Transient Ionizing Dose Radiation Measurements on GaAs SDHT FETs: Effect of Insulating Metallization Paths from Substrate.

The transient ionizing dose response of GasAs selectively doped heterostructure transistor technology (SDHT) FETs were measured using the LINAC at the Naval Research Laboratory. SDHT FETs (gate length=0.75micron, gate width=30micron) were evaluated both with and without an insulating layer between the metallization paths (including the bonding pads) and the GaAs substrate. During the 50 ns radiation pulse, the response of the SDHT FETs without an insulating layer was dominated by prompt substrate photocurrents between metallization areas of different potentials - typically 2 mA at 2x10 sup 9 rad (GaAs)/sec, saturating at about 6.6 mA by 1.3x10 sup 11 rad (GaAs)/sec. SDHT FETs with silicon nitride under the metallization paths showed less than half the photocurrent at 2x10 sup 9 rad (GaAs)/sec, reversed to a net positive photocurrent response at higher dose rates, and reached from 2 to 4.7 mA (depending on the gate bias) at 1.3x10 sup 11 rad (GaAs)/sec. For both types of FETs, electrical characteristics returned to pre-pulse values within 200 ns after the radiation pulse ended.