C-H diamond metal-oxide-semiconductor field effect transistors with different structures were fabricated on the same polycrystalline diamond plate. Devices A and B with 25-nm-thick high temperature (300°C) atomic layer deposition grown Al ₂ O ₃ dielectric have the same source-to-drain distance of 6 μm and different gate length of 2 μm and 6 μm, respectively. Both devices show ultra-high on/off ratio of over 1010 and ultra-low gate leakage of below 10 ^-10 A and continuous measurement stability. Device B with the source/drain-channel interspaces eliminated has achieved an on resistance of 46.20 Ω·mm, which is record low in the reported 6-μm H-diamond MOSFETs with the gate dielectric prepared at high temperature (≥ 300 °C). Meanwhile, device B shows larger drain current in a large portion of the linear region at VGS = -6 V, and a just slightly smaller IDmax compared with device A though its LG is three times of that of device A. A simple model of ID was used to explain the physics behind this phenomenon. In addition, the breakdown voltage is 145 V for device A and 27 V for device B, corresponding to the average breakdown field of about 0.72 MV/cm and 10.8 MV/cm, respectively.