Articles you may be interested inResponse to "Comment on 'Carrier trapping and current collapse mechanism in GaN metal-semiconductor field effect transistors'" [Appl. Phys. Lett.86, 016101 (2005)] Appl. Phys. Lett. 86, 016102 (2005); 10.1063/1.1844604 Photoionization cross-section analysis for a deep trap contributing to current collapse in GaN field-effect transistors J. Appl. Phys. 96, 715 (2004); 10.1063/1.1753076Carrier trapping and current collapse mechanism in GaN metal-semiconductor field-effect transistors Mechanisms of current collapse and gate leakage currents in AlGaN/GaN heterostructure field effect transistorsTwo-dimensional transient analyses of GaN metal-semiconductor field effect transistors ͑MESFETs͒ are performed in which a three level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor, and a deep acceptor are included. Quasipulsed current-voltage ͑I-V͒ curves are derived from the transient characteristics and are compared with steady-state I-V curves. It is shown that when the drain voltage V D is raised abruptly, the drain current I D overshoots the steady-state value, and when V D is lowered abruptly, I D remains at a low value for some periods, showing drain-lag behavior. These are explained by the deep donor's electron capturing and electron emission processes quantitatively. The drain lag could be a major cause of current collapse, although some gate lag is also seen due to the buffer layer. The current collapse is shown to be more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because the change of ionized deep-donor density becomes larger and hence the trapping effects become more significant. It is suggested that to minimize the current collapse in GaN-based FETs, an acceptor density in a semi-insulating layer should be made low, although the current cutoff behavior may be degraded.
PACS 71.55.Eq, 72.20.Jv, 85.30.De, 85.30.Tv Buffer-trapping effects in a GaN MESFET are studied by two-dimensional transient simulation. A threelevel compensation model is adopted for a semi-insulating buffer layer where a shallow donor, a deep donor and a deep acceptor are considered. It is shown that when the drain voltage V D is raised, 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 power compression in the GaN MESFET.
Two-dimensional transient simulations of GaNMESFETs are performed in which a three-level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor and a deep acceptor are considered. When the drain voltage V D is raised abruptly (while keeping the gate voltage V G constant), the drain current I D overshoots the steady-state value, and when V D is lowered abruptly, I D remains a low value for some periods, showing drain-lag behavior. These are explained by the deep donor's electron capturing and electron emission processes. We also calculate a case when both V D and V G are changed abruptly from an off point, and quasi-pulsed I-V curves are derived from the transient characteristics. It is shown that the drain currents in the pulsed I-V curves are rather lower than those in the steady state, indicating that so-called current collapse could occur due to deep levels in the semi-insulating buffer layer. It is also shown that the current collapse is more pronounced when V D is lowered from a higher voltage during turn-on, because the trapping effects become more significant.
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.
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