Date of Award

1994

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Physics

Supervisor

J.G. Simmons

Co-Supervisor

P.E. Jessop

Abstract

A novel optoelectronic switching device, the bistable field effect transistor (BISFET) has been developed. The BISFET is an inversion channel heterostructure device containing a positive feedback loop transverse to the channel, between the gate and collector terminals. This leads to the existence of two distinct operating states. In one state, the feedback loop is in a high-impedance OFF state, associated with which are a large carrier population in the conduction channel and a correspondingly high drain current. In the other state, the feedback loop is in a low-impedance ON state, characterized by a small carrier population and relatively low drain current. These transitions, referred to as switchup and switchdown, sweep out a large hysteresis loop in the drain characteristics of the BISFET, making the device strongly bistable. A mathematical model is developed to describe the operation of the device.

Implementation of the BISFET in the GaAs/AIGaAs material system is reported. A study of the electrical characteristics of the device has been carried out, using two separate device configurations. In one configuration, a separate collector terminal is used to supply feedback current in the device. In the other, known as the integral collector configuration, the source contact simultaneously serves as the collector terminal. The electrical characteristics reveal a sharp drop in gate current corresponding with that in the drain current. This confirms the feedback loop as the origin of the switching. The gate characteristics are shown to exhibit S-type negative differential resistance (NDR) associated with the feedback loop. The transfer characteristics of the BISFET are found to contain hysteresis corresponding with that seen in the drain characteristics.

A single set of transitions is observed in the BISFET when the integral collector configuration is used. The feedback loop in this case lies between the gate and source terminal. Two distinct sets of transitions have been observed in this mode. The previously-observed set resulting from the gate-source feedback loop (integral collector configuration) is still present. In addition to this, however, a second set of transitions is seen, which is associated with the feedback loop between the gate and the separate collector terminal.

An enhancement in drain current is reported in the BISFET at low drain voltage. This increase, seen at high gate bias, is believed to result from a geometric and electrical symmetry between the source and drain terminals at low drain voltages. The turn-off of the enhancement with increasing drain bias varies from a gradual decline at lower gate voltages to an abrupt drop at higher values.

Optical bistability in the GaAs/AIGaAs BISFET is reported. The device is found to emit light from the active region when it is in the ON state. The light output exhibits abrupt transitions and hysteresis corresponding directly with those seen in the electrical characteristics. Optical control of the bistability is also reported. The drain voltages at which the current transitions occur are found to change when optical excitation is applied to the device. As the optical intensity is increased, the transition voltages decrease, then saturate, then increase.

The structure and fabrication process used for the BISFET are compatible with a range of other devices, including light sources and detectors. The feasibility of constructing optoelectronic integrated circuits using the BISFET is demonstrated using a circuit consisting of an LED and a BISFET current driver.

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