Deep-level effects on slow current transients and current collapse in GaN MESFETs

Abstract

Slow current transients in a GaN MESFET are analyzed by two-dimensional simulation in which deep levels in a semi-insulating buffer layer is considered.It is shown that when the drain voltage V D is raised abruptly, the drain current overshoots the steady-state value, and when V D is lowered, the drain current remains at a low value for some periods, showing drain lag behavior. This drain lag is shown to become a cause of so-called current collapse in the GaN MESFET. IntroductionRecently, GaN-based FETs have received great interest because of their potential applications to high power and high temperature microwave devices [1]. However, slow current transients are often observed even if the drain voltage or the gate voltage is changed abruptly [2]. This is called drain lag or gate lag, and is problematic in circuit applications. The slow transients mean that the DC I-V curves and the AC I-V curves become quite different, resulting in lower AC power available than that expected from the DC operation [1,2]. This is called power compression or current collapse in the GaN-device field. These are serious problems, and there are many experimental results reported on these phenomena. But few theoretical results have been reported for GaN-based FETs, although several numerical analyses were made for GaAs-based FETs [3][4][5][6][7]. Therefore, in this work, we have made transient simulation of a GaN MESFET in which deep levels in a semi-insulating buffer layer is considered, and discussed how the slow current transients and the current collapse could be reproduced. 2.Physical Model Fig.1 shows a device structure analyzed in this study. The donor density in the active layer is 2x10 17 cm -3 , and its thickness is 0.2 µm. This structure has a semi-insulating buffer layer, and is similar to a GaAs MESFET that has a semi-insulating substrate. As a model for the semi-insulating buffer layer, we use a three level compensation model which includes a shallow donor, a deep donor and a deep acceptor. Some experiments show that two levels (E C -1.75 eV, E C -2.85 eV) are associated with current collapse (or power compression) in GaN-based FETs with a semi-insulating buffer layer [2], so that we use energy levels of E C -2.85 eV (or E V + 0.6 eV) for the deep acceptor and of E C -1.75 eV for the deep donor, as shown in Fig.2. Other experiments show shallower energy levels for the deep donor [8,9], and hence we vary the deep donor's energy level (E DD ) as a parameter. Here, the deep donor's density (N DD ) and the deep acceptor's density (N DA ) are typically set to 5x10 16 cm -3 and 2x10 16 cm -3 , respectively.