A study of deep levels of the emitter region of a heterojunction bipolar transistor is investigated using deep level transient spectroscopy (DLTS), deep level admittance spectroscopy (DLAS), thermally stimulated capacitance (TSCAP), and capacitance-voltage (C-V) profiling. The DX center, with an activation energy of 0.45 eV, is the only deep level detected. By varying the DLTS rate window and filling pulse widths, DX is found to be comprise of two closely spaced DX centers, denoted DX1 and DX2. A positive peak observed in the DLTS spectra is attributed to electron capture, not minority carrier emission, and, thus, is an experimental artifact. Finally, the reduction of current gain (β) at low collector current and the effect of the DX center on the switching characteristics of HBTs are briefly discussed. / Graduation date: 1993