Quasi Solution Research Articles

Effects of varying base and collector doping on high current behaviour in bipolar epitaxial transistors

High current, effects in bipolar epitaxial transistors of different relative base and epitaxial collector doping are studied by dividing the active base into incremental devices which are simulated in one dimension and the solutions combined to give a quasi two-dimensional solution. Increasing the collector/base doping ratio is shown to make base conductivity modulation dominate the current gain degradation at high current densities, while lower ratios make ‘ base widening ’ dominate. The cut-off frequency ∫T degrades, due to base widening for all cases, falling off at lower current densities for more lightly doped collectors. The seriousness of, and the rate at which emitter crowding occurs, are shown to be less in a device with lower base doping where base conductivity modulation more significantly reduces ohmic drop due to lateral base current flow. The quasi 2-D approach is shown to predict transistor terminal characteristics with reasonable accuracy.