Optical characterization of heterojunction bipolar transistors

We report the nondestructive, optical characterization of a heterojunction bipolar transistor (HBT) structure using photoreflectance spectroscopy (PR) and spectral ellipsometry (SE). The PR results show good agreement with capacitance-voltage (C-V) measurements for the dopant concentration in the n-GaAs collector layer. We find that PR provides a reasonable N_d for the emitter layer; whereas C-V analysis of the thin emitter layer provides only an upper-limit to the dopant concentration, PR provides a useful means of obtaining emitter and collector dopant concentrations quickly and nondestructively. We have developed a detailed SE model of the HBT structure and applied this model to obtain alloy compositions and layer thicknesses for the upper five of the nine HBT layers. Combining the structure and dopant concentration data obtained from critical layers of the HBT, we can screen the epitaxial material for potentially harmful structural deviations