| OCR Text |
Show 2 n+ GaAs layer is present between the ernitt r conta t and th Al aA lay r. Th nergy band diagram of this device is shown in Figur 4.1. The primary advantage of an HBT is its high emitter effici ncy. For an n-p-n structure the electron injection and hole injection current ratio an b xtr rn ly large because of the energy gap difference between the emitter and bas rnat rials. In homojunction bipolar transistors, the advantage of this band-gap dift ren cannot be used, instead the ratio of the doping concentration in the emitter and base rnust be very high. In HBT, the emitter doping concentration can be much lower than that of the base. This arrangement allows the designer to achieve a very high base doping and a very thin base without increasing the base layer sheet resistance. This also allows one to have a relatively large emitter-base depletion layer thickness, inside the lightly doped emitter body, and hence a low junction capacitance. The thin base layer allows one to achieve a low base transit time, and by selecting an appropriate collector doping, the collector depletion region transit time can be optimized, consistent with the need to support a high collector current AIGaAs 1 Conduction __ _ Band Valence Band Fermi Level Emitter GaAs r- - - - - - - - - - -- - - - . Barrier for Holes _j_ Base Figure 4.1. Energy Band Diagram of an HBT Structure |
