Mechanisms of current collapse and gate leakage currents in AlGaN/GaN heterostructure field effect transistors

In order to clarify the mechanisms of drain current collapse and gate leakage currents in the AlGaN/GaN heterostructure field effect transistor (HFET), detailed electrical properties of the ungated portion and Schottky-gated portion of the device were investigated separately, using a gateless HFET structure and an AlGaN Schottky diode structure. The gateless device was subjected to plasma treatments and surface passivation processes including our novel Al 2 O 3 -based surface passivation. dc I–V curves of gateless HFETs were highly nonlinear due to virtual gating by surface states. After drain stress, air-exposed, H 2 plasma-treated and SiO 2 -deposited gateless HFETs showed an initial large-amplitude exponential current transient followed by a subsequent smaller, slow, and highly nonexponential response. The former was explained by emission from deep donors at E c −0.37 eV , and the latter by emission from surface states. Capture transients with stress-dependent capture barriers were also observed. An x-ray photoelectron spectroscopy (XPS) study indicated that 0.37 eV-deep donors are N-vacancy related. On the other hand, no current transients took place in N 2 plasma treated and Al 2 O 3 -passivated samples. Temperature dependences of I–V curves of Schottky diodes were extremely small and reverse currents were anomalously large. They were explained by the “thin surface barrier” (TSB) model where thermionic field emission and field emission through the TSB region formed by deep donors produce leakage current paths. By combining the results on gateless HFETs and Schottky diodes, a new unified model of near-surface electronic states for the free surface and Schottky interface of AlGaN is proposed. It consists of a U-shaped surface state continuum and N-vacancy related near-surface discrete deep donors. The model can explain the observed large gate leakage and drain current collapse in AlGaN/GaN HFETs in a unified way. It is also shown that our novel Al 2 O 3 passivation, when also used as a gate insulator, can completely suppress current collapse and gate leakage.