Ballistic heterojunction bipolar transistor

Claim 1

A high-speed heterojunction bipolar semiconductor device, comprising: a base region of a first semiconductor material having a first conductivity type; emitter and collector regions each of a semiconductor material having a second conductivity type adjacent said base region and forming an emitter-base junction and a collector-base junction, respectively, said emitter region being doped to a first doping level; said base semiconductor material being doped to a doping level higher than the doping level of said emitter material to produce an emitter-base conduction band step having a high potential barrier and a sharp potential drop on the base side of said emitter-base junction, the height of said barrier being sufficient to produce a high kinetic energy level in electrons thermally injected by a forward bias applied across said emitter-base junction over said barrier from said emitter region into said base region, said barrier height being selected to produce in said injected electrons a kinetic energy level close to but slightly lower than the energy level required for injection of said electrons into an upper valley energy level in said base; said base region having a thickness substantially equal to the mean distance of travel of injected electrons so that said high kinetic energy level provides substantially ballistic transport of said electrons through said base region, substantially without collision; and an interface region formed in said emitter material at said emitter-base junction and including means to produce in said potential barrier a predetermined width gradient sufficient to prevent electron tunnelling through said barrier.