Yuhao Zhang

High-Performance WSe2 Complementary Metal Oxide Semiconductor Technology and Integrated Circuits

Because of their extraordinary structural and electrical properties, two-dimensional materials are currently being pursued for applications such as thin-film transistors and integrated circuit. One of the main challenges that still needs to be overcome for these applications is the fabrication of air-stable transistors with industry-compatible complementary metal oxide semiconductor (CMOS) technology. In this work, we experimentally demonstrate a novel high performance air-stable WSe2 CMOS technology with almost ideal voltage transfer characteristic, full logic swing and high noise margin with different supply voltages. More importantly, the inverter shows large voltage gain (∼38) and small static power (picowatts), paving the way for low power electronic system in 2D materials.

GaN-on-Si Vertical Schottky and p-n Diodes

This letter demonstrates GaN vertical Schottky and p-n diodes on Si substrates for the first time. With a total GaN drift layer of only 1.5-μm thick, a breakdown voltage (BV) of 205 V was achieved for GaN-on-Si Schottky diodes, and a soft BV higher than 300 V was achieved for GaN-on-Si p-n diodes with a peak electric field of 2.9 MV/cm in GaN. A trap-assisted space-charge-limited conduction mechanism determined the reverse leakage and breakdown mechanism for GaN-on-Si vertical p-n diodes. The ON-resistance was 6 and 10 mQ · cm2 for the vertical Schottky and p-n diode, respectively. These results show the promising performance of GaN-on-Si vertical devices for future power applications.

Origin and Control of OFF-State Leakage Current in GaN-on-Si Vertical Diodes

Conventional GaN vertical devices, though promising for high-power applications, need expensive GaN substrates. Recently, low-cost GaN-on-Si vertical diodes have been demonstrated for the first time. This paper presents a systematic study to understand and control the OFF-state leakage current in the GaN-on-Si vertical diodes. Various leakage sources were investigated and separated, including leakage through the bulk drift region, passivation layer, etch sidewall, and transition layers. To suppress the leakage along the etch sidewall, an advanced edge termination technology has been developed by combining plasma treatment, tetramethylammonium hydroxide wet etching, and ion implantation. With this advanced edge termination technology, an OFF-state leakage current similar to Si, SiC, and GaN lateral devices has been achieved in the GaN-on-Si vertical diodes with over 300 V breakdown voltage and 2.9-MV/cm peak electric field. The origin of the remaining OFF-state leakage current can be explained by a combination of electron tunneling at the p-GaN/drift-layer interface and carrier hopping between dislocation traps. The low leakage current achieved in these devices demonstrates the great potential of the GaN-on-Si vertical device as a new low-cost candidate for high-performance power electronics.

Electrothermal Simulation and Thermal Performance Study of GaN Vertical and Lateral Power Transistors

In this paper, we present self-consistent electrothermal simulations of single-finger and multifinger GaN vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and lateral AlGaN/GaN high-electron-mobility transistors (HEMTs) and compare their thermal performance. The models are first validated by comparison with experimental dc characteristics, and then used to study the maximum achievable power density of the device without the peak temperature exceeding a safe operation limit of 150°C (P150°C). It is found that the vertical MOSFETs have the potential to achieve a higher P150°C than the lateral HEMTs, especially for higher breakdown voltages and higher scaling level designs.

Formation of low resistance ohmic contacts in GaN-based high electron mobility transistors with BCl3 surface plasma treatment

A BCl3 surface plasma treatment technique to reduce the resistance and to increase the uniformity of ohmic contacts in AlGaN/GaN high electron mobility transistors with a GaN cap layer has been established. This BCl3 plasma treatment was performed by an inductively coupled plasma reactive ion etching system under conditions that prevented any recess etching. The average contact resistances without plasma treatment, with SiCl4, and with BCl3 plasma treatment were 0.34, 0.41, and 0.17 Ω mm, respectively. Also, the standard deviation of the ohmic contact resistance with BCl3 plasma treatment was decreased. This decrease in the standard deviation of contact resistance can be explained by analyzing the surface condition of GaN with x-ray photoelectron spectroscopy and positron annihilation spectroscopy. We found that the proposed BCl3 plasma treatment technique can not only remove surface oxide but also introduce surface donor states that contribute to lower the ohmic contact resistance.

Design space and origin of off-state leakage in GaN vertical power diodes

Variable-range-hopping through dislocations was identified as the main off-state leakage mechanism for GaN vertical diodes on different substrates. The behavior of leakage current for vertical devices as a function of dislocation density and electric field was derived by TCAD simulations, after careful calibration with experiments and literature data. Designed GaN vertical diodes demonstrate 2-4 orders of magnitude lower leakage current while supporting 3-5 times higher electric field, compared to GaN lateral, Si and SiC devices.

Enhancement-mode single-layer CVD MoS2 FET technology for digital electronics

2D nanoelectronics based on single-layer (SL) MoS2 offers great advantages for ubiquitous electronics. With new device technology, highly uniform E-mode FETs using SL CVD MoS2 with positive VT, large mobility, excellent subthreshold swing are achieved. The integrated inverter shows excellent voltage transfer characteristic, close to rail-to-rail operation, high noise margin, large voltage gain (~45) and small static power. The combinational and sequential digital circuits shown here serve as a toolbox of building blocks for realizing wide range of digital circuitry.

Novel GaN trench MIS barrier Schottky rectifiers with implanted field rings

We demonstrate a novel GaN vertical Schottky rectifier with trench MIS structures and trench field rings. The new structure greatly enhanced the reverse blocking characteristics while maintaining a Schottky-like good forward conduction. The reverse leakage current improved beyond 104-fold and the breakdown voltage increased from 400 V to 700 V, while the low turn-on voltage (0.8 V) and on-resistance (2 mΩ·cm2) were retained. High-temperature operation up to 250 oC and fast switching performance were also demonstrated. This new device shows great potential for high-power and high-frequency applications.

Beyond Thermal Management: Incorporating p-Diamond Back-Barriers and Cap Layers Into AlGaN/GaN HEMTs

This work explores the use of p-diamond back-barriers (BBs) and cap layers to enhance the performance of GaN-based high electron mobility transistors (HEMTs). Diamond can offer a heavily doped p-type layer, which is complementary to GaN electronics. Self-consistent electrothermal simulations reveal that the use of p-diamond BBs and cap layers can increase the breakdown voltage of GaN-based HEMTs by fourfold, at the same time that they enhance the 2-D-electron-gas confinement and reduce short channel effects. These results highlight that p-diamond layers can improve the performance of GaN HEMTs for high-power and high-frequency applications beyond the thermal improvements pursued until now.

Trench formation and corner rounding in vertical GaN power devices

Trench formation and corner rounding are the key processes to demonstrate high-voltage trench-based vertical GaN devices. In this work, we developed a damage-free corner rounding technology combining Tetramethylammonium hydroxide wet etching and piranha clean. By optimizing the inductively coupled plasma dry etching conditions and applying the rounding technology, two main trench shapes were demonstrated: flat-bottom rounded trench and tapered-bottom rounded trench. TCAD simulations were then performed to investigate the impact of trench shapes and round corners on device blocking capability. GaN trench metal-insulator-semiconductor barrier Schottky rectifiers with different trench shapes were fabricated and characterized. A breakdown voltage over 500 V was obtained in the device with flat-bottom rounded trenches, compared to 350 V in the device with tapered-bottom rounded trenches and 150 V in the device with non-rounded trenches. Both experimental and simulation results support the use of rounded flat-b...