GaN-based HEMTs on Low Resistivity Silicon Technology for Microwave Applications

This paper investigates the effect of insertion AlN spacer between the GaN channel and buffer in a sub-micron gate (0.3 μm) AlGaN/GaN HEMTs on a low-resistivity (LR) (σ < 10 Ω.cm) silicon substrates on RF performance. Enhancement in short circuit current gain (fT) and maximum frequency of oscillation (fMAX) was observed in the HEMT with a 1 nm AlN spacer, where (fT) and (fMAX) were increased from 47 GHz to 55 GHz and 79 GHz to 121 GHz, respectively. Small-signal-modelling analysis was carried out to study this improvement in performance. We found that the AlN interlayer played a crucial role in reducing the gate-source capacitance, Cgs, by 36 % and delay, τ, by 20 % under the gate, as a result of an increase in mobility and a reduction in trap-related effects

Effect Of AlN Spacer In The Layer Structure On High Rf Performance GaN-Based HEMTs On Low Resistivity Silicon At K-Band Application

AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown on Si substrate are emerging as an attractive devices for many RF applications. This is due to lower circuits realization cost and multifunction chips integration. In this study we investigate the effect of AlN spacer between AlGaN and GaN of a sub-micron gate (0.3 μm) AlGaN/GaN and AlGaN/AlN/GaN HEMTs on a Low Resistivity LR Si substrates on RF performance. We have observed an enhancement in RF performance fT and fMAX in the HEMT with of AlN spacer; (fT) was increased from 47 GHz to 55 GHz and (fMAX) was increased from 79 GHz to 121 GHz. This enhancement in performance is mainly due to the increase in the mobility in the channel and confinement of the carriers reducing Cgs, and delay τ under the gate. We believe this is the first RF study of this type as previous studies were based on the effects of the DC characteristic of the devices [1]

Terahertz Monolithic Integrated Circuits (TMICs) Array Antenna Technology On GaN-on-Low Resistivity Silicon Substrates

In this paper, we have demonstrated a viable microstrip array patch antenna technology for the first time on GaN-on-low resistivity silicon (LR-Si) substrates (ρ <; 40 Ω.cm) at H-band frequencies (220-325 GHz). The developed technology is compatible with standard MMIC technology with no requirement for high temperature processes. To mitigate the losses presented by the substrate and to enhance the performance of the integrated array antenna at THz frequencies, the driven patch is shielded by silicon nitride and gold layer in addition to a layer of benzocyclobutene (BCB). The demonstrated 4×1 array integrated antenna showed a measured resonance frequency in agreement with our simulation at 0.27 THz; a measured S11 as low as -41 dB was obtained. A directivity, gain and radiation efficiency of 11.2 dB, 5.2 dB, and 20% respectively was observed from the 3D EM model for a 5 μm BCB inset. To the authors' knowledge, this is the first demonstration of a THz integrated microstrip array antenna for TMIC technology; this developed technology is promising for high performance III-V electronic material on low resistivity/high dielectric substrates

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

Passive Components Technology for THz-Monolithic Integrated Circuits (THz-MIC)

In this work, a viable passive components and transmission media technology is presented for THz-Monolithic Integrated Circuits (THz-MIC). The developed technology is based on shielded microstrip (S-MS) employing a standard monolithic microwave integrated circuit compatible process. The S-MS transmission media uses a 5-μm layer of benzocyclobutene (BCB) on shielded metalized ground plates avoiding any substrate coupling effects. An insertion loss of less than 3 dB/mm was achieved for frequencies up to 750 GHz. To prove the effectiveness of the technology, a variety of test structures, passive components and antennas have been design, fabricated and characterized. High Q performance was demonstrated making such technology a strong candidate for future THz-MIC technology for many applications such as radar, communications, imaging and sensing

Terahertz Microstrip Elevated Stack Antenna Technology on GaN-on-Low Resistivity Silicon Substrates for TMIC

In this paper we demonstrate a THz microstrip stack antenna on GaN-on-low resistivity silicon substrates (ρ < 40 Ω.cm). To reduce losses caused by the substrate and to enhance performance of the integrated antenna at THz frequencies, the driven patch is shielded by silicon nitride and gold in addition to a layer of benzocyclobutene (BCB). A second circular patch is elevated in air using gold posts, making this design a stack configuration. The demonstrated antenna shows a measured resonance frequency in agreement with the modeling at 0.27 THz and a measured S11 as low as −18 dB was obtained. A directivity, gain and radiation efficiency of 8.3 dB, 3.4 dB, and 32% respectively was exhibited from the 3D EM model. To the authors' knowledge, this is the first demonstrated THz integrated microstrip stack antenna for TMIC (THz Monolithic Integrated Circuits) technology; the developed technology is suitable for high performance III-V material on low resistivity/high dielectric substrates

