Diamond Field-Effect Transistors with 1.3 A/mm Drain Current Density by Al₂O₃ Passivation Layer

Using nitrogen-dioxide (NO₂) adsorption treatment and Al₂O₃ passivation technique, we improved drain current (I_DS) of hydrogen-terminated (H-terminated) diamond field-effect transistors (FETs). The Al₂O₃ passivation layer also serves as a gate-insulator in a gate region. Maximum I_DS (I_DSmax) of -1.35 A/mm was obtained for the diamond FETs with NO₂ adsorption and the Al₂O₃ passivation layer. This I_DSmax is the highest ever reported for diamond FETs and indicates that the Al₂O₃ passivation layer can stabilize adsorbed NO₂, which increases the hole carrier concentration on the H-terminated diamond surface. In RF small-signal characteristics, the diamond FETs with NO₂ adsorption and the Al₂O₃ passivation layer showed high cutoff-frequency (f_T) and maximum frequency of oscillation (f_max) in a wide gate–source voltage (V_GS) range (>10 V). This is because the Al₂O₃ 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.