−400 mA mm−1Drain Current Density Normally-Off Polycrystalline Diamond MOSFETs

This letter reports a high drain current density and normally-off operation metal-oxide-semiconductor field-effect transistors (MOSFETs) with a gate insulator of 100 nm-Al₂O₃. A heavily boron-doped layer as the source/drain region was deposited on a (110) polycrystalline diamond substrate to achieve a low ohmic contact resistance. The MOSFETs demonstrate a maximum current density of −400 mA mm $^{-{1}}$ normalized by gate width and a maximum current density of $- 2000\,\,\mu \text{m}$ mA mm⁻¹ normalized by gate length and gate width, which are the highest values for normally-off diamond FETs. The Grain boundaries (GBs) and the nitrogen impurities ( $\sim {3}\,\,\times \,\,{10}^{{17}}$ cm $^{-{3}}$ ) as ionized donors in the channel region caused the threshold voltage ( ${V}_{\text {th}}$ ) to shift in the negative direction, exhibiting normally-off characteristics. This technique provides a promising method to achieve high-performance diamond devices, and help improve safety and save energy in switching systems.