Effect Of AlN Spacer In The Layer Structure On High Rf Performance GaN-Based HEMTs On Low Resistivity Silicon At K-Band Application

AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown on Si substrate are emerging as an attractive devices for many RF applications. This is due to lower circuits realization cost and multifunction chips integration. In this study we investigate the effect of AlN spacer between AlGaN and GaN of a sub-micron gate (0.3 μm) AlGaN/GaN and AlGaN/AlN/GaN HEMTs on a Low Resistivity LR Si substrates on RF performance. We have observed an enhancement in RF performance fT and fMAX in the HEMT with of AlN spacer; (fT) was increased from 47 GHz to 55 GHz and (fMAX) was increased from 79 GHz to 121 GHz. This enhancement in performance is mainly due to the increase in the mobility in the channel and confinement of the carriers reducing Cgs, and delay τ under the gate. We believe this is the first RF study of this type as previous studies were based on the effects of the DC characteristic of the devices [1]

Modeling and simulation of ultrahigh sensitive AlGaN/AlN/GaN HEMT based hydrogen gas detector with low detection limit

Presented through this work is a steady state analytical model of the GaN HEMT based gas detector. GaN with high chemical and thermal stability provides promises for detectors in hazardous environments. However, HEMT sensor resolution must be improved to develop high precision gas sensors for automotive and space applications. The proposed model aids in systematical study of the sensor performance and prediction of sensitivities. The linear relation of threshold voltage shift at thermal equilibrium is used in predicting the sensor response. Numerical model for the reaction rates and the electrical dipole at the adsorption sites at the surface and metal/semiconductor interface have been developed and the sensor performance is analyzed for various gas concentrations. The validation of the model has been achieved through surface and interfacial charge adsorption-based gate electrode work function, Schottky barrier, 2DEG and threshold voltage deduction using MATLAB and SILVACO ATLAS TCAD. Further the applicability of gd (channel conductance) as gas sensing metric is also presented. With high ID and gd percentile sensitivities of 118.5% and 92 % for 10 ppm hydrogen concentration. The sensor shows capability for detection in sub-ppm levels by exhibiting a response of 0.043% for 0.01ppm (10 ppb) hydrogen concentration. The detection limit of the sensor (1% sensitivity) presented here is 169 ppb and the device current increases by 34.2 μA for 1ppb hydrogen concentration

Novel Shielded Coplanar Waveguides on GaN-on-Low Resistivity Si Substrates for MMIC Applications

Shielded-Elevated Coplanar Waveguides (SE-CPWs) with low loss have been successfully developed for the first time for RF GaN on low-resistivity silicon (LR-Si) substrates (σ < 40 Ω.cm). Transmission losses (S 21 ) of less than 0.4 dB/mm at X-band and better than 2 dB/mm at K-band with less than 20 dB return loss were exhibited by the developed SE-CPW, making them comparable in performance to those on traditional (semi-insulating) SI substrates. The developed waveguides use air-bridge technology to suspend CPW tracks above the HEMT GaN layer on LR-Si, directly above an additional thin layer of SiN and shielded ground planes. EM simulation was used to adjust structure parameters for performance optimization. In this work, we eliminated RF energy coupled into the substrate, paving the way for a cost-effective and higher integration GaN MMICs on LR-Si.This work was supported in part by the EPSRC III-V national center pump-priming scheme

GaN-HEMT on Si as a robust visible-blind UV detector with high responsivity

This work presents performance evaluation of GaN High Electron Mobility Transistor (HEMT) based ultraviolet (UV) detector on Si substrate. In addition to the fabrication and characterization, a systematic study is presented here using simulations extensively to investigate the UV detection mechanism. Output current has been chosen as the sensing metric, the fabricated device exhibits a high UV responsivity of 1.62 x 107 A/W at 2.5 x 10-10 W, VGS=0.5 V. Simulations have been done using optical modules available in Silvaco ATLAS TCAD to analyze the energy band bending, Two-Dimensional Electron Gas (2DEG), channel potentials and electric fields in the device. This model can aid in systematic study of HEMT based detectors in terms of dimensional and epi layer design optimizations for sensitivity enhancements. The UV response of the device is found to decrease as the wavelength approaches the visible light wavelength. This makes the photodetectors blind to visible light ensuring selective detection of UV wavelengths. It has been observed that as the area for UV absorption is increased by increasing the W/L ratio, the increases. For a W/L ratio of 100, the detector exhibits a responsivity of 1.86 x 107 A/W