Millimeter-wave GaN high electron mobility transistors and their integration with silicon electronics

Abstract

In spite of the great progress in performance achieved during the last few years, GaN high electron mobility transistors (HEMTs) still have several important issues to be solved for millimeter-wave (30 ~ 300 GHz) applications. One of the key challenges is to improve its high frequency characteristics. In this thesis, we particularly focus on fT and fma, two of the most important figures of merit in frequency performance of GaN HEMTs and investigate them both analytically and experimentally. Based on an improved physical understanding and new process technologies, we aim to demonstrate the state-of-the-art high frequency performance of GaN HEMTs. To maximize fmax, parasitic components in the device (Ri, R, Rg, Cgd, and go) are carefully minimized and the optimized 60-nm AlGaN/GaN HEMT shows a very high fmax of 300 GHz. The lower-than-expected fT observed in many AlGaN/GaN HEMTs is attributed to a significant drop of the intrinsic transconductance at high frequency (RF gm) with respect to the intrinsic DC g. (called RF gm-collapse). By suppressing RF gm-collapse and harmoniously scaling the device, a record fT of 225 GHz is achieved in the 55-nm AlGaN/GaN HEMT. Another important challenge for the wide adoption of GaN devices is to develop suitable technology to integrate these GaN transistors with Si(100) electronics. In this thesis, we demonstrate a new technology to integrate, for the first time, GaN HEMTs and Si(100) MOSFETs on the same chip. This integration enables the development of hybrid circuits that take advantage of the high-frequency and power capability of GaN and the unsurpassed circuit scalability and complexity of Si electronics.