3659 V 0.37 A/mm NO₂-Doped P-Channel Diamond MOSFETs Fabricated on Diamond Grown on Misoriented Sapphire Substrates

Diamond is an ultra-wide-gap semiconductor for prospective high-power and high-frequency transistors, because it possesses high bandgap energy of 5.47 eV and a breakdown field of &gt;10 MV/cm. A 1-inch-diameter diamond wafers were grown on (11-20) sapphire just substrate without cracking using the micro-needle technique [1]. Very recently, we have demonstrated a 2-inch-diameter (001) diamond wafer without cracking by using a misoriented (11-20) sapphire substrate. The diamond wafer showed the highest crystal quality; the lowest XRC FWHM of 98 arcsec and low dislocation density of 1~2 × 10⁷ cm^-2[2]. In this study, we fabricated a NO₂-doped p-channel diamond MOSFETs on a high-quality misoriented diamond substrate, which showed the highest breakdown voltages of 3659 V among diamond FETs.<br/>We used a (001) diamond as a substrate which was grown on a misoriented (11-20) sapphire substrate by 5^o toward the [0001] c-direction. The H-terminated diamond substrate was exposed to 2% NO₂ gas diluted in N₂ to perform NO₂ p-type doing. After forming Au ohmic contact, 16-nm-thick Al₂O₃ bi-layer was deposited as a gate insulator layer, and Au gate was formed with a gate length (L_G) of 2 µm with gate-to-drain distance (L_GD) varied from 12 µm to 50 µm. Finally, the diamond p-channel was passivated with 16-nm-thick Al₂O₃ bi-layer.<br/>The DC output characteristics of a MOSFET showed a maximum drain current of 372 mA/mm. The gate leakage current was &lt;10^-4 mA/mm and an on/off ratio was determined as 10⁷. The on-resistance was 98.3 Ωmm and transconductance was obtained as 81 mS/mm at V_GS of -3 V. The threshold voltage was 4.1 V indicative of normally-ON operation. The maximum field-effect mobility was estimated as 134 cm²/Vs. The maximum off-state breakdown voltage reached 3659 V, which is the highest for diamond FETs.<br/>[1] S.-W. Kim, Y. Kawamata, R. Takaya, K. Koyama, and M. Kasu, Appl. Phys. Lett. <b>117</b>, 202102 (2020).<br/>[2] S.-W. Kim, R. Takaya, S. Hirano and M. Kasu, Appl. Phys. Express <b>14</b> 115501 (2021).