The effect of emitter layer variations on the current gain of AlGaAs-GaAs heterojunction bipolar transistors grown by chemical beam epitaxy

Abstract

AlGaAs-GaAs heterojunction bipolar transistors have been fabricated with current gains > 200 for devices with emitter dimensions of 2×2 μm 2. The epitaxial layers were grown using chemical beam epitaxy and were npn single heterojunction transistors with aluminum grading in the base. The high current gains observed in these devices are attributed to leaving a thin AlGaAs emitter layer (30–60 nm) over the entire base mesa and alloying through this layer to make base contact. This reduces the surface recombination currents to the extent that no significant decreases of current gain were observed with decreasing emitter size. Values of f t and f max from 50–110 GHz were measured with f max values dependent on the base doping which was in the (2−6) × 10 19 cm -3 range. The epitaxial layers were grown on 3 inch diameter semi-insulating substrates in a VG Semicon 4200 CBE system. Room temperature photoluminescence intensities were used to determine the optimum growth conditions for both the n-Al 0.3Ga 0.7As emitters and the p-Al x Ga 1− x As graded bases. SIMS, DCD X-ray diffraction, Hall effect measurements and photoreflectance were also used to evaluate the grown layers. Variations in emitter composition, doping and thickness were studied as well as the effects of growth interrupts at the emitter-base junction. Digital circuits that operate at data rates above 10 Gb/s> have been demonstrated. These include 4:1 multiplexers, 1:4 demultiplexers, dividers, preamplifiers and decision circuits.