An Investigation of Direct-Current Characteristics of Composite-Emitter Heterojunction Bipolar Transistors

Direct-current characteristics of the composite-emitter heterojunction bipolar transistor (CEHBTs) having a composite emitter formed of a 0.04 mu m In0.5Ga0.5P bulk layer and a 0.06 mu m Al0.45Ga0.55As/GaAs digital graded superlattice (DGSL) were investigated. InGaP/DGSL-passivated and DGSL-passivated CEHBTs were fabricated for comparison with a AlGaAs-passivated CEHBT having an InGaP/AlGaAs emitter and a conventional InGaP/GaAs SHBT. In addition to having a small collector-emitter offset voltage, the InGaP/DGSL-passivated and DGSL-passivated CEHBTs exhibit small base-emitter turn-on voltages, which are 130 and 370 mV lower than those of the InGaP/GaAs SHBT and comparison CEHBT, resp., at a 1 A/cm2 collector current. Results reveal that the DGSL structure forms a gradedlike wide-gap emitter to effectively smooth out the potential spike at the base-emitter heterointerface. The current gain of the InGaP/DGSL-passivated CEHBT is 250 and is enhanced to 385 by directly removing the InGaP layer. Authors qualitatively explain these improvements introducing the concept of a built-in electric field within the DGSL structure. 19 refs.