Characterization of enhanced low dose rate sensitivity (ELDRS) effects using Gated Lateral PNP transistor structures

The high and low dose rate responses of bipolar transistors in a bipolar linear circuit process technology have been studied with specially designed gated lateral pnp test transistors that allow for the extraction of the oxide trapped charge (N_ot) and interface trap (N_it) densities. The buildup of N_ot and N_it with total dose is investigated as a function of the irradiation gate voltage at 39 rad/s and 20 mrad/s for three variations of the final passivation layer (all variations had the same oxide covering the active region of the devices). The three variations in final passivation were selected to exhibit minimal degradation at high and low dose rate (no passivation), significant degradation at high and low dose rate (p-glass/nitride) and enhanced low dose rate sensitivity (ELDRS) (p-glass only). It is shown that the increase in base current is dominated by increased N_it and the "true" low dose rate enhancement in the ELDRS parts occurs for zero and negative gate voltage, but is eliminated for large positive gate voltage and elevated temperature irradiation. Implications for ELDRS models are discussed.