Design and fabrication of GaAs/GaAIAs heterojunction bipolar transistors with symmetrical characteristic

GaAlAs (N)/GaAs (p⁺)/GaAlAs (N) Double Heterojunction Bipolar Transistor (DHBTs) with single regrown base (p⁺ -GaAs or p⁺ -Ga₀.₈Al₀.₂As), double regrown base (p⁺) -GaAs /p⁺ -Ga₀.₈Al₀.₂As) and diffused base have been designed and fabricated by Liquid Phase Epitaxy (LPE) super cooling technique. Most of DHBTs with the single regrown base and diffused base showed the asymmetrical (I, V) characteristics with normal mode gain of 10-40 and inverted mode gain of 5-18. The offset voltage was in between 100mV to 600mV which conformed to the large difference between the CB and EB junction voltages. The asymmetric characteristics with high offset voltage are owing to either high electron injections from the collector to the external base region under inverted mode and then loss by recombination in GaAs (p⁺) regrown base or high intrinsic recombination centers in the Ga₀.₈Al₀.₂As (p⁺) external base layer. Moreover some DHBTs with single regrown base exhibited a knee-shape characteristic in inverted mode due to influence of spike at emitter-base heterojunction. In contrary, double regrown base DHBTs showed the symmetrical characteristics with the gain around 18 and the very low offset voltage of 60mV. This is because, in inverted mode, the lower (p⁺) -Ga₀.₈Al₀.₂As regrown base layer in the double regrown base DHBTs can suppress the electron injections from the collector in the external base region and also the upper p⁺ -GaAs regrown base layer can reduce the electron recombinations. In addition, the aluminum contents which relate to not only the injection efficiency but also the spike height at each heterojunction of these DHBTs were optimized. Finally very low offset voltage for double regrown base DHBTs was proved by small difference between junction voltages.