Improving the RF Performance of InP DHBT Using fT-Doubler Technique

This letter presents the application of the $f_T$ -doubler technique to improve the cut-off frequency ( $f_T$ ) of InP heterojunction bipolar transistors (HBTs), and reported dc and ac characteristics in detail for the first time. A $f_T$ -doubler structure was implemented by the InGaAs/InP DHBTs technique in the Institute of Microelectronics Chinese Academy of Science (IMECAS), which uses three InP DHBTs with the same emitter area of 0.8 $\times$ 10 $\mu$ m. The forward Gummel plot and output characteristic of the single DHBT and $f_T$ -doubler were compared and analyzed. The cut-off frequency (peak $f_T$ ) of $f_T$ -doubler is obtained as 263 GHz by extrapolating $H_{21}$ , while the peak $f_T$ of single DHBT is 165 GHz. In addition, the small-signal parameters of the $f_T$ -doubler were extracted, resulting in a good fit. This demonstrates that the reduction of $C_{\pi}$ is responsible for the increase in $f_T$ , and confirms the applicability of the $f_T$ -doubler structure as a single unit for circuit design.