On The Emitter Resistance Of High-performance Gaas- And Inp-based Heterojunction Bipolar Transistors

摘要

Emitter resistance R_(EE) and collector current ideality factor n_C of inGaP/GaAs heterostructure bipolar transistors (HBTs) and InP/InGaAs double-HBTs (DHBTs) were investigated from the viewpoints of DC and RF characteristics. It was found that the apparent ideality factor of collector current n_C~(app) increases with the collector current I_C for all HBTs. The increase in n_C~(app) is more conspicuous in the InP/InGaAs DHBTs than in the InGaP/GaAs HBTs. The most likely explanation is that the R_(EE) consists of two components: one is the well-known contact resistivity R_(EE)~0 and the other is band-profile-dependent resistivity R_(Ei), which decreases as I_C increases. In the InP/InGaAs DHBTs, the increase in n_C~(app) with I_C is made remarkable by the insertion of an InGaAs etching stop layer (ESL) that makes it easy to form a ledge structure indispensable for high-reliability and high-performance HBTs. However, with the increase of I_C, the difference in R_(EE) between the InP/InGaAs DHBTs with and without the ESL becomes small. The insertion of an ESL is considered acceptable for high-speed IC applications. Using an emitter structure with an ESL, we developed self-aligned InP/InGaAs DHBTs with a ledge passivation structure that attained an f_T of 302 GHz, f_(max) of 388 GHz, and BV_(CEO) of 6.2 V.