Suppression of Impact Ionization by Carbon Doping in the GaN Buffer Layer in InAlN/GaN-Based High Electron Mobility Transistors

Herein, the impact ionization in InAlN/GaN-based high electron mobility transistors (HEMTs) is studied for the first time. It is shown that the carbon doping in the GaN buffer layer can suppress impact ionization by reducing the electric field at the drain side gate edge. For comparison, two wafers are taken. In one wafer (D1), the GaN buffer layer is unintentionally doped, and in another wafer (D2), the buffer layer is carbon doped with a doping concentration of 2x1019 cm-3 at a distance of 500 nm from the InAlN-GaN interface. The impact ionization is probed from the bell-shaped nature of the I-G - V-GS characteristics. The bell-shaped nature is observed in D1 for the drain-to-source voltage (V-DS) >= 6 V, whereas it is absent in D2. Floating source measurements confirm the occurrence of impact ionization in D1. At a lower temperature (-60 degrees C), the impact ionization rate of D1 increases even though there is no sign of impact ionization in D2. A technology computer-aided design simulation shows that the peak electric field at the drain side gate edge of D2 is lower than that of D1.