A Simulation Study on the Transient Leakage Current Analysis of a GaN Epitaxial Layer

Abstract

GaN epitaxial layers on Si substrate are being used as semiconductors for power devices owing to its wide bandgap, a high breakdown voltage with low on-resistance [1]. Due to the presence of crystal defect both in energy and its depth profile, a transient behavior is commonly observed in the channel current. Some of characterization methods, including deep level transient spectroscopy (DLTS), can extract the energy levels with its depth profile, by adopting a Schottky electrode to the epitaxial layer. Therefore, the extracted profile is limited to the surface of the GaN layers. In this work, we conducted a simulation study to extract the defect density profile from a direct current obtained from a GaN epitaxial layer on Si substrate with an Ohmic contact electrode to the surface. While applying a step voltage to the electrode, the transient current was measured and characterized.The proposed defect extraction method can be analytically expressed by the change in the depletion capacitance (ΔC) in the epitaxial layer upon a voltage of V applied to the electrode at t=0 as shown in equation (1).Here, ε, Wo d , W∞ d , q, N d and N t are the permittivity, depletion width at t=0, depletion width at steady state, doping density of the GaN layer and the trap density, respectively. The change in the charge accumulated to the electrode (ΔQ) can be written as the equation (2). Therefore, by the integral of the transient current, one can obtain the relationship between V and N t, which can be transformed into the defect density depth profile of the epitaxial layer.The simulated model is shown in fig. 1(a), where two GaN layers of each N d=1016 /cm3 are stacked with in two metal electrodes. The area of the electrode is 0.1 mm2. Defect levels of E3 and E7 with a density of N t,E3 and N t,E7 are introduced in the top and bottom GaN layers, respectively. By applying a positive V to the Ohmic top electrode, the depletion width increases from the Schottky bottom electrode, depending on the N d . By the presence of E3 and E7, a transient current with a time constant depending on the defect levels is obtained as is shown in fig. 2(b). By obtaining the relationship between the ΔQ and the V, as shown in fig. (2), firstly we can extract the N d of each layer from the change in the slope. The extract N d values showed a nice agreement to the values set in the simulations. Also, from the ΔQ itself, we can deduce the N t of each layer.In conclusions, we have made a simulation of the transient current response of a stacked GaN layers with different defect levels. From the transient current integral, we can extract the doping density and the defect density of each layer.