InAlAs/InGaAs heterojunction bipolar transistor with a 300 A˚ spacer inserted between emitter and base grown by molecular beam epitaxy was characterized by using photoreflectance spectroscopy. The energy features observed above the InGaAs fundamental band gap are attributed to the quantum confined subband transition of two‐dimensional electron gas which was confined in the spacer channel. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the two‐dimensional electron gas concentration. The behavior of two‐dimensional electron gas at temperatures between 10 and 300 K was also characterized and the sheet‐density in proportion to temperature was observed. Furthermore, using the temperature dependence of effective mass measured by cyclotron resonance combined with photoreflectance to analyze built‐in electric field is also reported. ©1995 American Institute of Physics.