UBC Theses and Dissertations
Parasitic substrate effects in gallium arsenide monolithic MESFETs Shulman, David Dima
The present large scale GaAs integrated circuit industry is based on the fabrication of metalsemiconductor field-effect-transistors (MESFETs) on semi-insulating GaAs substrates which provide the device isolation. High-resistivity in semi-insulating GaAs is achieved by the delicate balance between shallow donors and acceptors, and deep levels. The semi-insulating substrates, however, do not provide perfect isolation and do allow crosstalk between neighboring MESFETs. One source of the crosstalk is sidegating, usually defined as the change in drain current of one MESFET as a result of applying a negative potential to a nearby contact of another MESFET. In addition, the interaction of each MESFET with the semi-insulating substrate is strong enough to affect the electrical properties of the device, the most important being the change of the output conductance with frequency. This work is concerned with the above two parasitic effects with the main focus on sidegating, which is the major obstacle for developing large scale GaAs integrated circuits. Electron injection into the vicinity of a MESFET from a nearby contact via a semi-insulating substrate is known to produce the sidegating effect. This process is known as single injection, because the injection is due to a single carrier type. In this work we present a novel study of sidegating in the frequency domain (AC sidegating) and a new mechanism of DC sidegating in which holes are injected into a semi-insulating substrate from the gate of a MESFET and electrons are injected into a semi-insulating substrate from a nearby contact. This process is known as double injection. We distinguish between high- and low-level double injection where the low-level injection is referred to a condition in which the excess carrier concentration is much smaller than the majority carrier concentration in semi-insulating GaAs, while the low-level injection is referred to a condition in which the concentration of injected excess carriers exceeds the majority carrier concentration in semi-insulating GaAs. High-level double injection results in a drastic variation of a MESFET drain current at voltages lower that those predicted by the single-carrier injection model. It also results in hysteresis in current voltage characteristics as observed in experiments. It is shown that sidegating may occur under conditions of low-level double injection, because of the resultant excess trapped charge distribution which produces non-linear potential profiles across the semi-insulating substrate. The contribution of hole injection and recombination processes to the non-linear potential profile is discussed. We found that AC sidegating at least up to and including the kHz range is related to DC sidegating, in a way that upon increasing a negative sidegate voltage the AC drain current is decreasing. Upon applying small negative or positive sidegate voltages, thus preserving the conditions of low-level injection, this work predicts a strong sidegating effect in the kHz-MHz range due to the decrease by a few orders of magnitude of the resistance of semi-insulating substrates. This is because semiinsulating GaAs transfonns in this frequency range from what is called a “lifetime semiconductor”, in which quasi-neutrality of free carriers is preserved, to a “relaxation semiconductor”, in which separation of electrons and holes in space exists through zero local recombination. The present treatment predicts that this form of AC sidegating will be only weakly sensitive to hole injection, and will increase and start at lower frequencies on decreasing the distance between the MESFETs. The peculiar electrical properties of the semi-insulating GaAs in the frequency-domain are used to explain the frequency dependence of the output conductance of GaAs MESFETs on semi-insulating substrates. One result of the model developed in this thesis for the output admittance of GaAs MESFETs is that while the magnitude of the admittance can change by a factor of two or three, the variation of its phase is negligible. The results of this work indicate that device performance is strongly influenced by the properties of the semi-insulating substrate. One result is that device characteristics are not determined solely by the most dominant trap in the undoped SI substrate EL2, but also by recombination centers (which are not EL2) and shallower traps.
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