QUANTITATIVE CORRELATION BETWEEN OXYGEN IMPURITY AND CURRENT GAIN-BETA OF ALGAAS GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS GROWN BY MOLECULAR-BEAM EPITAXY/

The experimental correlation between current gain beta of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) and oxygen in the n-AlGaAs e mitter layer is discussed quantitatively. It has been observed that be ta decreases monotonically with an increase of the oxygen concentratio n in the n-AlGaAs layer. It has been found that the growth rate is one of the dominant factors which determine the crystal quality including the oxygen concentration, and that the optimum growth rate for the Al GaAs/GaAs HBT structure is around 0.7 mu m/h under the constant growth temperature used. It has been thought that the variation of beta is m ainly ascribed to the recombination center at the emitter/base junctio n due to the oxygen impurity. The value of 3 x 10(17) cm(-3) is conseq uently obtained as the upper limit of the oxygen concentration for hig h and stable beta by extrapolating the linear relation between the oxy gen concentration and 1/beta(Brs) (the reciprocal of the recombination component of beta at the emitter/base junction).