Over 450-GHz ft and fmax InP/InGaAs DHBTs With a Passivation Ledge Fabricated by Utilizing SiN/SiO2 Sidewall Spacers

This paper describes InP/InGaAs double heterojunction bipolar transistor (HBT) technology that uses SiN/SiO₂ sidewall spacers. This technology enables the formation of ledge passivation and narrow base metals by i-line lithography. With this process, HBTs with various emitter sizes and emitter-base (EB) spacings can be fabricated on the same wafer. The impact of the emitter size and EB spacing on the current gain and high-frequency characteristics is investigated. The reduction of the current gain is <;5% even though the emitter width decreases from 0.5 to 0.25 μm. A high current gain of over 40 is maintained even for a 0.25-μm emitter HBT. The HBTs with emitter widths ranging from 0.25 to 0.5 μm also provide peak f_t of over 430 GHz. On the other hand, peak f_max greatly increases from 330 to 464 GHz with decreasing emitter width from 0.5 to 0.25 μm. These results indicate that the 0.25-μm emitter HBT with the ledge passivaiton exhibits balanced high-frequency performance (f_t = 452 GHz and f_max = 464 GHz), while maintaining a current gain of over 40.