CARBON-DOPED BASE GAAS/ALGAAS HETEROJUNCTION BIPOLAR-TRANSISTOR GROWN BY METALORGANIC CHEMICAL VAPOR-DEPOSITION USING CARBON-TETRACHLORIDE AS A DOPANT SOURCE

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/D'Alembertian 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.