InAlN/GaN high electron mobility transistors on Si for RF applications

Abstract

Conventional AlGaN/GaN High Electron Mobility Transistors (HEMTs) have been proven to be a strong competitor in both high voltage and high frequency applications resulting from the intrinsic material properties of GaN such as large bandgap, high electron mobility, high electron saturation velocity and high thermal conductivity. In the past decades, GaN HEMTs have emerged as one of the hottest research topics and intensively studied. The performance of conventional AlGaN/GaN HEMTs have been improved significantly and continuously in the past decades, such as high output power, high operation frequency and low noise figure. Recently, a novel heterostructure with a thin layer of InAlN on top of GaN have been demonstrated to further improve the high frequency performance of GaN HEMTs. Benefiting from the unique properties of InAlN/GaN hetrostructure such as very thin top barrier thickness, high electron density and lattice match, the high frequency performance of GaN HEMTs have been pushed to the next level. On the other hand, there are still some critical challenges limiting the applications of GaN HEMTs. On key issue is that most of the high frequency results, especially those above 200 GHz, were reported from devices grown on SiC substrates. SiC has the advantages of small lattice mismatch to GaN eplilayers, very high resistivity and thermal conductivity. Thus, GaN HEMTs grown on SiC can achieve higher RF performance than those grown on Si. However, GaN HEMT on SiC is not cost-effective and is only available in smaller sizes (≥ 6 inch) which make it less attractive to be adopted commercially. To reduce the cost of GaN HEMTs, Si substrates have attracted increasing interest in recent years, not only in power electronics applications but also in RF applications. Significant efforts have been made on improving the epitaxial quality of GaN on Si substrates as well as the device fabrication technology. As a result, the performance of RF GaN HEMTs on Si has improved significantly. However, the high frequency performance of GaN HEMTs on Si still lags behind their counterparts on SiC. The best reported AlGaN/GaN HEMT on Si only exhibited a fT of 176 GHz with for a gate length of 80 nm. Another drawback is the poor linearity performance of deeply scaled GaN HEMTs. Linearity is an important parameter for GaN HEMTs to be applied as amplifiers in modern communication system. GaN HEMT is expected to maintain high operation frequency at high gate bias to support its application for large signal RF operation. However, poor linearity characteristics have been observed in the conventional GaN HEMTs. It is manifested by a non-flat transconductance (gm) and fT, fmax versus gate bias (or drain current). After reaching its maximum point, gm or fT, fmax decrease drastically with the increasing gate bias. Linearity of GaN transistors ultimately limits the power density and efficiency of these devices in many applications, as the operating point of the device typically needs to be backed-off to meet the linearity…