Periodically-recessed ohmic contacts on GaN heterostructures

Abstract

Gallium nitride high electron mobility transistors (HEMTs) are widely used for microwave high power applications thanks to their high current driving capability and high operating voltages enabled by high sheet carrier concentration, high electron saturation velocity, and the high breakdown field. High-performance ohmic contacts are crucial for GaN-based HEMTs for high frequency and low loss power switching characteristics. The parasitic resistance associated with the source ohmic contact sets the limitation on the maximum oscillation frequency (fmax), the maximum unity-gain frequency (fT), and on-resistance (Ron) of the transistors. Various approaches to achieve low contact resistance include the epitaxial regrowth, recessed ohmic contacts, and pre-metallization annealing. Among these methods, the recessed ohmic contacts have succeeded to demonstrate low contact resistance. In particular, the depth of etching prior to the ohmic metallization is studied to achieve optimum contact resistance. The best performance can be achieved when 2-dimensional electron gas (2-DEG) is completely removed beneath the ohmic metallization. Two sidewalls formed by recess etching enable the ohmic metallization to contact 2DEG directly without a barrier layer. In this thesis, pattern-recessed ohmic contacts have been demonstrated to be an effective method to reduce the ohmic contact resistance in GaN-based HEMTs. Periodically-recessed patterns are introduced in the source and drain contact area to achieve the lowest contact resistance by making sidewall contact as well as top contact on to the active mesa. The additional process of pre-metallization etching using a Cl2-based inductively-coupled plasma reactive ion etching (ICP-RIE) removes the 2DEG beneath the ohmic metallization in part by introducing the periodic patterns. Due to the sidewalls formed by pattern-recessed etching, the effect of sidewall contacts is enhanced. The performance improvement of the pattern-recessed ohmic contacts is investigated by comparing the electrical characteristics of linear transmission line model (LTLM) patterns and the HEMT devices fabricated with and without pattern-recessed ohmic contacts.