ELECTRON-TRANSPORT IN BIPOLAR-TRANSISTORS WITH BIAXIALLY STRAINED BASE LAYERS

In silicon npn bipolar junction transistors grown on (100) oriented su bstrate, at base doping levels in excess of 10(20) boron atoms/cm(3), strain induced splitting of the normally sixfold degenerated conductio n band minimum becomes important and needs to be considered in modelin g of injection currents. The biaxial tensile strain, originating in th e smaller covalent radius of boron compared to silicon, induces a lowe ring of two valleys with heavy effective mass in vertical direction wh ereas the remaining four valleys are raised in energy. Using a coupled set of equations for the electron gas systems in the twofold and four fold degenerated valleys, emitter and collector current formulas fire derived, In die relevant ease of strong f-type intervalley scattering rates compared to Anger recombination rates (which holds at least up t o about 10(21) cm(-3)) collector currents are described by(V-EC = 0V) jc = -eD(n4)n(4,0)+D(n2)n(2,0)/w(eV(BE)/V-th-1) provided that the elec tron diffusion length is large compared to the base width w., D-n4 D-n 2, and n(4,0), n(2,0) are diffusion constants and equilibrium minority carrier concentrations in the two electron gas systems, respectively. In Si/SiGe heterojunction bipolar transistors the conduction band sit uation in the base is similar to that fn extremely heavily boron doped (homojunction) base layers as presence of Ge also causes die conducti on band minimum to split (splitting is, however, of opposite sign), Th us, the transport model discussed here likely applies also to that kin d of device.