A diamond field-emission triode with low gate turn-on voltage of 10 V and high gain factor of 250 is reported. The gated diamond triode was fabricated with a, self-aligning gate technique from a silicon-on-insulator wafer. I-a - V-g plot of emission characteristics from four tips shows a very low gate turn-on voltage of 10 V and high emission current of 4 muA at gate voltage of 20 V. I-a - V-a plots of emission characteristics demonstrate the desired saturation behavior of field-emission transistor with a high voltage gain of 250. The low turn-on gate voltage and high gain factor ate comparable to solid-state metal-oxide-semiconductor field-effect transistor devices, confirming the diamond field-emission. triode has significant. potential for integrated circuit-compatible vacuum microelectronic applications. (C) 2003 American Vacuum Society.