Simulation of Breakdown Voltage Enhancement in AlGaN/GaN HEMTs with Double Passivation Layers

K. Horio, H. Hanawa
Shibaura Institute of Technology,

Keywords: GaN HEMT, breakdown voltage, high-k passivation layer, two-dimensional analysis


It is well known that the introduction of field plate increases the breakdown voltage of AlGaN/GaN HEMTs [1, 2]. However, it increases the parasitic capacitance, leading to the degradation of high-frequency performance. As another way to improve the breakdown voltage, using a high-k passivation layer is proposed and analyzed [3]. But, it should have high interface-state densities, degrading the device performance. Therefore, in this study, we propose a structure with double passivation layers where the first passivation layer is a thin SIN layer having low interface-state densities and the second passivation layer is a high-k dielectric, and study how the breakdown voltage is enhanced. The analyzed device structure has the gate length LG of 0.3 μm and the gate-to-drain distance LGD of 1.5 μm. The thickness of first passivation layer (SiN) d1 is 0.01 μm and the relative permittivity is 7. The thickness of second passivation layer d2 and its relative permittivity εr2 are varied as parameters. In a buffer layer, we consider a deep donor and a deep acceptor [4, 5], and the deep-acceptor density NDA is set to 10^{17} cm^{-3}. We compare the calculated off-state drain current ID – drain voltage VD curves of AlGaN/GaN HEMTs as a parameter of d2 with a single passivation layer (SiN, εr2 = 7) and double passivation layers with a high-k dielectric (εr2 = 20). In the case with a single passivation layer of SiN, ID increases suddenly due to impact ionization, showing breakdown. In the case with double passivation layers, ID increases suddenly at thin d2. But when d2 becomes thick, ID increases gradually, reaching a critical value of ID (1 mA/mm) which corresponding to the breakdown. In this case, the buffer leakage current determines the breakdown voltage. The breakdown voltage Vbr versus the thickness of second passivation layer d2 curves clearly shows that Vbr becomes higher for the double passivation layers (εr2 = 20), particularly for thicker d2. This is because the electric field at the drain edge of the gate is reduced in the case of double passivation layers with a high-k dielectric. Vbr becomes nearly 300 V when d2 is 0.2 μm. We also calculate ID-VD curves of AlGaN/GaN HEMTs with double passivation layers as a parameter of relative permittivity of second passivation layer εr2, where d2 = 0.09 μm (d1 + d2 = 1 μm). When εr2 is low, the drain current increases suddenly due to impact ionization, as in a case of single passivation layer. But, as εr2 becomes high, the drain current increases gradually and reaches the current level of breakdown. The breakdown voltage Vbr increase as εr2 becomes high. Vbr can be nearly 300 V when εr2 is 30 even for thin d2. The value of Vbr = 300 V corresponds to an average electric field of 2 MV/cm between the gate and the drain.