Analysis of semiconductors with field-dependent mobility
Using the time domain analysis of Lagrangian approach, the small-signal behavior of a semiconductor with field-dependent mobility under various contact conditions is analyzed. A generalized closed form solution for an impulse current response is derived when the carrier velocity characteristic is expressed by a piecewise linear function. This result gives an accurate description of the transient performance of the device and can provide good insight into device operation. Numerical examples are given for a negative differential mobility GaAs crystal, considering the conditions under which negative resistances occur. A generalized small-signal impedance of the semiconductor is given, from which previous results reported for particular devices can be obtained as special cases.
Extensive electrostatic investigation of workfunction-modulated SOI tunnel FETs
An analytical model for the silicon-on-insulator (SOI) tunnel field-effect transistor (FET) with linearly graded workfunction-modulated gate is proposed to improve device performance through subthreshold slope (SS) optimization. The surface potential of the suggested model is analyzed using the two-dimensional (2-D) Poisson equation with imposed channel boundary conditions. Other electrical parameters such as the electric field, drain current, transconductance, and SS are evaluated to examine the performance of the model. Moreover, the performance in terms of the SS and $$I_{60}$$I60 values for the proposed model with downscaling of gate oxide thickness and silicon body thickness are also investigated and the results compared with results for a conventional tunnel FET (TFET) model. The present model exhibits significant reduction in subthreshold slope ($$\sim 14\,\hbox {mV/decade}$$∼14mV/decade) and improvement in $$I_{60} $$I60 performance. The accuracy of the model is verified against 2-D technology computer-aided design (TCAD) model simulations.
Band Structure and Electron Transport of GaAs
Existing experimental data on GaAs are reviewed and analyzed to yield the band structure in the vicinity of the band edges as well as the parameters characterizing the bands summarized in Fig. 1 of this paper. On the basis of presently existing experimental evidence, chiefly the behavior of the optical band gap in Ga(As, P) alloys and the deduced pressure shift and density of states effective mass, it is thought likely that the subsidiary conduction band minima lie along [100] directions. Analytical expressions including nonparabolic effects are given for the energy and density of states of the [000] conduction band and used to obtain a better value of the effective mass from optical reflectivity data. The experimentally observed structure in the Hall effect in $n$-type material at elevated temperatures is shown to result from excitation of carriers into the subsidiary conduction band. Changes of resistivity with pressure are explained on the basis of an increase of the [000] effective mass at low pressures and the transfer of carriers to the subsidiary minima at higher pressures. The scattering mechanisms, which are important in connection with transport phenomena, are shown to be polar lattice scattering and charged impurity scattering in the highest mobility samples. The transport calculations leading to the mobility and thermoelectric power as a function of temperature and impurity concentrations are performed using variational techniques, and shown to agree well with experiment. The apparently low mobility in the subsidiary minima is attributed at least in part to the large effective mass and relatively small anisotropy ratio. An estimate shows scattering between the two conduction bands probably to be unimportant.