Browsing by Author "Ma, Yunwei"

Now showing 1 - 7 of 7
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    1 kV GaN-on-Si Quasi-Vertical Schottky Rectifier
    Qin, Yuan; Xiao, Ming; Zhang, Ruizhe; Xie, Qingyun; Palacios, Tomás; Wang, Boyan; Ma, Yunwei; Kravchenko, Ivan; Briggs, Dayrl P.; Hensley, Dale K.; Srijanto, Bernadeta R.; Zhang, Yuhao (IEEE, 2023-07)
    This work demonstrates quasi-vertical GaN Schottky barrier diodes (SBDs) on 6-inch Si substrate with a breakdown voltage (BV) over 1 kV, the highest BV reported in vertical GaN-on-Si SBDs to date. The deep mesa inherently in quasi-vertical devices is leveraged to form a self-aligned edge termination, and the mesa sidewall is covered by the p-type nickel oxide (NiO) as a reduced surface field (RESURF) structure. This novel termination enables a parallel-plane junction electric field of 2.8 MV/cm. The device also shows low turn-on voltage of 0.5 V, and low specific on-resistance of 1.1 m ·cm2. Moreover, the device exhibits excellent overvoltage robustness under the continuous 800 V stress in the unclamped inductive switching test. These results show the good promise of the low-cost vertical GaN-on-Si power diodes.
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    1 kV Self-Aligned Vertical GaN Superjunction Diode
    Ma, Yunwei; Porter, Matthew; Qin, Yuan; Spencer, Joseph; Du, Zhonghao; Xiao, Ming; Wang, Yifan; Kravchenko, Ivan; Briggs, Dayrl P.; Hensley, Dale K.; Udrea, Florin; Tadjer, Marko; Wang, Han; Zhang, Yuhao (IEEE, 2024-01)
    This work demonstrates vertical GaN superjunction (SJ) diodes fabricated via a novel self-aligned process. The SJ comprises n-GaN pillars wrapped by the charge-balanced p-type nickel oxide (NiO). After the NiO sputtering around GaN pillars, the self-aligned process exposes the top pillar surfaces without the need for additional lithography or a patterned NiO etching which is usually difficult. The GaN SJ diode shows a breakdown voltage (B V) of 1100 V, a specific on-resistance ( RON) of 0.4 mΩ⋅ cm2, and a SJ drift-region resistance ( Rdr) of 0.13 mΩ⋅ cm2. The device also exhibits good thermal stability with B V retained over 1 kV and RON dropped to 0.3 mΩ⋅ cm2 at 125oC . The trade-off between B V and Rdr is superior to the 1D GaN limit. These results show the promise of vertical GaN SJ power devices. The self-aligned process is applicable for fabricating the heterogeneous SJ based on various wide- and ultra-wide bandgap semiconductors.
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    10-kV Ga2O3 Charge-Balance Schottky Rectifier Operational at 200 ◦C
    Qin, Yuan; Xiao, Ming; Porter, Matthew; Ma, Yunwei; Spencer, Joseph; Du, Zhonghao; Jacobs, Alan G.; Sasaki, Kohei; Wang, Han; Tadjer, Marko; Zhang, Yuhao (IEEE, 2023-08)
    This work demonstrates a lateral Ga2O3 Schottky barrier diode (SBD) with a breakdown voltage (BV) over 10 kV, the highest BV reported in Ga2O3 devices to date. The 10 kV SBD shows good thermal stability up to 200◦C, which is among the highest operational temperatures reported in multi-kilovolt Ga2O3 devices. The key device design for achieving such high BV is a reduced surface field (RESURF) structure based on the p-type nickel oxide (NiO), which balances the depletion charges in the n-Ga2O3 channel at high voltage. At BV, the chargebalanced Ga2O3 SBD shows an average lateral electric field (E-field) over 4.7 MV/cm at 25 ◦C and over 3.5 MV/cm at 200◦C, both of which exceed the critical E-field of GaN and SiC. The 10 kV SBD shows a specific on-resistance of 0.27 ·cm2 and a turn-on voltage of 1 V; at 200◦C, the former doubles and the latter reduces to 0.7 V. These results suggest the good potential of Ga2O3 devices for mediumand high-voltage, high-temperature power applications.
