AC SIDEGATING THROUGH SEMI-INSULATING GALLIUM-ARSENIDE

The mechanisms of crosstalk between GaAs MESFETs due to sidegating through the semi-insulating GaAs were investigated over a frequency range of 100 Hz-500 MHz. The sidegating mechanism was found to be dependent on the frequency and on the distance between the sidegate and the output contacts. The crosstalk can be represented as a parallel RC network, where the capacitance is associated with the interaction of fringing fields around the input and output electrodes. For shorter distances the resistive component represents a conductive path through the semi-insulating GaAs substrate normally associated with the trap-filled space-charge-limited current. In general the resistance will be bias-dependent. Furthermore, it is expected to be frequency-dependent because of the finite time constant associated with the charging and discharging of the traps in semi-insulating GaAs. For longer distances the capacitive coupling is reduced. Also a higher biasing voltage is required in order to initiate the space-charge-limited current for the distant input and output contacts. Thus at low voltages the resistive component is associated with the ohmic leakage current through the substrate. This resistance exhibits a frequency dependent conductivity, which is a result of potential fluctuations in the compensated semiconductor. At low frequencies the capacitive coupling is reduced and the resistive coupling tends to prevail. At high enough frequencies capacitive coupling dominates disregarding the distance between the input and output electrodes.