Breakdown mechanism in AlGaN/GaN HEMTs on Si substrate Citation

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates have attracted a great interest for power electronics applications. Despite the low cost of the Si substrate, the breakdown voltage (V bk) of AlGaN/GaN HEMTs grown on Si (less than 600 V for 2 μm total nitride epilayer [1, 4]) is much lower than that grown on SiC (1.9 kV for 2 μm total epi-layer [2]). Although several approaches have been reported to improve V bk [1, 3 and 4], the breakdown mechanism in these transistors is still not well understood. This paper studies for the first time the breakdown mechanism in AlGaN/GaN HEMTs on Si substrates. In addition, by transferring the AlGaN/GaN HEMTs grown on Si to a glass wafer, we have achieved devices with V bk in excess of 1.45 kV and specific on-resistance of 5.3 mΩ.cm 2. The devices analyzed in this paper were fabricated using our standard fabrication technology [4] on Al 0.26 Ga 0.74 N/GaN structures grown on intrinsic Si(111) by Nitronex Corporation. The device structure had a 2 nm GaN cap, 20 nm Al 0.26 Ga 0.74 N barrier and a 2 μm thick GaN/AlGaN buffer. The devices have a gate width of 100 μm, a 2 μm gate length, and varying gate-to-drain distances. It is suggested in [4] that the GaN-to-Si substrate vertical leakage limits the maximum breakdown of the AlGaN/GaN HEMTs on Si. To study this phenomena we measured the vertical leakage current between an ohmic contact on the GaN surface and the Si substrate (Fig.1). The leakage current shows a strong asymmetry on the polarity of the bias voltage. The leakage current starts to increase at a positive bias of 100 V between the GaN and the substrate, while a negative bias of 290 V is needed for the same amount of leakage. This asymmetry of the ohmic contact results in an unbalanced voltage distribution in AlGaN/GaN power transistors, as shown in the lateral buffer breakdown measurement in Fig.2. In this measurement, the Si substrate potential is recorded using a high impedance (20 MΩ) voltage meter while increasing the drain-to-source voltage. As shown in …