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    2 kV, 0.7 mΩ·cm2 Vertical Ga2O3 Superjunction Schottky Rectifier with Dynamic Robustness
    Qin, Yuan; Porter, Matthew; Xiao, Ming; Du, Zhonghao; Zhang, Hongming; Ma, Yunwei; Spencer, Joseph; Wang, Boyan; Song, Qihao; Sasaki, Kohei; Lin, Chia-Hung; Kravchenko, Ivan; Briggs, Dayrl P.; Hensley, Dale K.; Tadjer, Marko; Wang, Han; Zhang, Yuhao (IEEE, 2023)
    We report the first experimental demonstration of a vertical superjunction device in ultra-wide bandgap (UWBG) Ga2O3. The device features 1.8 μm wide, 2×1017 cm-3 doped n-Ga2O3 pillars wrapped by the charge-balanced p-type nickel oxide (NiO). The sidewall NiO is sputtered through a novel self-align process. Benefitted from the high doping in Ga2O3, the superjunction Schottky barrier diode (SJ-SBD) achieves a ultra-low specific on-resistance (RON,SP) of 0.7 mΩ·cm2 with a low turn-on voltage of 1 V and high breakdown voltage (BV) of 2000 V. The RON,SP~BV trade-off is among the best in all WBG and UWBG power SBDs. The device also shows good thermal stability with BV > 1.8 kV at 175 oC. In the unclamped inductive switching tests, the device shows a dynamic BV of 2.2 kV and no degradation under 1.7 kV repetitive switching, verifying the fast acceptor depletion in NiO under dynamic switching. Such high-temperature and switching robustness are reported for the first time in a heterogeneous superjunction. These results show the great potential of UWBG superjunction power devices.
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    Junction Based Gallium Nitride Power Devices
    Ma, Yunwei (Virginia Tech, 2023-09-05)
    Power electronics plays an important role in many energy conversion applications in modern society including consumer electronics, data centers, electric vehicles, and power grids, etc. The key components of power electronic circuits are power semiconductor devices including diodes and transistors, which determine the performance of power electronics circuits. Traditional power devices are based on the semiconductor silicon (Si), which have already reached the silicon's material limit. Gallium nitride (GaN) is a wide bandgap semiconductor with high electron mobility and high critical electric field. GaN-based power devices promise superior device performance over the Si-based counterpart. The primary design target of a unipolar power device is to achieve low on-resistance and high breakdown voltage. Although GaN high electron mobility transistor (HEMT) is commercially available in a voltage class from 15 V to 900 V, the performance of GaN devices is still far below the GaN material limit, due to several reasons: 1) To achieve the normally-off operation in a GaN HEMT, the density of two-dimensional electron gas (2DEG) channel cannot be too high; this limits the on-resistance reduction in the access region. 2) The gate capacitance of GaN HEMT is usually low so that the carrier concentration in the channel underneath the gate is relatively low, limiting the on-resistance reduction in the gated channel region. 3) The electric-field distribution in the drift region is not uniform, resulting in a limited breakdown voltage. We proposed to use the junction-based structure in GaN power devices to address the above problems and fully exploit GaN's material properties. The first part of this dissertation characterizes nickel oxide (NiO) as a p-type material to construct the junction-based GaN power devices. Although the homogenous p-GaN/n-GaN junction is preferred in many devices, the selective-area, p-GaN regrowth can lead to excessive leakage current; in comparison, the p-NiO/n-GaN junction is stable without leakage. This section describes the optimization of NiO deposition as well as the NiO characterization. Although acceptor in NiO is not generated by impurity doping, the acceptor concentration modulation is realized by tuning the O2 partial pressure during the sputtering process. Practical breakdown electric field is also characterized and confirmed to be higher than GaN. These results provide the design guidelines for NiO-GaN junction-based power devices. The second part of this dissertation demonstrates the 3D NiO-GaN junction gate to improve the GaN HEMT's on-resistance. The 3D junction gate structure enables a high carrier concentration under the gate region in the device on-state. Meanwhile, the strong depletion effect of the junction-based gate allows for a robust normally-off operation; as a result, the GaN wafer with a higher 2DEG concentration can be used to achieve both normally-off and low on-state resistance in HEMT devices. Simulation is also performed to project the performance space of trigate GaN junction HEMTs using the p-GaN instead of NiO. The third part of this dissertation presents the application of the p-GaN/n-GaN junction in the drift region of the multi-channel lateral devices to achieve the high breakdown voltage. Here p-GaN is grown in-situ with the multi-channel AlGaN/GaN structure, and there is no leakage problem. The structure is designed to achieve charge balance between the acceptor in p-GaN and the net