Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Engineering Physics


J.G. Simmons


P.E. Jessop


The fabrication and characterization of the double heterojunction opto-electronic switch (DOES) and the heterojunction field effect transistor (HFET) in the Si/Si_(1-x)Ge_(x) materials system is described. The significant achievements of this work are summarized below. The theory of operation of the DOES and HFET are reviewed. A computer-based, one-dimensional, analytical model is used to calculate the current-voltage characteristics and operational parameters of the DOES. From this, the design of the inversion-channel Si/Si_(1-x)Ge_(x) heterostructure is shown to contain a manifold parameter space. For example, the switching voltage of the DOES is shown to be extremely sensitive to the magnitude of doping in the charge sheet. In the HFET, the threshold voltage is calculated to have a non-linear dependence on the magnitude of the doping in the charge sheet. The measured switching voltages and currents compare well with the calculations of the model. Separately, the effects of illumination and temperature on the dc electrical characteristics of the device are investigated. Illumination is seen to reduce the switching voltage and holding current. With decreasing temperature, the switching voltage and holding current are observed to increase. The effects of illumination and temperature are explained as phenomena related to carrier injection (or the lack thereof). An account of the fabrication of these devices is given in detail. A self-aligned technology for the Si/Si_(1-x)Ge_(x)-based HFET was developed using the aluminum gate as a mask for reactive ion etching and ion implantation procedures. Oscillatory electrical behavior of the DOES is examined. Self-induced oscillations are shown to be correlated to the regime of negative differential resistance. The dc current-voltage characteristics of the DOES are demonstrated to be affected by this oscillatory behavior. In addition, an enhancement in the optical emission over a narrow range of drive currents in the DOES is shown to be a result of device oscillations. Changes in the electrical characteristics of the DOES in response to third terminal current injection are measured for both active layer contact and inversion layer contact. Contact to the inversion layer is shown to be more effective than a third terminal contact to the active layer. These experimental observations are supported by the computer-based model. Concurrent usage of optical and electrical injection to affect the I-V characteristic of the device is demonstrated. Electrical extraction via the third terminal is shown to negate the effect of optical injection. The highest reported transconductance in a Si/SiGe-based metal-semiconductor FET is found in the HFETs reported in this thesis. Peak transconductance of 8mS/mm is measured for a depletion mode device. Subthreshold slope is measured as 720mV/decade and a high frequency 3-dB point for voltage gain is seen to be 1.8GHz. The suitability of this technology for integration is examined by demonstrating a Reset-Set Flip-Flop which is comprised of two HFETs and a three-terminal DOES. The range of operational voltages for the circuit are shown to be determined primarily by the operational characteristics of the DOES. Gate leakage from the HFET is seen to hinder circuit performance.

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