Y. Satoh, H. Hanawa, K. Horio
Shibaura Institute of Technology, Japan
pp. 113 - 117
Keywords: GaN HEMT, breakdown voltage, high-k passivation layer, buffer leakage current, two-dimensional analysis
It is well known that the introduction of filed plate enhances the breakdown voltage in AlGaN/GaN HEMTs. This is because the electric field at the drain edge of the gate is reduced. However, the field plate increases the parasitic capacitance, leading to the degradation of high frequency performance. As another way to improve the breakdown voltage, the introduction of high-k passivation layer can be considered. This is because the electric field profiles between the gate and the drain can be smoother. In this work, we analyze I-V characteristics of AlGaN/GaN HEMTs as a function of relative permittivity of the passivation layer εr, and studied how the buffer leakage current affects the off-state breakdown voltage Vbr. We consider an AlGaN/GaN HEMTs on a semi-insulating GaN buffer layer. The gate length and the gate-to-drain distance are typically 0.3 and 1.5 μm, respectively. The thickness of passivation layer is 0.1 μm. In a buffer layer, we consider a deep donor and a deep acceptor, and the deep-acceptor density is set to 1017 cm-3. We have calculated ID-VD curves at VG = 8 V with and without impact ionization. The drain currents without impact ionization correspond to the normal buffer leakage currents, which are found to be lower for higher εr. When εr is low (< 20), an abrupt increase in ID due to impact ionization determines the breakdown voltage Vbr. However, when εr is relatively high (> 30), the buffer leakage current reaches 1 mA/mm before the abrupt increase in ID and the bufffer leakage current determines Vbr. Vbr is higher for higher εr. We have also calculated and compared Vbr versus εr for the two cases with VG = - 8 V and - 10 V. When VG = - 10 V, Vbr becomes higher in the region where εr is high (> 30). This is because the buffer leakage current is reduced in the case of VG = - 10 V. Therefore, the buffer leakage current plays an important role in determining the breakdown voltage of AlGaN/GaN HEMTs.