MM-wave frequencies GaN-on-Si HEMTs and MMIC technology development

Gallium Nitride (GaN)-based High Electron Mobility Transistors (HEMTs) grown on Silicon (Si) substrates technology is emerging as one of the most promising candidates for cost effective, high-power, high-frequency Integrated Circuit (IC) applications; operating at Microwave and Millimetre (mm)-wave frequencies. To capitalise on the advantages of RF GaN technology grown on Low resistivity (LR) Si substrates; RF losses due to the Si substrate must be eliminated at the active devices, passive devices and interconnect. Low resistivity Si substrates are intrinsic prone to RF losses and high resistivity (HR) Si substrates shown to exhibit RF losses as a result of operating substrate temperature at the system level. Therefore, obtaining a viable high-performance RF GaN-on both LR and HR Si device remains a challenge for this technology. In an attempt to overcome these issues, Microwave Monolithic Integrated Circuit (MMIC)-compatible technology was developed for the first time aiming to eliminate the substrate coupling effect for the realisation of high performance passive and active devices on GaN-on-Si substrates for mm-wave MMIC applications. To validate the novel RF GaN-on-Si substrates developed technology in this work, several fabrication techniques approaches were investigated and developed in order to improve the DC and RF performance of developed AlGaN/GaN HEMTs. The electrical characteristics were analysed based on the extracted small-signal equivalent circuit model from the measured data using on-wafer probes. Device parasitic effects associated with input/output contact pads were minimised by optimising the layout of the device. Consequently, using a proper device layout design, downscaling the AlGaN Schottky barrier and inserting an AlN interlayer in the material structure were found to have effectively improve the RF performance, where a maximum cutoff frequency, fT of 79.75 GHz and maximum oscillation frequency, fMAX of 82.5 GHz were obtained. To our knowledge, these results were the best performance AlGaN/GaN HEMTs grown on LR Si, and comparable to AlGaN/GaN HEMTs grown on Semi Insulating (SI)-SiC and HR Si substrates with similar gate lengths. Novel low-loss transmission media technology on GaN-on-LR Silicon was also developed and demonstrated in this work. Two design structures were successfully realised providing complete isolation of the conductive substrate by employing a ground plane, a 5 µm-thick II benzocyclobutene (BCB) and an additional elevation of elevated line traces supported by airbridges. Consequently, an attenuation constant, α, of better than 0.06 Np/mm and 0.45 Np/mm were achieved at frequencies of up to 76 GHz and 750 GHz, respectively, with matching (S11) of better than -15 dB over the whole frequency range. These results for the passive components and transmission media interconnect performance exhibited a better performance than those currently used in MMICs’ conventional transmission media technology, such as Microstrip and Coplanar waveguide (CPW) on a standard SI-GaAs substrates. To prove the capabilities and efficiency of the developed transmission media, low-loss in-line series and shunt MetalInsulator-Metal (MIM) capacitors were presented. In addition, High-Q on-chip inductors employing elevated traces and a BCB interface layer were also realized. A peak Q-factor of 22 at 24 GHz and fSRF of 59 GHz was achieved for 0.81 nH inductors. The realised MIM capacitors and spiral inductors were characterized based on the extracted small-signal equivalent circuit model. The developed transmission media and passive devices technology offered a promising platform for integrated RF GaN-HEMTs on Si for the realisation of high-performance monolithic integrated circuits for mm-wave applications