Numerical simulation and comparison of Si BJT's and Si(1x)Ge(x) HBT's
Abstract
Using the Monte Carlo method for the solution of the Boltzmann transport equation, the authors analyze the lowfield carrier mobilities of strained layer and bulk Si and Si(1x)Ge(x) alloys. Strained alloy layers exhibit higher lowfield mobility compared with bulk Si at doping levels greater than 10 to the 18the cu cm and for a Ge mole fraction x less than or equal to 0.2, while the unstrained alloy bulk lowfield mobility is always lower than that of Si for any doping level or mole fraction. These mobilities are then used in a twodimensional driftdiffusion equation solver to simulate the performance of Si BJTs (bipolar junction transistors) and Si(1x)Ge(x) HBTs (heterojunction bipolar transistors). The substitution of a Si(0.8)Ge(0.2) layer for the base region leads to a significant improvement in current gain, turnon voltage, and highfrequency performance. Maximum unity current gain frequency fT increases two times over that of an Si BJT if the bulk alloy mobility is used for the alloy base layer; it increases three times if strainedlayer mobility is used. Maximum frequency of oscillation also improves, but not as dramatically as fT.
 Publication:

IEEE Transactions on Electron Devices
 Pub Date:
 October 1989
 DOI:
 10.1109/16.40892
 Bibcode:
 1989ITED...36.2129P
 Keywords:

 Bipolar Transistors;
 Carrier Mobility;
 Digital Simulation;
 Germanium Alloys;
 Junction Transistors;
 Silicon Alloys;
 Boltzmann Transport Equation;
 Current Density;
 Heterojunctions;
 Monte Carlo Method;
 Poisson Equation;
 Electronics and Electrical Engineering