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 homo junction 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 α values of 0.75 have been obtained at 77 K in experimental devices with base widths of 870 Å. Microwave measurements from 2 to 18 GHz on these devices imply a unity common-emitter current gain frequency fTof ∼ 40 GHz. The observed α values in other devices are unfortunately lower than those predicted by recent Monte Carlo simulations, and an unexpectedly strong dependence of α 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 10¹⁷cm^-3.