No abstract

Ti/TiN/Cu is established to be an enabling alternative to the better-known Au-based ohmic contact metals such as Ti/Al/Ni/Au. The Cu-based option delivers lower contact resistance and smoother surface morphology and is proven to be compatible with AlGaN/GaN HEMTs device processing. The TiN layer serves as an effective Cu-diffusion barrier as no detectable Cu-diffusion was observed when subjected to thermal treatment up to 600 ℃. There is a tendency of N-diffusion across the Ti/GaN interface near which N-deficiency in the GaN epitaxial layer and formation of a nano-sheath of TiN was found. This ultrathin layer of TiN works to further improve the ohmic performance of the electric contact, as reflected in lowered contact resistivity ρC. It is possible to manufacture the TiN thin films with low sheet resistance at a high deposition rate by adjusting the ratios between argon and nitrogen gas flows during sputtering deposition. Contact resistivity ρC, tested for the AlGaN/GaN HEMT devices fabricated on Si substrate according to the TLM standard were found to be as low as 3.65×10-6 Ω-cm2 (RC = 0.54 Ω-mm). The outcomes benchmark favourably against many reported metal-stacking structures for ohmic contacts. The robustness of surface morphology and interface sharpness against thermal treatments make the established ohmic stack structures suitable for scalable device fabrications.

An AlGaN/GaN/Si high electron mobility transistor (HEMT) using a GaN:C buffer with a 2 nm AlGaN electron-blocking layer (EBL) is investigated for the first time for millimeter-wave applications. Compared with the double heterostructure field effect transistor (DHFET), the AlGaN/GaN HEMT with the GaN:C/EBL buffer has a lower vertical leakage, higher thermal stability, and better RF performance. In addition, AlGaN EBL can prevent carbon-related traps from GaN:C and improve electron confinement in 2DEG during high-frequency operation. Finally, a Pout of 31.2 dBm with PAE of 21.7% were measured at 28 GHz at 28 V. These results demonstrated the great potential of HEMTs using GaN:C with AlGaN EBL epitaxy technology for millimeter-wave applications.

Recently, high-frequency devices for Ka band and above are becoming more important for wireless communication due to 5 G and beyond applications. The highfrequency AlGaN/GaN high-electron-mobility transistors (HEMTs) are widely considered for millimeter-wave applications due to high electron mobility, high critical field, excellent electron peak velocity, and wide bandgap of GaN material. [1][2][3][4] To reduce the production cost, AlGaN/GaN HEMTs grown on Si substrates are popularly investigated for Ka band applications. [5] Besides, the AlGaN/GaN HEMTs on Si technology has the potential to be integrated into complementary metaloxide-semiconductor (CMOS) circuits on the same platform. [6] Over the past, several GaN-based technologies for applications at Ka band or above have been reported. Demonstration of highefficiency monolithic microwave integrated circuits (MMIC) power amplifiers using 40 nm gate length technology was presented. [7] A 60 nm gate length with graded-channel technology was adopted, exhibiting peak power-added efficiency (PAE) up to 70%. [8,9] While the short gate length approaches effectively led to high efficiency at the desired frequency bands, the power density and overall reliability may be an issue due to the limited breakdown voltage. GaN-on-SiC configuration with very thin barrier of 3 nm was reported, showing a power density of 4 W mm À1 under continuous wave (CW) mode at 40 GHz. [10] In general, gold (Au) is used as the thick metallization metal for III-V HEMTs in the past. Moreover, the price of Au is expensive. To reduce the production cost for mass production of GaN-based device, copper (Cu) is considered to be an ideal candidate to replace Au as metallization metal. Compared to Au, Cu has lower resistivity, and the better thermal conductivity of Cu suggests it to be a suitable replacement. Nevertheless, when Cu is in direct contact with a semiconductor, it tends to diffuse into the semiconductor, sabotaging the substrate and the device. [11][12][13] Therefore, a diffusion barrier layer is needed to overcome this problem.Diffusion barrier layers such as tantalum (Ta), tantalum nitride (TaN x ), tungsten nitride (WN x ), Ta/MnSi x O y bilayer structures, and platinum (Pt) have been studied in the past years. [13][14][15][16][17] According to the previous reports, Pt is an effective diffusion barrier to prevent Cu diffusion into III-V devices. [8,12,13] Reports have also shown that addition of titanium (Ti) layer to the Au/WN x or WN x /Cu interfaces can improve the adhesion between Cu and the semiconductor. [18]

Substrate voltage (VSUB) effects on GaN-on-Si high electron mobility transistors (HEMTs) power application performance with superlattice transition layer structure was investigated. The 2DEG conductivity and buffer stack charge redistribution can be affected by neutral/ionized donor and acceptor traps. As the donor/acceptor traps are excessively ionized or de-ionized by applying VSUB, the depletion region between the unintentionally doped (UID)/Carbon-doped (C-doped) GaN layer may exhibit a behavior similar to the p–n junction. An applied negative VSUB increases the concentration of both the ionized donor and acceptor traps, which increases the breakdown voltage (BV) by alleviating the non-uniform distribution of the vertical electric field. On the other hand, an applied positive VSUB causes the energy band bending flattener to refill the ionized traps and slightly improves the dynamic Ron degradation. Moreover, the amount of electrons injected into the buffer stack layer from the front side (2DEG channel/Ohmic contact) and the back side (AlN nucleation layer/superlattice transition layer) are asymmetric. Therefore, different VSUB can affect the conductivity of 2DEG through the field effect, buffer trapping effect, and charge redistribution, which can change the electrical performance of the device.