Yi-Wei Lian

AlGaN/GaN Schottky Barrier Diodes on Silicon Substrates With Selective Si Diffusion for Low Onset Voltage and High Reverse Blocking

In this letter, a selective Si diffusion approach is proposed to improve both the forward and reverse characteristics of AlGaN/GaN Schottky barrier diodes on Si substrates. The Si diffusion layer forms a dual Schottky barrier anode structure, which results in a low Schottky barrier portion to reduce the onset voltage VON from 1.3 to 1.0 V (23%). In the same process step, the selectively diffused Si is adopted in the cathode to reduce the ohmic contact resistance RC and improve the breakdown voltage VBK. A low RC of 0.21 Ω·mm and enhanced VBK up to 20% (from 1250 to 1500 V) are demonstrated, which can be attributed to the alleviated electric-field peaks around the alloy spikes beneath the ohmic contact.

2.07-kV AlGaN/GaN Schottky Barrier Diodes on Silicon With High Baliga’s Figure-of-Merit

In this letter, we demonstrate high-performance AlGaN/GaN Schottky barrier diodes (SBDs) on Si substrate with a recessed-anode structure for reduced turn-on voltage VON. The impact of the surface roughness after the recessed-anode formation on device characteristics is investigated. An improved surface condition can reduce the leakage current and enhance the breakdown voltage simultaneously. A low turn-on voltage of only 0.73 V can be obtained with a 50-nm recess depth. In addition, the different lengths of Schottky extension acting like a field plate are investigated. A high reverse breakdown voltage of 2070 V and a low specific ON-resistance of 3.8 mΩ · cm2 yield an excellent Baligas figure of merit of 1127 MW/cm2, which can be attributed to the low surface roughness of only 0.6 nm and also a proper Schottky extension of 2 μm to alleviate the peak electric field intensity in the SBDs.

AlGaN/GaN HEMTs on Silicon With Hybrid Schottky–Ohmic Drain for High Breakdown Voltage and Low Leakage Current

In this letter, a hybrid Schottky-ohmic drain structure is proposed for AlGaN/GaN high-electron-mobility transistors on a Si substrate. Without additional photomasks and extra process steps, the hybrid drain design forms a Γ-shaped electrode to smooth the electric field distribution at the drain side, which improves the breakdown voltage and lowers the leakage current. In addition, the hybrid drain provides an auxiliary current path and decreases the on-resistance, in contrast to the devices with a pure Schottky drain. Compared with the conventional ohmic drain devices, the breakdown voltage could be improved up to 64.9%, and the leakage current is suppressed by one order of magnitude without degradation of the specific on-resistance.

101-GHz InAlN/GaN HEMTs on Silicon With High Johnson’s Figure-of-Merit

In this brief, the InAlN/GaN high-electron mobility transistors (HEMTs) on silicon substrate with high Johnsons figure-of-merit (J-FOM) are presented. A trilayer photoresist of polymethylmethacrylate (PMMA)/copolymer/PMMA associated with a T-shaped gate is used to reduce the parasitic resistance while maintaining high current gain cutoff frequency. The small dc-to-RF transconductance dispersion of only 1.1% suggests a good quality SiNx passivation layer, and the fMAX of 101 GHz and fT of 60 GHz can be simultaneously obtained with a 0.11-μm foot length and 1.5-μm source-drain distance. In addition, the three-terminal OFF-state breakdown measurements reveal a source-drain breakdown voltage (BVDS) of 21 V (VDG = 31 V). The results lead to a high J-FOM of 1.3 THz · V, which has not been reported for the InAlN/GaN HEMTs on silicon substrate.

Contact engineering of GaN-on-silicon power devices for breakdown voltage enhancement

Two contact engineering approaches are proposed and investigated to enhance the breakdown voltage VBK in GaN-on-Si power devices, including the hybrid Schottky–ohmic-drain structure for AlGaN/GaN HEMTs and the selective Si-diffusion structure for AlGaN/GaN SBDs. With the hybrid Schottky–ohmic drain, the devices showed a zero onset voltage and reduced off-state leakage current by one order of magnitude, compared with that of the traditional ohmic-drain devices. The breakdown voltage was also enhanced with comparable on-resistance. For the SBDs with the selective silicon-diffusion layer underneath ohmic metal at the cathode, a low-contact resistance Rc of 0.2 Ω mm and a smooth ohmic metal morphology were obtained. The SBDs with the additional silicon diffusion layer showed enhanced VBK, compared with that of the conventional SBDs. The results demonstrate that the proposed contact engineering approaches are useful for the breakdown voltage enhancement of GaN-on-Si power devices.

Drain E-Field Manipulation in AlGaN/GaN HEMTs by Schottky Extension Technology

The proposed hybrid Schottky-ohmic drain structure is analyzed in detail for AlGaN/GaN power high-electron mobility transistors on the Si substrate. Without any additional photomasks and process steps, the hybrid drain design can alter the electric field distribution to improve the breakdown voltage VBK. In addition, it provides an additional current path to achieve zero onset voltage and reduce the ON-resistance. It was found that the Schottky extension Lext is critical to VBK, RON, and also the current collapse phenomena of the transistors. The extended Schottky electrodes for optimized transistor characteristics are investigated, and the physics behind are discussed. With an Lext ~ 2-3 μm, VBK can be improved up to 60% with an RON degradation below 3%.

GaN-on-silicon devices and technologies for RF and microwave applications

This paper presents our recent studies on GaN-on-Si devices and technologies for RF and microwave applications. The considerations of layer structure and fabrication technology are reviewed including two different GaN-based heterostructures and optimization of ohmic and Schottky contacts. Also, the proposed novel approaches for achieving high speed GaN-on-Si HEMTs are addressed such as the hybrid-drain structure and silicon substrate removal technology. Finally, the small-signal equivalent circuit model is employed to analyze the parasitic effects of silicon substrate for the device at high frequencies.

AlGaN/GaN HEMTs on Silicon With Hybrid Schottky–Ohmic Drain for RF Applications

In this paper, the AlGaN/GaN high electron mobility transistors on a low resistivity Si substrate with the hybrid drain structure for RF applications are analyzed in detail, based on measurements, TCAD simulation, model extraction, and delay time calculation of the transistors. Owing to the E-field redistribution of the Schottky extension, both the leakage current and the breakdown voltage can be improved. Also, the enhanced RF performance can be attributed to the reduced transit time and increased transconductance, resulting from the increased electron velocity and reduced drain depletion width. With a 3-μm extension length and a 0.2-μm gate length, fT and fMAX of transistors can be improved from 32.7 to 49.9 GHz (52.6%) and from 35.8 to 49.2 GHz (37.4%), respectively, with ON-OFF ratio enhancement by four orders of magnitude. The breakdown voltage was improved from 21 to 38 V (80.9%).