Abstract
The effects of grading on the performance of heterojunction bipolar transistors (HBTs) are studied by numerically solving the basic semiconductor equations. The numerical modelling is based on the extended drift–diffusion formulation with inclusion of thermionic emission current at the heterointerface of abrupt emitter HBTs. The results of the simulation show that the graded layer improves significantly the current driving capability of the HBT and lowers its offset voltage. However, owing to the increase in the recombination in the graded layer, the current gain of the graded HBT is lower and depends strongly on the bias at low and medium bias range. The simulation also reveals the presence of a potential minimum in the conduction band of the graded emitter HBT, which results in an electron accumulation in the region. This accumulation increases the emitter-base capacitance and its charging time, leading to a smaller unity-gain frequency f T.
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