Over 1 A/mm drain current density and 3.6 W/mm output power density in 2DHG diamond MOSFETs with highly doped regrown source/drain

This paper reports the direct current and radio frequency characteristics of two-dimensional hole gas (2DHG) diamond metal-oxide-semiconductor field-effect transistors (MOSFETs) with microwave-plasma-chemical-vapor-deposition regrown P⁺⁺-diamond (B ≈ 1 × 10²² cm⁻³) layers as their source and drain regions. To evaluate the ohmic contact resistance, two patterns of transmission line method were fabricated—one with P⁺⁺-diamond/2DHG diamond interfaces and the other with only the P⁺⁺-diamond layer. Three resistance components (metal/P⁺⁺-diamond contact resistance (0.39 Ωmm), P⁺⁺-diamond access resistance, and P⁺⁺-diamond/2DHG contact resistance (∼0.65 Ω mm) were evaluated for the first time. The introduction of the P⁺⁺-diamond ohmic contacts generated a drain current density of 1 A/mm and an output power density of 3.6 W/mm at a quiescent drain voltage (V_{DS, Q}) of −50 V at 1 GHz; an output power of 1.7 W was achieved at a V_{DS, Q} of −40 V at 1 GHz with a 1 mm gate width. These values were obtained in 2DHG diamond MOSFETs with a regrown P⁺⁺-diamond layer with 1 μ m gate length. These results indicate that regrown P⁺⁺-diamond has the potential to improve the drain current density and output power density of 2DHG diamond field-effect transistors.