Importance of electron scattering with coupled plasmon-optical phonon modes in GaAs planar-doped barrier transistors

The GaAs planar-doped barrier (PDB) transistor is an MBE-grown structure which employs two unipolar homojunction barriers. One barrier, the emitter, injects energetic electrons into a thin n-type base region where these electrons are intended to experience negligible energy relaxation and thereby surmount the second (collector) barrier. Maximum common-base current gain or alpha values of 0.75 have been obtained at 77 K in experimental devices with base widths of 870 A. Microwave measurements from 2 to 18 GHz on these devices imply a unity common-emitter current gain frequency f(T) of approximately 40 GHz. The observed alpha values in other devices are unfortunately lower than those predicted by recent Monte Carlo simulations, and an unexpectedly strong dependence of alpha on ambient electron density in the base is noted. These are attributed to the previously overlooked electron scattering with coupled plasmon-optical phonon modes, and to electron-electron scattering. These scattering mechanisms must be included in any accurate model of energetic electron transport in GaAs in regions where the concentration of cooler ambient electrons is above 10 to the 17th per cu cm.