Stable, low-resistance, 1.5 to 3.5 kΩ sq⁻¹, diamond surface conduction with a mixed metal-oxide protective film

Semiconducting diamond has the potential for an order-of-magnitude increase in power handling over currently used semiconductors. This is made possible by diamond's higher thermal conductivity and a higher breakdown voltage than any other device-quality semiconductor. Diamond power devices have numerous potential applications in the power grid and in high-power high-frequency RF applications. One approach to leverage diamond's abilities is through the fabrication of field-effect transistors (FETs). The FET is made by forming a p-type surface conductive layer on the diamond surface. This is accomplished by terminating the diamond surface with hydrogen atoms and then coating the surface with a material that contains negative charges to compensate for the positive holes in the p-type layer. Impressive drain current (1.3 A mm⁻¹), maximum operational voltages (>2000 V), and frequencies of unity current gain (f_T of 75 GHz) have been demonstrated with this surface conductance method. This surface layer, however, is not stable and FET performance degrades over time on the scale of hours to days. This paper describes an encapsulating layer with a mixed oxide, Al₂O₃-SiO₂, which maintains the resistance of the conductive layer in the range of 1.5 to 3.5 kΩ sq.⁻¹ by protecting the diamond surface while maintaining a stable negative charge.