Static and dynamic behavior of a Si/Si0.8Ge0.2/Si heterojunction bipolar transistor using Monte Carlo simulation

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Abstract
A theoretical study of a Si/Si1−xGex/Si heterojunction bipolar transistor using Monte Carlo simulations is reported. The geometry and composition of the emitter‐base junction are optimized using one‐dimensional simulations with a view to improving electron transport in the base. It is proposed to introduce a thin Si‐P spacer layer, between the Si‐N emitter and the SiGe‐P base, which allows launching hot electrons into the base despite the lack of natural conduction‐band discontinuity between Si and strain SiGe. The high‐frequency behavior of the complete transistor is then studied using 2D modeling. A method of microwave analysis using small signal Monte Carlo simulations that consists of expanding the terminal currents in Fourier series is presented. A cutoff frequency fT of 68 GHz has been extracted. Finally, the occurrence of a parasitic electron barrier at the collector‐base junction is responsible for the fT fall‐off at high collector current density. This parasitic barrier is lowered through the influence of the collector potential.