

Microwave amplification and negative conductance are discussed theoretically and experimentally for those semiconductors that exhibit the Gunn effect, e.g., GaAs, InP, and CdTe. Field‐induced transfer of carriers, from the high‐mobility, main conduction band minimum to energetically higher, low‐mobility valleys, gives the medium a negative differential bulk conductivity. The resultant growing space‐charge waves impart to the semiconductor structure a calculated negative conductance which is in good agreement with experiment. The regime of stable amplification in n‐GaAs is found to be (2×107/l)<−(1+γ)n1l<0.65×1011 cm−2, where n1 is the carrier concentration in the lower valley, l is the sample length, and γ is the intervalley field rate of transfer of carriers. Static field distortion imposes a lower limit at n1l=1010 cm−2 on the above relation. Amplification under conditions of strong field distortion is also discussed, as well as other modes of amplification that exist in the bulk semiconductor.