The growth of GaN transistors on Si substrate has received tremendous attention due to large size availability of Si substrates at low cost. However, it is imperative to demonstrate a high breakdown AlGaN/GaN HEMTs on Si grown by MOCVD as high power device applications are the primary significant contribution expected of a GaN based devices. In the past, we have demonstrated high breakdown on AlGaN/GaN HEMTs grown on Si by thickening the buffer layers [1–2]. All our previous reports were based on the 3-terminal OFF breakdown voltage (3TBV) measured on devices with short gate-drain (Lgd = 3 or 4 µm) spacing which limited the breakdown voltage due to Schottky gate leakage current [3]. Therefore, in the current investigation, we prepared HEMTs with various Lgd and studied its dependence on 3TBV. We observed a 3TBV of 1.4 kV for an AlGaN/GaN HEMT grown on Si having Lgd of 20 µm.