Metallic electrons can only go over barrier by thermionic emission tunnel or or can tunnel through hermio if it is narrow (highly doped If pm>p = Schottky contact with n-type semiconductor Ohmic with p-type If m< o Schottky contact with p-type semiconductor Ohmic with n-type ll/703 3.155J6.l521 should determine contact type but in practice surface contamination and traps nd usually have Schottky barrier To obtain Ohmic contact, make barrier very narrow very heavy doping (n*or p*- 1020 cm-3 adjacent to the metal Al is a p dopant 11/17/03 3.155J6.l52JMetallic electrons can only go over barrier by thermionic emission, tunnel or or can tunnel through thermioic if it is narrow (highly doped). If fm s > f => Schottky contact with n-type semiconductor Ohmic with p-type If fm s < f => Schottky contact with p-type semiconductor Ohmic with n-type 11/17/03 3.155J/6.152J 11 The metal work function should determine contact type, but in practice, surface contamination and traps pin Fermi level and usually have Schottky barrier when fB = 0.5 - 0.8 eV. To obtain Ohmic contact, make barrier very narrow very heavy doping (n+ or p+ ~ 1020 cm-3) adjacent to the metal. Al is a p dopant. 11/17/03 3.155J/6.152J 12 6