Secondary Electroluminescence of GaN-on-Si RF HEMTs: Demonstration and Physical Origin

This paper demonstrates that-for high-electric fields and drain current levels-the electroluminescence (EL) versus VGS curves of GaN-on-Si radio frequency HEMTs significantly deviate from the well-known bell-shape, due to the turn-on of a secondary EL process that has not been described so far in the literature. Based on the combined EL measurements, electrical characterization, and thermal analysis, we demonstrate that: 1) for moderate gate and drain voltages, the EL versus VGS curve has the characteristic bell shape, and light emission originates from hot electrons injected from the source; EL signal is stronger at the edges of the gate, due to the higher electric field; 2) at high drain/gate bias and temperature, a second process induces a monotonic increase in the EL versus VGS curve; in this case, the EL signal is stronger at the center of the gate, and the EL intensity is directly correlated with the gate leakage current. Based on the experimental evidence collected within this paper, the secondary luminescence process is ascribed to: 1) the injection of carriers from the gate metal to the channel; 2) the acceleration of these electrons by the high gate-drain field; and c) the subsequent radiative emission.