First AlGaN/GaN MOSFET with photoanodic gate dielectric

We present the first GaN based MOSFET with wet chemical processed gate oxide. The oxide was grown photoelectrochemically (PEC) in KOH based aqueous solutions and was determined to be AlxGa2−xO3. This process offers low surface damage. The gate contact for our created PEC-MOSHFET (metal oxide semiconductor heterostructure field effect transistor) was fabricated by e-beam evaporation of tungsten on the AlxGa2−xO3 layer, followed by a lithographic step and wet etch by H2O2. Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance (gm) are 64 mS mm−1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm2 V−1 s−1. We achieve a maximum drain current IDmax of 540 mA mm−1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages Vth of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure Vth is about −9 V and for the SiO2-MOSHFETs it varies between −11.5 and −14 V depending on the wet chemical pre-treatment. Leakage currents depend on device isolation and on gate currents, which are lowest for the SiO2-MOSHFETs (∼2 pA) and several orders of magnitude bigger for the HFET (∼4 μA). Gate currents for the PEC-MOSHFET depend on the oxide growth and vary between microamperes and a few picoamperes.