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Finite Element Modeling and Simulation of Photoconductive and Ballistic Semiconductor Nanodevices

Gregg Guarino

Prof. Roman Sobolewski

CSB 523

Abstract

This thesis presents models, simulation techniques, and simulation results for two types of semiconductor nanodevice; the metal-semiconductor-metal (MSM) photodetector, and the ballistic deflection transistor (BDT). Our simulation tools were developed using the commercial ComsolT finite element analysis (FEA) field solver to obtain the numeric solutions. Finite element models have been developed for both an alloyed- and surface-contact MSM photodetector. The simulation results agree with previously reported experimental data. The alloyed device, despite having a somewhat larger capacitance, has a non-illuminated region of lower resistance with a more-uniform and deeper-penetrating electric field and carrier transport current. The latter explains, in terms of the equivalent lumped parameters, the experimentally observed faster response of such device. The model was further used to predict improved responsivity, based on electrode spacing and antireflective coating, as well as the optimal depth of the alloyed contact being approximately half of the optical penetration depth. For the BDT, novel simulation techniques, also based on the FEA method, have been developed. The results demonstrate that diffusive transport is capable of predicting the current vs. voltage characteristics of the current-generation of BDTs, as well as the effects of selected changes in the BDT geometry. Simulation results were used to predict the characteristics of several variations of scaled-down and geometry-modified devices and other physical parameters. Also, the newly introduced concept of ballistic conductivity predicts behavior consistent with ballistic transport, relative to the effect of the deflector, for small devices.