Jon Zhang

Progress in Silicon Carbide Power Devices

SiC materials and device technology has entered a new era with the commercialization and acceptance of 600 V/10 A and 1200 V/10 A Schottky Barrier Diodes (SBDs) in the marketplace. These diodes are finding applications in the Power Factor Correction (PFC) stage of Switch Mode Power Supplies (SMPS). SiC power MOSFETs with ratings of 800-1200 V up to 10 A will soon be commercially available. The next step is to integrate the SiC MOSFET and Schottky diodes in a power module for PFC and motor control applications. For high temperature applications, greater than 200°C, a bipolar switch such as a SiC BJT offers superior performance over the MOSFETs. The lack of gate oxide in the BJT offers better reliability at such extreme temperatures, in addition to the lowest combined switching and conduction losses.

SiC power devices for Smart Grid systems

A Smart Grid with distributed generation is critical for reducing greenhouse gas emissions. However, current power converters and circuit breakers built with silicon switches are very bulky and inefficient, making their use difficult in practical Smart Grid systems. The development of high voltage power devices based on SiC will be a critical development in building a Smart Grid with distributed and fluctuating sources of power generation. In this paper, the physics and technology of high voltage (> 10kV) 4H-SiC power devices, namely MOSFETs, IGBTs, and GTOs, are discussed. A detailed review of the current status and trends in these devices is given with respect to materials growth, device design, and the potential future ranges for use.

15 kV, Large Area (1 cm2), 4H-SiC p-Type Gate Turn-Off Thyristors

In this paper, we report our recently developed 1 cm2, 15 kV SiC p-GTO with an extremely low differential on-resistance (RON,diff) of 4.08 mΩ•cm2 at a high injection-current density (JAK) of 600 ~ 710 A/cm2. The 15 kV SiC p-GTO was built on a 120 μm, 2×1014/cm3 doped p-type SiC drift layer with a device active area of 0.521 cm2. Forward conduction of the 15 kV SiC p-GTO was characterized at 20°C and 200°C. Over this temperature range, the RON,diff at JAK of 600 ~ 710 A/cm2 decreased from 4.08 mΩ•cm2 at 20°C to 3.45 mΩ•cm2 at JAK of 600 ~ 680 A/cm2 at 200°C. The gate to cathode blocking voltage (VGK) was measured using a customized high-voltage test set-up. The leakage current at a VGK of 15 kV were measured 0.25 µA and 0.41 µA at 20°C and 200°C respectively.

High Speed Switching Devices in 4H-SiC - Performance and Reliability

Silicon carbide (SiC) is a very attractive material for high temperature semiconductor devices because of its very wide bandgap (3.26 eV). Due to the wide bandgap, thermal carrier generation is very low in SiC, resulting in negligible junction leakage currents for temperatures up to 500 degC. Other advantage of SiC is high breakdown strength (10times that of silicon) and high thermal conductivity. This allows the drift layers in SiC power devices to be 10times thinner with 100times the doping concentration, compared to a silicon device for a given blocking voltage. Thus, high voltage majority carrier power devices with reasonably low on-resistances are possible in SiC (Ryu et al., 2004). Due to lack of excess minority carriers, these devices can operate at much higher switching frequencies with acceptable switching losses. The ability to operate at higher frequencies reduces the passive components in a power system. In addition, higher temperature capability of SiC devices can translate into more relaxed heat sinking requirements. This means that smaller heat sinks and/or passive cooling can be used for SiC power devices. It is expected that the size and weight of power electronics utilizing SiC switching devices and diodes will be significantly reduced by means of passive cooling, and smaller and lighter passive components

Development of Low RON,Diff, 12 kV, 4H-SiC GTOs For High-Power and High-Temperature Applications

In this paper, we report our recently developed 1 × 1 cm2, 12 kV SiC GTOs with a very low differential on-resistance (RON,Diff) of 4 mΩ·cm2 with respect to the device active area at high injection level current of 100 A/cm2 or higher, which is more than a 40% reduction from our previously reported work. This significant reduction in the on-resistance was attributed to an improvement of carrier lifetime in the SiC bulk region. The SiC GTO was wire-bonded and attached to a high-voltage package before the high-temperature measurement. Forward characteristics of the device were then measured using a Tektronics 371 curve tracer from room temperature up to 400°C. Over the temperature range, the RON,Diff of the 4H-SiC GTO increased modestly from 4 mΩ·cm2 at 20°C to 4.7 mΩ·cm2 at 400°C, while the forward voltage drop at 100 A decreased slightly from 3.97 V at 20°C to 3.6 V at 400°C. The gate to cathode blocking voltage (VGK) was measured using a customized high-voltage test set-up. The leakage current was measure...

