Cryogenic operation of field-effect transistors (FETs) fabricated on hydrogen-terminated (H-terminated) diamond surface conductive layers is investigated. 5-μm gate-length metal-insulator- semiconductor FETs (MISFETs) is fabricated using CaF2 film as a gate insulator. The MISFETs operate successfully even at 4.4 K. At low temperature, the contact between source/drain electrode and H-terminated diamond surface cannot maintain ohmic characteristics, because the thermal activation energy of the carriers is not high enough to overcome the barrier height at the interfaces between the source electrode and the H-terminated diamond. Estimated channel mobility increases from 63 cm2/V-s to 137 cm2/V-s and the maximum transconductance increases from 10.5 mS/mm to 14.5 mS/mm, as the temperature decreases from 300 K to 4.4 K, indicating reduced phonon scattering of the channel.
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