A study of the influence of the substrate and epilayers on the InP HEMT gate current

The excess gate current usually measured on InP HEMT structures is mainly due to an impact ionization mechanism developing in the InGaAs channel. Recent investigations noted on the presence of a second ionization mechanism near pinch-off condition. In this communication we report studies on both mechanisms. We verified that the first one is effectively due to InGaAs channel, depending on the gate-to-drain distance. The second one is more complex to investigate. From our experiments, a contribution of the substrate/buffer layer interface is postulated. Holes are injected toward this interface.