An epitaxial planar structure for the unijunction transistor

A unijunction transistor with fast recovery time and high interbase resistance is made by growing a thin high-resistivity boron-doped epitaxial layer on a low-resistivity arsenic-doped substrate. Ring-and-dot base contacts are made directly on the p-type epitaxial layer with the beam-lead contact process. With the dot grounded and a negative bias on the ring, the most positive point along the edge of the space-charge region in the epitaxy is centered below the dot. When the substrate is adequately biased in the forward direction with respect to this point, conductivity modulation of the spreading resistance under the dot begins because of minority charge injection from the substrate. Experimental devices have been made by growing a 7.1-micron 53.1-Ω.cm epitaxial layer on a 5Ω.cm substrate. Interbase resistance was ∼ 100k Ω, intrinsic stand-off ratio ∼ 0.5, holding current ∼ 1 mA, and recovery time <20 ns. The paper concludes with possible memory array and shift register circuit applications that incorporate the unit described as well as a junction-isolated version that is free of parasitic transistors.