Structure optimization of trench-isolated SiGe HBTs for simultaneous improvements in thermal and electrical performances

The current level in the modern high-speed SiGe heterojunction bipolar transistors (HBTs) continues to increase for operation speed enhancement, but the resultant self-heating and elevated junction temperature emerge as a growing concern for device reliability as well as performance. To address such thermal issues, the optimization of SiGe HBT structures to achieve simultaneous improvements in thermal and electrical performance is carried out in this study. As a foundation for the study, an R/sub th/ measurement method and a geometry-based fast analytic thermal model were first developed for trench-isolated SiGe HBTs. Based on the method and model, a set of device design points for lowered R/sub th/ without compromising the RF performance have been successfully proposed and experimentally verified on IBM's 200-GHz SiGe HBTs. The details of the proposed structures and acquired results will be described in detail in the paper. The results obtained in this study shed a light on the possibility of the simultaneous optimization of thermal and electrical performance of SiGe HBTs.

I. Introduction

From a historical perspective, the development of semiconductor technologies has been driven by the ever-increasing need for improved electrical performance. Interestingly, however, it has been the thermal performance that often induced major changes to the direction of the technology development in recent years. Fig. 1. Measured effect of ambient temperature on cutoff-frequency of high-speed SiGe HBTs, which shows the importance of thermal effects on semiconductor devices. The rapid replacement of the bipolar logic with the CMOS logic for high-speed digital applications in early 1990s was triggered by excessive heat generation from bipolar transistor-based integrated circuits. A decade later, the most advanced CMOS-based integrated circuits of today are confronting the same issue, the long-term direction of CMOS technology development being affected by the approaches on how to cope with such thermal problems. The thermal effects, or heat generation from self-heating and consequent junction temperature rise, adversely affect semiconductor device performance in terms of both functional operation and reliability.