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The influence of ambient temperature and applied electric field on the electrical properties of high resistivity (1–30 kΩ cm), semi-insulating (>100 kΩ cm), and insulating (1011–1012 Ω cm) single-crystal 6H–SiC is reported. Current–voltage (I–V) characteristics of lateral metal-semiconductor-metal test structures were measured in vacuum in a temperature range of 295–730 K and under moderate pulsed electric fields (0.5–80 kV/cm). It is shown that the resistivity of the undoped 6H–SiC varies strongly with the ambient temperature after a temperature/field function dominated by a factor containing the activation (ionization) energy of residual boron of 0.35 eV. The dominant activation energy of semi-insulating Vanadium-compensated material (6H–SiC:V) varies with the ambient temperature, increasing from ∼0 eV at 295–320 K to ∼0.8 eV at T⩾600 K. This result can explain the relatively low decrease of the resistivity of insulating 6H–SiC at very high ambient temperatures and its viability as a substrate for next-generation high temperature microwave integrated circuits based on large band gap semiconductors.

We have simulated and measured 60–120fs time jitter of photoelectron pulses emitted by a nitride photocathode at 100GHz rate as in order to evaluate the resolution performance of a previously proposed photonic analog to digital converter. Recently, there has been an increasing demand for high speed analog-to-digital converters (ADCs) for microwave bandwidth signals. State of the art electronic ADCs have reached 10Gigasamples∕second (GS/s), 6–12bit performance [P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L. Wasserman, F. J. O’Donnell, K. G. Ray, and R. C. Williamson, IEEE Microwave Theory Tech. 49, 1840 (2001)]. We have previously introduced a photoelectronic ADC implementation with measured performance of 3bits at 100GS∕s [K. Ioakeimidi, R. Leheny, S. Gradinaru, K. Ma, R. Aldana, J. Clendenin, J. S Harris, R. F. W. Pease, IEEE Trans. Microwave Theory Tech. (to be published); Conference on Lasers and Electro-Optics, Baltimore MD, 1–6 June 2003]. The basic operating principle of the ADC is based on a miniaturized cathode ray tube where a bunch of photoemitted electrons passing through an electric deflection system is directed to a specific detector whence a digital code word emanates. The electron bunch samples the analog deflecting voltage that is then quantized according to the position of the detector receiving the bunch. The fundamental limit of the number of distinguishable voltage levels is the ratio of the deflecting voltage to the energy spread due to diffraction of the electron beam. This allows for up to 12bits at 100GS∕s [R. F. Pease, K. Ioakeimidi, R. Aldana, and R. Leheny, J. Vac. Sci. Technol. B 21, 2826 (2003)]. At a more practical level, the bit resolution is primarily limited by the uncertainty in the emission of each electron bunch (temporal jitter). For 100fs time uncertainty 5bits of resolution are attainable with the nitride cathode for a 50GHz bandwidth analog signal.

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We investigated the effect of an accelerating field on the spin polarization of photogenerated electrons in a 100nm thick GaAs based photocathode active region. By decreasing the transport time of the electrons and the number of scattering events that cause depolarization, we expected to increase the polarization as was indicated by Monte Carlo simulations of the scattering and transport time statistics of the electrons.A tungsten (W) grid was deposited on the cathode surface to provide a uniform voltage distribution across the cathode surface. The metal grid formed a Schottky contact with the semiconductor surface. The bias voltage was primarily dropped at the metal semiconductor interface region, which is the cathode active region. For positive surface bias, the accelerating voltage not only increased the polarization, but it also enhanced the quantum efficiency of the photocathode. Preliminary results verify the bias effect on both quantum efficiency and polarization by a factor of 1.8 and 1% respectively.