On the performance limits of cryogenically operated SiGe HBTs and its relation to scaling for terahertz speeds

The goal of achieving terahertz (THz) transistors within the silicon material system has generated significant recent interest. In this paper, we use operating temperature as an effective way of gaining a better understanding of the performance limits of SiGe HBTs and their ultimate capabilities for achieving THz speeds. Different approaches for vertical profile scaling and reduction of parasitics are addressed, and three prototype fourth-generation SiGe HBTs are compared and evaluated down to deep cryogenic temperatures, using both dc and ac measurements. A record peak f_T/f_max of 463/618 GHz was achieved at 4.5 K using 130-nm lithography (309/343 GHz at 300 K), demonstrating the feasibility of reaching half-THz f_T and f_max simultaneously in a silicon-based transistor. The BV_CEO of this cooled SiGe HBT was 1.6 V at 4.5 K (BV_CBO = 5.6 V), yielding a record f_T × BV_CEO product of 750 GHz · V (510 GHz · at 300 K). These remarkable levels of transistor performance and the associated interesting device physics observed at cryogenic temperatures in these devices provide important insights into further device scaling for THz speeds at room temperature. It is predicted in a new scaling roadmap that f_T/f_max} of room-temperature SiGe HBTs could potentially achieve 782/910 GHz at a BV_CEO of 1.1 V at the 32-nm lithographic node.