There is a growing interest in vacuum macro/nano-electronics in which very small structures are fabricated in a well-defined array of gate apertures. The arrays are biased to emit electron currents from the surface of emitter tips into vacuum by field emission. Field emission is a quantum mechanical process in which electrons with energies below the top of the potential barrier tunnel through the barrier at the tip/vacuum interface. The small dimension enables high fields at relatively low voltages, typically from 10--200 V. Field emission from p-Si exhibits a light-sensitive saturation region, which can be optically modulated. Unlike electrical modulation, optical modulation can potentially bypass the large gate-to-substrate capacitance. As a result, ultra-short electron bunches are achievable when excited with ultra-short optical pulses. Ultra-fast optically modulated p-Si field emitter arrays (FEAs) can be used beneficially in microwave tube applications.; Microfabrication steps using standard silicon processes have been refined to consistently produce functional devices without an undesired "burn-out" process to achieve low leakage currents. Together with better conditioning techniques, devices showing superior performance have been demonstrated. The improved FEAs possess emission currents achieving 1800 times the gate currents. Current-voltage characteristics exhibit a clear transition from normal field emission at low voltages to saturation at high voltages, consistent with reported models of the Fowler-Nordheim theory and energy trap levels, respectively. Quantum efficiencies of 55% have been achieved. Dynamic range of 160 and photocurrent approaching 0.5 mA have been obtained. Photo-excitation with laser pulses at 12-kHz repetition rate results in field emission current pulses having the same rate. Temporal response analyses of a gated FEA suggest that photo-excited electrons can possess three different responses: sub-nanosecond response by drift electrons right beneath the emission site, sub-microsecond response by diffusion electrons, and microsecond response by the RC time constant from the gate capacitor. A FEA device having a high packing density of emitters per unit area is expected to increase the fast signal. These findings have paved the way for a future gated FEA photocathode capable of sub-nanosecond responses and high peak currents.
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