Numerical studies of large-signal power characteristics of AlGaAs/GaAs heterojunction bipolar transistors

The class B large-signal power characteristics of AlGaAs/GaAs heterojunction bipolar transistors are simulated using a one-dimensional model. The effects of various kinds of structure with uniform base, bandgap graded base and doping graded base on power characteristics are studied. It is found that the power gain reaches a maximum value at a moderate level of Al mole fraction x = 0.1 at the base-emitter metallurgical boundary. To maintain the average base doping constant, the power gain is increased with the graded exponent due to the reduced input power and the slightly increased output power for a given input signal. Based on the assumption of uniform heat distribution over the device, the self-heating effects on power characteristics are also investigated. It is shown that if the mismatching problem between the device and its external circuits is not considered and the temperature dependence of the electron mobility is taken as unαT−0.2, a rise of temperature leads to an increase in the output power, which is opposite to the results of the current gain in dc steady-state.