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The first successful demonstration of a delta-doped InAlGaP/GaAs heterojunction bipolar transistor (HBT) is reported. A comparison to a baseline InAlGaP/GaAs HBT without a delta-doping layer is made. Both of these devices exhibit near-ideal current gain (beta) versus the collector current (I C) characteristics (i.e., beta independent of I C) at high currents. The delta-InAlGaP/GaAs HBT exhibits a 40% reduction in offset voltage (V CE, offset) and a 250-mV reduction in knee voltage (V k) without sacrificing beta compared with the baseline InAlGaP/GaAs HBT. At a higher I C, the decrease in beta of the InAlGaP/GaAs HBTs with increasing temperature is significantly smaller than the corresponding effect measured in the formerly reported GaAs-based HBTs. The rather temperature-insensitive characteristics of these two InAlGaP/GaAs HBTs originate from their large valence-band discontinuity (DeltaE V) at the emitter-base (E-B) junction. Furthermore, at intermediate base current I B levels (0.4-1.6 mA), V CE, offset falls as I B increases, which is a trend contrary to that of most HBTs in the literature. Finally, the experimental dependence of V CE, offset on temperature, I B, and the effective barrier height at the E-B junction is explained with reference to an extended large-signal model.