THE EFFECT OF EMITTER LAYER VARIATIONS ON THE CURRENT GAIN OF ALGAAS-GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS GROWN BY CHEMICAL BEAM EPITAXY

AlGaAs-GaAs heterojunction bipolar transistors have been fabricated wi th current gains > 200 for devices with emitter dimensions of 2X2 mum2 . The epitaxial layers were grown using chemical beam epitaxy and were npn single heterojunction transistors with aluminum grading in the ba se. The high current gains observed in these devices are attributed to leaving a thin AlGaAs emitter layer (30-60 nm) over the entire base m esa 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) va lues dependent on the base doping which was in the (2-6)X10(19) cm-3 r ange. The epitaxial layers were grown on 3 inch diameter semi-insulati ng substrates in a VG Semicon 4200 CBE system. Room temperature photol uminescence intensities were used to determine the optimum growth cond itions for both the n-Al0.3Ga0.7As emitters and the p-AlxGa1-xAs grade d bases. SIMS, DCD X-ray diffraction, Hall effect measurements and pho toreflectance were also used to evaluate the grown layers. Variations in emitter composition, doping and thickness were studied as well as t he effects of growth interrupts at the emitter-base junction. Digital circuits that operate at data rates above 10 Gb/s have been demonstrat ed. These include 4:1 multiplexers, 1:4 demultiplexers, dividers, prea mplifiers and decision circuits.