T. Iwamoto, D. Kubota, S. Akiyama, K. Horio
Shibaura Institute of Technology,
Keywords: GaN HEMT, breakdown voltage, field plate, passivation layer
Summary:AlGaN/GaN HEMTs are now receiving a great attention for applications to high-power microwave devices and high-power switching devices. It is well known that the introduction of field plate improves the power performance of AlGaN/GaN HEMTs. This is because the field plate mitigates the so-called current collapse and increases the off-state breakdown voltage. The increase in breakdown voltage occurs because the electric field at the drain edge of the gate is reduced. Several works have been made how the breakdown voltage is influenced by the field-plate length. However, few works have been reported how the thickness of passivation layer under the field plate affects the breakdown voltage. Therefore, in this work, we analyze how the SiN passivation-layer thickness affects the off-state breakdown voltage of AlGaN/GaN HEMTs. Here, the gate length is 0.3 um and the gate-to-drain distance is 1.5 um. The field-plate length LFP is varied between 0 and 1 um, and the thickness of SiN passivation layer d is varied between 0.01 and 0.2 um. The drain current ID – drain voltage VD curves and the gate current IG - VD curves are calculated. As a result, it has been shown that when d is relatively thick (> 0.1 um), the breakdown voltage increases as LFP increases, and takes a maximum value around 400 V at LFP = 0.3 um. On the other hand, when d is relatively thin (< 0.03 um), the breakdown voltage becomes less than 100 V at all LFP. This is attributed to the fact that when d is too thin, the field plate becomes acting like a gate electrode and the drain voltage becomes entirely applied at the field-plate edge. This is similar to the case without a field plate where the drain voltage is entirely applied along the drain edge of the gate. Therefore, to achieve a high breakdown voltage in AlGaN/GaN HEMTs, the passivation-layer thickness should be thick to some extent (0.1 um). However, it should be noted that when the passivation layer is too thick, effects of field plate become weak and hence the breakdown voltage should be reduced.