Energy transport modeling of graded AlGaAs/GaAs HBTs: importance of giving adequate transport parameters

AlGaAs/GaAs HBTs have received great interest for applications to high-speed and high-frequency devices and ICs. Since nonequilibrium carrier transport becomes important in the HBTs, carrier energy must be considered in the modeling of them. For this purpose, an energy transport model that uses hydrodynamic equations derived from the Boltzmann equation is attractive and has been applied to AlGaAs/GaAs HBTs. In the HBTs, material parameters should depend on material composition as well as carrier energy and the doping densities. However, up to now, methods of giving parameters in the transport equations were too crude. A constant value of energy relaxation time was assumed and parameters for GaAs at a certain doping density were also assumed for all x in Al_xGa_1-xAs. In this work, a method of giving composition-, doping-, and energy-dependent transport parameters is proposed, and successfully applied to the simulation of AlGaAs/GaAs HBTs.