| Summary: | This thesis is concerned with novel modeling approaches to three GaAs-based devices,
namely: the heterojunction bipolar transistor (HBT), the metal-semiconductor field-effect
transistor (MESFET) and the modulation-doped field-effect transistor (MODFET).
In GaAs-based HBTs, the transit time of carriers across the collector-base space-charge
region plays an important role in determining the performance of these devices at high frequency. In this thesis, a novel phenomenological approach is developed to modeling transport phenomena in GaAs-based HBTs. The model takes the dynamics of intervalley electron
transfer into account so that effects such as velocity overshoot can be studied. The results from this computationally-effective phenomenological model are in good agreement with
those from the time-consuming Monte Carlo approach. This suggests that the phenomenological model could be a useful tool for HBT design. The model is extended to treat the
practically-important case of operation at very high collector currents. It is revealed, perhaps for the first time, that the holes in a n-p-n device play a very significant role in the
decline of the device’s speed performance at high current levels.
A one-dimensional model for MESFETs is developed in which the characterizing system
of ordinary differential equations is analyzed by the phase portrait method. The model yields
two criteria for the formation of a stationary dipole domain in MESFETs. One of these
criteria pertains to the product of the doping density and the gate length, and produces a
length-dependent value which is different from the constant value appropriate to travelling
domains and which has been widely-used in the past for stationary domain formation. The
new criterion should prove useful in analysing the dynamic performance of MESFETs.
A new method for obtaining the electron surface charge density in the potential well of
MODFETs is presented. The surface charge density is needed in calculations of the I—V
and C—V
characteristics of the device. The new method uses the WKB method to solve
Schrödinger’s equation for the bound states, and introduces a modified effective density
of states function to solve Poisson’s equation for the potential profile. The method yields
excellent agreement with the much more computationally—intensive full quantum approach
to the solution.
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