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Title: InAlN/GaN high electron mobility transistors on Si for RF applications
Authors: Xing, Weichuan
Keywords: DRNTU::Engineering::Electrical and electronic engineering
Issue Date: 12-Sep-2018
Source: Xing, W. (2018). InAlN/GaN high electron mobility transistors on Si for RF applications. Doctoral thesis, Nanyang Technological University, Singapore.
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 specifications. In fact, as the operating frequency increases into the mm-wave range by shrinking the gate length, the linearity is expected to degrade even further. This thesis mainly focuses on these two issues. Novel approaches are employed to resolve them and much improved device performance were obtained. The major contributions of the thesis are listed as below. (1) The factors that limit the devices’ high frequency performance are investigated. A thin InAlN top barrier is applied instead of conventional AlGaN top barrier in order to minimize the impact of decreased gate length-to-barrier thickness ratio and thereby degradation of the gate modulation efficiency. Sub-100 nm gate was developed using electron beam lithography (EBL) technology to minimize gate induced intrinsic delay. Parasitic charging delay was minimized benefiting from the short source-to-drain distance down to 300 nm and low contact resistance Rc of 0.2 Ω.mm. Maximum fT of 250 GHz was obtained in a 40-nm gate device, which is the highest among any other GaN HEMTs demonstrated on Si substrate previously. Surface passivation effects on DC and RF performance were also investigated. (2) The mechanism of the poor linearity performance of the gm and fT at high gate bias was investigated. A novel planar-nanostrip GaN HEMTs structure using ion implantation technology was developed to improve the linearity performance and maintain fT at a high level without introducing too much gate parasitic capacitance. The fabrication process was described in details including As ion implantation for isolation application, nanostrip-channel formation using different approaches. Moreover, the planar-nanostrip device also showed much improved maximum drain current Idmax up to 2.6 A/mm, which is close to the theoretical limit. Also, device geometries including gate length, line-to-space ratio of the nanostripchannel and gate-to-source distance have been studied. These results do not only identify the origin of the non-linear performance in GaN HEMTs, but also illustrate the direction of design improvement of RF GaN HEMTs for high linearity application. (3) A Planar nanostrip-channel Al2O3/InAlN/GaN MISHEMTs on Si was demonstrated. A thin layer of oxide between the metal gate and the thin InAlN barrier and form a metal-insulator-semiconductor (MIS) gate in the Planar nanostrip-channel GaN HEMT, gate leakage current was reduced and thus increase the gate voltage swing and drain current swing. The results show that the Planar nanostrip-channel Al2O3/InAlN/GaN MISHEMTs is able to work at up to Vg = +4 V and the linearity performance was further improved. Two-tone intermodulation characterizations are discussed for conventional HEMT, Planar nanostrip-channel HEMT and MISHEMT. The lower IM3-to-carrier ratio (– C/IM3) and larger third order intercept OIP3 values of the Planar nanostripchannel MISHEMT clearly indicate that the linearity of GaN HEMT was improved by the planar nanostrip-channel structure as well as the insertion of 6- nm Al2O3 gate insulator.
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