Metal-oxide-semiconductor field-effect transistors (FETs) have been fabricated using B-doped diamond thin films deposited on polycrystalline, (100) highly-oriented, and single crystal diamond insulating substrates. Diamond films were grown using a microwave plasma chemical vapor deposition technique. Various electrical and materials characterization techniques were employed to confirm that the films exhibited properties suitable for FET fabrication. Devices with gate lengths and widths of 2 μm and 314 μm respectively, were processed using standard photolithography. Silicon dioxide was used as the gate dielectric. Current-voltage characteristics of these devices have been measured during variable temperature cycling in air. Devices fabricated on the randomly oriented polycrystalline diamond substrates have been operated to 285°C. Field-effect transistors fabricated using the highly-oriented diamond substrates have been characterized to 400°C. Single crystal diamond devices exhibited saturation and pinch-off of the channel current at temperatures up to 500°C. These devices have been biased in amplifier circuit configurations that have been characterized from 20 Hz to 1 MHz. Single crystal FETs exhibited voltage gain over an extended temperature range. Transconductances as large as 1.7 mS/mm have been observed. The electronic properties, fabrication technologies, and performance of devices fabricated on the three diamond substrate materials will be discussed and compared.