SILICON-ON-SAPPHIRE EPITAXIAL BIPOLAR TRANSISTORS

All-epitaxial bipolar transistors have been fabricated in silicon-on-sapphire films which exhibit a common-emitter low frequency current gain hfe of 10 at room temperature. The value of hfe increases approximately as the square of the absolute temperature, which suggests recombination by charged coulomb centers in the base region. Monotonic increase in hfe with collector current up to 100 mA (emitter dimensions are 0.8 × 1.2 mil) is probably due to trap-filling in the base region. No conductivity modulation, which would result in beta fall-off, is seen up to 500 mA of collector current. This is a consequence of the narrow (0.6 μ), heavily-doped (∼ 1017 cm-3) base region. High-frequency measurements indicate an fT of 350 MHz at 15 mA and extrapolation of a plot of 1 fT vs. 1 Ie yields a base transit time of 350 psec. Minority carrier lifetime in the base region is measured to be 0.6-0.9 nsec. The silicon is deposted on single crystal sapphire oriented in the [11̄02] plane which results in the growth of [100]-oriented silicon. Arsenic and boron are used as the dopants in the n+, n, p, n+ structure. The total film thickness is approx 6 μ and the transistor structure contains a mesa-emitter and mesa-collector. © 1968.