The effect of the material velocity—Field characteristic on the small-signal microwave impedance of punchthrough devices

The theoretical dependence of the small-signal impedance of microwave punchthrough diodes (BARITT) on the shape of the velocity-field characteristic is studied with the aid of a small-signal computer simulation and a simple analytical model, in an attempt to assess what semiconductor material might give the smallest negative Q factor. Four different types of velocity-field characteristics are considered showing, respectively, soft saturation (e.g., holes in Si), negative mobility (e.g., electrons in GaAs), constant mobility (e.g., electrons in GaP), and hard saturation (e.g., electrons in GaAs
1-x
P
x
). A clear picture emerges of the effect of the velocity-field characteristics on the microwave negative resistance with Si appearing to be the near optimum material; GaAs provides little if any negative resistance due to poor phasing of the injected ac space charge and the other two materials are intermediate.