THE PERMEABLE BASE TRANSISTOR

Among the high speed electronic devices which have been developed in the last two decades the permeable base transistor (PBT) is one of the most attractive due to its conceptual simplicity. One of its prime advantages, however, is that it can be fabricated with silicon as the basic material thus pushing further the limits of Si-technology in terms of speed. We shall concentrate in this review on one particular version of the PBT employing a buried grid of CoSi2 as the gate. The implementation of this transistor rests on recent advances in Si heteroepitaxy, allowing high quality epitaxial growth of metallic silicides, of which CoSi2 is the most promising to date. Heterostructures of the kind Si/CoSi2/Si(111) are grown by molecular beam epitaxy and subsequently overgrown with additional Si by low pressure vapour phase epitaxy after the structuring of the gate metal. Strained-layer epitaxy of CoSi2 is shown to be of utmost importance for the following Si overgrowth. The lattice mismatch of 1.2% limits the growth of CoSi2 with a low density of dislocations on Si to a few nanometers. The small gate thicknesses may be shown to affect the transistor characteristics in a significant way.