Insight into Buffer Trap-Induced Current Saturation and Current Collapse in GaN RF Heterojunction Field-Effect Transistors

Buffer traps in GaN RF heterostructure field-effect transistors (HFETs) provide a high dc output resistance on one hand but induce the current collapse effect on the other hand. Here, we show that traps do not merely confine the carriers into the channel, but serve to pin the Fermi level. Only by pinning the Fermi level a high saturated dc output resistance can be obtained in short channel devices. If carriers are confined into the channel by a back-barrier, and no traps are present, then short channel effects still severely reduce the saturated dc output resistance. In addition, we show that the buffer-related current collapse, which manifests as an output conductance under RF or pulse conditions, can straightforwardly be explained by a simple capacitive coupling model between the drain and the channel under the gate. The trapped charge under the gate results in a pulse voltage step-dependent shift in threshold voltage. Based on this picture, current collapse at pulsed condition can be approximately evaluated from dc simulations or from capacitance measurements.