Summary: | 碩士 === 國立成功大學 === 電機工程研究所 === 82 === The Heterojunction wide-gap emitter provides a large barrier to
emitter reverse current and so allows the base region to be
heavily doped, and therefore lower in resistance than for a
homojunction transistor. The general view is that the lower
base resistance obviously results in a substantially improved
frequency response. Hence, HBTs based on Npn AlGaAs/GaAs
material system have attracted considerable attention for
microwave power and digital application due to their high speed
and high current handling capabilities. In the thesis, an
analytical model based on the Npn AlGaAs/GaAs HBT structure for
carrier transport is present. The numerical simulation analyze
the I-V characteristics of the HBT, and optimizing the device
structure and parameters to increase the current gain. We also
present the calculation for the high frequency performance. Two
figures of merit of power HBT operation : cut-off and maximum
oscillation frequency are calculated. Besides, thermal
instability is a phenomenon peculiar important to power HBT,
since GaAs has a smaller thermal conductivity (Ks=0.47 W/(cm‧
K)) than Si BJT (Ks=1.45) .The heat generated in the device was
hard to dissipated then increase the junction temperature, thus
cause self-destruction of the transistor. Therefore an iso-
thermal HBT model is derived to calculate the junction
temperature, I-V characteristics, and frequency response when
to achieve a more realistic power HBT model.
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