PERMEABLE-BASE TRANSISTORS WITH ION-IMPLANTED COSI2 GATE

Permeable-base transistors (PBTs) with a buried CoSi2 gate overgrown by low pressure vapour phase epitaxy have been fabricated by high dose cobalt ion implantation through a gridded mask into n-type Si(111) and Si(100). SiO2 and SiO2/W/Cr were tested as implantation mask materials. For the first time different CoSi2/Si Schottky barrier heights, deduced from I-V and C-V measurements, for the top and the bottom diodes are observed in Si(100) with values of 0.67 +/- 0.03 eV and 0.78 +/- 0.03 eV respectively. In Si(111) the barrier heights of the top and bottom diodes exhibit a value of 0.78 +/- 0.03 eV. The ideality factor of the diodes depends strongly on implantation dose and energy. PBTs on Si(111) obtained by implantation into low doped n-type wafers show a transconductance per gate finger length of 15 mS mm-1, whereas PBTs on Si(100), obtained by implantation into a low doped epitaxial silicon layer on a highly doped wafer, exceed 50 mS mm-1 for gate spacings of 1-mu-m. Pinch-off was achieved in these PBTs.