Impact of carrier lifetime enhancement using high temperature oxidation on 15 kV 4H-SiC P-GTO thyristor

The impact of the lifetime enhancement process using high temperature thermal oxidation method on 4H-SiC P-GTOs was investigated. 15 kV 4H-SiC P-GTOs with 140 μm thick drift layers, with and without 1450°C lifetime enhancement oxidation (LEO) process, were compared. The LEO process increased the average carrier lifetime in p-type epi layer from 0.9 μs to 6.25 μs, and it was observed that the effectiveness of the lifetime enhancement process was very sensitive to the doping concentration. The device with the LEO process showed a significant reduction in forward voltage drop and a substantially lower holding current, as expected from the carrier lifetime measurements. However, a slight reduction in blocking capability was also observed from the devices treated with LEO process. The common emitter current gain (β) of the wide base test NPN BJT was approximately 10X higher for the wafer with LEO process.

4H-SiC DMOSFETs for power conversion applications successes and ongoing challenges

Power devices fabricated in 4H-SiC are poised to significantly impact the field of power electronics. There has been great interest in SiC as a material in which to fabricate power electronic devices for quite some time based on its very promising fundamental materials properties. However, it has been far more recently that the potential of SiC is being appreciated as a result of the recent advances in material quality, fabrication processes and device design. Based on the high critical breakdown electric field, high bandgap and high thermal conductivity of SiC, systems that are specifically designed to take advantage of these characteristics offer superior power density, lower cooling requirements, and prolonged survivability in adverse conditions when compared to systems fabricated with Si power devices.

Fast turn-off of high voltage 4H-SiC npn BJTs from the saturation on-state regime

Fast turn-off of high-voltage (breakdown voltage 3 kV) 4H‐SiC npn bipolar junction transistors driven in a deeply saturated regime has been reported. In the conventional turn-off mode (base current break), the turn-off delay and current fall times are 80 ns and 100 ns, respectively. It is shown that these times can be made as short as 20 and 4 ns, respectively, if a reverse base current pulse of appropriate amplitude is applied to sweep out minority carriers from the base. The experimental values of delay and turn-off times well coincide with those calculated in terms of the charge control model.

Transient switch-off of a 4H-SiC bipolar transistor from the deep-saturation mode

The transient switch-off of a bipolar 4H-SiC transistor from the deep-saturation mode is studied by performing 1D numerical simulation. Switch-off in the zero base current mode and in the mode of switching-off with a negative base current is examined. It is shown that at quite real values of the switching-off base current, the switch-off time can be made ~40 times shorter than the switch-off time at zero base current. The delay time can also be made substantially (several times) shorter. It is noted that, in the deep saturation mode in which the conductivity of the collector layer is highly modulated by minority carriers, the bipolar transistor can operate in the continuous mode at a rather high current density.

The benefits and current progress of SiC SGTOs for pulse power applications

Pulse power systems require power switches to operate at higher speeds and higher temperatures to meet the demand for smaller and higher power density systems. 4H-Silicon Carbide (4H-SiC) offers several advantages that can make these goals feasible. Benefits of 4H-SiC compared to Si include wider band gap (3.2 eV), higher critical field (2.2×106 V/cm), and higher thermal conductivity (3.0–3.8 W/cm∗K) [1]. Compared to other devices made on SiC such as BJTs, JFETs, MOSFETs, and IGBTs, SiC GTOs are the favorable devices for pulse power applications due to its ability to operate at high current and high voltage levels, which is attributed to conductivity modulation in the drift layer of the device. Furthermore, SiC SGTOs offers several advantages over Si thyristors and Si GTOs such as compactness, higher current density, faster switching, and higher temperature operation. These devices have demonstrated excellent current sharing capability under pulsed conditions. Figure 1 and figure 2 displays the current sharing of two 0.5 cm2 SiC SGTOs with a pulse width of 1 ms [2–3].