Deeply Scaled InAlN/GaN-on-Silicon High Electron Mobility Transistors for RF Applications

Abstract

Wide bandgap gallium nitride (GaN) based high electron mobility transistors (HEMTs) are promising candidates for next-generation radio frequency (RF) power amplifier applications owing to high electron saturation velocity and large breakdown field. Conventional RF GaN HEMT stacks are epitaxially grown on semi-insulating silicon carbide (SiC) substrates. The high thermal conductivity of SiC and better crystal quality of epitaxial GaN-on-SiC helps in achieving excellent RF and power performance. However, the application of this technology in the emerging wireless communication sector is held back by the high cost of SiC substrates. Besides, GaN-on-SiC wafer size being limited to 6-inch diameter is a major bottleneck for large-scale production. Integrating GaN on silicon (Si) lowers the production cost and enhances scalability. This work aims to explore the RF and power performance of GaN-on-Si HEMTs. To enable deep scaling and achieve high cut-off frequencies, InAlN/GaN heterostructures are used as they offer high channel charge density with extremely thin InAlN barriers. This helps to maintain the aspect ratio while scaling down the device dimensions. One of the important parameters that limit the RF performance in deeply scaled devices is the Ohmic contact resistance. A dual approach for contact resistance minimization is attempted. First, using tetra-methyl ammonium hydroxide (TMAH) surface treatment and high-temperature annealing of a Ti-based metal stack, the Ohmic metal diffusion through the dislocations is enhanced to form TiN contacts in the GaN channel region. With this technique, a contact resistance of 0.23 Ohm-mm is achieved, which is three-fold lower compared to contacts fabricated without TMAH pre-treatment. Second, the polarization in the material is exploited to form Ohmic contacts using a novel Sc-based metal stack. A contact resistance of 0.39 Ohm-mm is obtained using this metal scheme. The mechanisms for the resistance reduction are investigated using transmission electron microscopy in each case, and a correlation is established between the microstructure and the contact resistance. Next, scaling studies are performed to enhance the cut-off frequency of the transistor. Using Ti-based Ohmic contacts for the source and the drain regions, a unity current/power gain cut-off frequency (fT/fmax) of 101/87 GHz is obtained in HEMTs with 200 nm gates. A record high effective electron velocity of 1.56 x 10^7 cm/s is estimated, which helps in achieving a state-of-the-art fT-LG product of 20.2 GHz-µm in GaN-on-Si devices. An fT/fmax of 240/47 GHz is achieved by scaling down the gate length to 40 nm. Finally, X-band (10 GHz) power performance is demonstrated in GaN-on-Si HEMTs. Two device configurations, the Schottky gate HEMT and the recessed gate metal-insulator semiconductor HEMT (MIS-HEMT), are explored. An output power density of 1.44 W/mm and a power added efficiency of 33% are obtained in the Schottky gate HEMTs with 100 nm gates. However, the low breakdown voltage limits the output power in these devices. Considerable breakdown enhancement is achieved in the MIS-HEMTs using an ex-situ metal organic chemical vapor deposited SiN gate dielectric. A high output power density of 2.75 W/mm and a power-added efficiency of 22% are demonstrated in the MIS-HEMT devices. To summarize, methods to minimize the Ohmic contact resistance in GaN-on-Si HEMTs are explored. Using InAlN/GaN heterostructures, RF and X-band power performance is demonstrated in highly scaled devices.