The major advantages of epitaxial transistors are the virtual elimination of the collector series resistance and a large decrease in storage and turn-off times. The basic process consists of depositing a thin (about 1/2-mil) layer of high resistivity semiconductor material on a thicker substrate of low resistivity material. Three epitaxial transistors were recently introduced:
Rheem Semiconductor's RT409 silicon unit is similar to the 2N697, with a 60-v collector breakdown. The typical collector-to-emitter saturation voltage at 150 ma has been reduced by a factor of two. The typical storage time for the epitaxial unit is less than 100 nsec, while for the 2N697 it is 400 nsec.
Motorola Semiconductor is offering a low-cost type 2N834 silicon-mesa transistor. Breakdown voltages are in the order of 90 v and storage time as low as 12 nsec. They are capable of delivering 0.5 w power output with 10-db gain at 70 mc.
Sylvania's germanium epitaxial transistors resemble the 2N702 and 2N711, with improved performance in saturation voltage and switching time. (Electronic Design, Oct. 26, 1960, p. 82)
The epitaxial process also was key to the development of ICs.