NONEQUILIBRIUM CARRIER TRANSPORT IN THE BASE OF HETEROJUNCTION BIPOLAR-TRANSISTORS

The conventional drift-diffusion equation is inadequate to analyze bal listic effects. In most device analysis, thermoelectric effects associ ated with gradients in the carrier temperature are assumed to be small . The base thickness dependence of current gain as a result of non-equ ilibrium transport of electrons in the base was experimentally shown b y many researchers. We present a simulation to analyze the electron tr ansport in the base region of an HBT based on the carrier density bala nce, momentum balance, and energy balance equations. Transistors with base thickness of 1000 Angstrom and 2500 Angstrom are investigated. Si mulation results of electron transport in the base region indicates th at tunneling electrons give rise to unique transport characteristics i n the base of HBTs. The density profile is found as a result of transi tion from ballistic to diffusive transport in the base region. The ran ge of observed non-equilibrium transport and the carrier density and t emperature profiles in the base region are functions of average moment um and energy relaxation times. The non-equilibrium effects are shown to exist up to 1000 Angstrom in the base region of AlGaAs/GaAs HBTs. T he simulation results are in good agreement with experimental data.