Using nitrogen-dioxide (NO 2) adsorption treatment and Al 2O 3 passivation technique, we improved drain current (IDS) of hydrogen-terminated (Hterminated) diamond field-effect transistors (FETs). The Al 2O 3 passivation layer also serves as a gate-insulator in a gate region. Maximum IDS (I DSmax) of -1:35A/mm was obtained for the diamond FETs with NO 2 adsorption and the Al 2O 3 passivation layer. This I DSmax is the highest ever reported for diamond FETs and indicates that the Al 2O 3 passivation layer can stabilize adsorbed NO 2, which increases the hole carrier concentration on the H-terminated diamond surface. In RF small-signal characteristics, the diamond FETs with NO 2 adsorption and the Al 2O 3 passivation layer showed high cutoff-frequency (fT) and maximum frequency of oscillation (f max) in a wide gate-source voltage (VGS) range (>10 V). This is because the Al 2O 3 gate insulator with a high potential barrier against hole carriers can confine and control the high concentration of hole carriers and then high forward-bias voltage can be applied without noticeable gate leakage current.
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