Carbon-doped base GaAs/AlGaAs heterojunction bipolar transistor grown by metalorganic chemical vapor deposition using carbon tetrachloride as a dopant source

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Abstract
Carbon tetrachloride (CCl4) has been used as a carbon doping source for the base region of a GaAs/AlGaAs Npn heterojunction bipolar transistor (HBT) grown by low‐pressure metalorganic chemical vapor deposition (MOCVD). Transistors were fabricated and characterized for dc current gain, emitter‐base junction ideality factor, base contact resistance, and external base resistance. Microwave characterization by S‐parameter measurement was performed to determine the common emitter current gain and maximum available gain as a function of frequency. Transistors with the base contact area self‐aligned to a 3×10 μm emitter finger had a dc current gain as high as 50, an emitter‐base junction ideality factor of n=1.2, and a current gain cutoff frequency of ft=26 GHz. Transistors of equal emitter area without self‐alignment exhibited dc current gain as high as 86, n=1.2, and ft=20 GHz. A base contact resistance of Rc=2.85×10−6 Ω cm2 and an external base sheet resistance of Rs=533.4 Ω/⧠ were measured. These preliminary results indicate that carbon doping from CCl4 may be an attractive substitute for Zn or Mg in GaAs/AlGaAs HBT structures grown by MOCVD.