Extraction of interface states at emitter–base heterojunctions in AlGaAs/GaAs heterostructure bipolar transistors using sub-bandgap photonic excitation

Distribution of interface states at the emitter–base heterojunctions in heterostructure bipolar transistors (HBTs) is characterized by using current–voltage characteristics using sub-bandgap photonic excitation. Sub-bandgap photonic source with a photon energy Eph which is less than the energy bandgap Eg (Eg,GaAs = 1.42, Eg,AlGaAs = 1.76 eV) of emitter, base, and collector of HBTs, is employed for exclusive excitation of carriers only from the interface states in the photo-responsive energy range at emitter–base heterointerface. The proposed method is applied to an Al0.3Ga0.7As/GaAs HBT (AE = WE × LE = 250 × 100 μm2) with Eph = 0.943 eV and Popt = 3 mW. Extracted interface trap density Dit was observed to be Dit,max ∼ 4.2 × 1012 eV−1 cm−2 at emitter–base heterointerface.