donor in the multichannel AlGaN/GaN. This design enables a uniform electric field distribution and breakdown voltage over 10 kV. The fourth part of this dissertation presents the application of the p-NiO/n-GaN junction in vertical superjunction (SJ) devices. We show the design and simulation of this heterojunction structure in a SJ and confirm the uniform electric field and high breakdown voltage under the charge balance. Then the device fabrication is presented in detail, which mainly comprises the deep GaN trench etch, NiO self-aligned lift off, and photoresist trench planarization. The optimized device shows a trade-off between its drift region specific on-resistance versus breakdown that exceeds the 1D GaN's limit. The last part of this dissertation is exploring the design and fabrication of p-GaN/n-GaN based SJ devices. First, the challenges in p-GaN regrowth especially the introduction of interface impurities are discussed, followed by device simulation and modeling to optimize the SJ performance considering these interface impurities. The activation of regrown p-GaN in deep trenches is more difficult than planar p-GaN, and we present the characterization and physical model for the activation of the deep buried p-GaN. Last, the results of p-GaN filling regrowth and the acceptor concentration calibration in the lightly doped p-GaN are presented and discussed. In summary, our work combines experimental device fabrication and characterization, TCAD simulation, and device modeling to demonstrate the benefit of multi-dimensional, junction-based GaN power devices as compared to the traditional GaN power devices. The junction-based structure at gate region can provides stable normally-off operation and low on-resistance. When being applied to the drift region, the multidimensional junction structure can push the device specific on-resistance versus breakdown voltage trade-off near or even exceeding the material limit. These results will advance the performance and application spaces of GaN power devices.
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    Superjunction Power Transistors With Interface Charges: A Case Study for GaN
    Ma, Yunwei; Xiao, Ming; Zhang, Ruizhe; Wang, Han; Zhang, Yuhao (2019-12-13)
    Recent progress in p-GaN trench-filling epitaxy has shown promise for the demonstration of GaN superjunction (SJ) devices. However, the presence of n-type interface charges at the regrowth interfaces has been widely observed. These interface charges pose great challenges to the design and performance evaluation of SJ devices. This work presents an analytical model for SJ devices with interface charges for the first time. In our model, two approaches are proposed to compensate interface charges, by the modulation of the SJ doping or the SJ geometry. Based on our model, an analytical study is conducted for GaN SJ transistors, revealing the design windows and optimal values of doping concentration and pillar width as a function of interface charge density. Finally, TCAD simulation is performed for vertical GaN SJ transistors, which validated our analytical model. Our results show that, with optimal designs, interface charges would only induce small degradation in the performance of GaN SJ devices. However, with the increased interface charge density, the design windows for pillar width and doping concentration become increasingly narrow and the upper limit in the pillar width window reduces quickly. When the interface charge density exceeds similar to 3X10(12) cm(-2), the design window of pillar width completely falls into the sub-micron range, indicating significant difficulties in fabrication. Vertical GaN SJ transistors with interface charges retain great advantages over conventional GaN power transistors, but have narrower design windows and require different design rules compared to ideal GaN SJ devices.
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    A Thermal Switch from Thermoresponsive Polymer Aqueous Solutions
    Ma, Yunwei (Virginia Tech, 2018-11-29)
    Thermal switch is very important in today’s world and it has varies of applications including heat dissipation and engine efficiency improving. The commercial thermal switch based on mechanical design is very slow and the structure is too complicated to make them smaller. To enable fast thermal switch as well as to make thermal switch more compact, I try to use second-order phase transition material to enable our thermal switch. Noticing the transition properties of thermoresponsive polymer for drug delivery, its potential in thermal switch can be expected. I used Poly(N-isopropylacrylamide) (PNIPAM) as an example to show the abrupt thermal conductivity change of thermoresponsive polymer solutions below and above their phase transition temperature. A novel technique, transition grating method, is used to measure the thermal conductivity. The ratio of thermal switch up to 1.15 in transparent PNIPAM solutions after the transition is observed. This work will demonstrate the new design of using second-order phase transition material to enable fast and efficient thermal switch.