SIDEGATING IN GAAS METAL-SEMICONDUCTOR FIELD-EFFECT TRANSISTORS UNDER LOW-LEVEL INJECTION

Sidegating in GaAs metal-semiconductor field-effect-transistors (MESFETs) is widely attributed to space-charge-limited (SCL) conduction through the semi-insulating (SI) substrate. But often sidegating is accompanied by low sidegate currents and occurs at sidegate voltages that are too small to initiate SCL conduction. We suggest that this is due to low-level hole injection from a MESFET into the substrate. An analytical treatment of carrier, field, and potential distribution in a SI substrate under conditions of low-level injection shows that an electric field overshoot may develop in the vicinity of the MESFET when hole injection occurs. This results in a large portion of the applied voltage being dropped across the channel-substrate interface and, consequently, in sidegating. One result for short structures is the electric field outside the narrow region of overshoot is below the value expected for ohmic conduction. Consequently, over a wide range of applied voltages the field in most portions of the structure is below the critical value required for intervalley electron transfer.