For high switching speed HV Schottky diodes, with very high work function metal and extremely lightly doped epi, the built-in potential may be too high for thermionic emission to occur, when the applied external voltage is quite low (near VF = 0.07V). If the epi is lightly doped p type, the built-in potential (VBuilt-in: potential difference between the metal and silicon Fermi levels) is 1.0V (measured with CV). If the external bias is 0.1V, near the measured VF, it is not enough to overcome the built-in potential for thermionic emission as illustrated. It is likely that in addition to thermionic emission, tunnelling and diffusion currents also contribute to the total HV Schottky diode forward current. TCAD simulation of HV Schottky diodes with N+ guard bands suggests the potential barrier and electric fields at the Schottky junction are relatively high for thermionic emission to occur, when external bias V ≈ VF. In this paper we report HV Schottky diodes fabricated with various metals, metal alloys and epitaxial films. Metal work functions and epi doping profiles are extracted with high frequency Capacitance-Voltage (CV) technique. 150V of breakdown voltage and very low forward voltage (VF = 0.07V) are demonstrated. The measured data indicate very high work function metal or metal alloy is needed to achieve high switching speed and low forward voltage.