Improvements in direct-current characteristics of Al(0.45)Ga(0.55)As-GaAs digital-graded superlattice-emitter HBTs with reduced turn-on voltage by wet oxidation

• Published in: IEEE transactions on electron devices Vol. 50; no. 2; pp. 303 - 309
• Main Authors: Tsai, Ming-Kwen, Tan, Shih-Wei, Wu, Yen-Wei, Yang, Ying-Jay, Lour, Wen-Shiung
• Format: Journal Article
• Language: English
• Published: 01.02.2003
• ISSN: 0018-9383
• DOI: 10.1109/TED.2002.808428

Heterojunction bipolar transistors (HBTs) having an Al(0 45)Ga(0 55)As-GaAs digital-graded superlattice (DGSL) emitter along with InGaP sub-emitter are reported. The band diagram of the DGSL emitter is analyzed by using a transfer matrix method and the theoretical result is consistent with the experimental observation that the DGSL emitter smoothes out the potential spike at the emitter-base junction. Such passivated HBTs with a high Al-fraction passivation layer exhibit a small offset voltage of 50 mV, a turn-on voltage of 0.87 V, and a current gain of 385. The HBTs are examined by wet-oxidizing the exposed passivated region under various conditions. Experimental results reveal that the HBTs with an exposed high Al-fraction emitter are sensitive to the ambient air. However, with InGaP capped upon the high Al-fraction emitter, the HBTs exhibit better oxidation quality. The wet oxidation brings forth the most remarkable improvements for the InGaP-capped HBTs when the passivation layer is totally wet oxidized. Further-more, some devices from the same chip have undergone nitrogen treatment for comparison. (Author)