Bradley Heath

A Comparison of High Temperature Performance of SiC DMOSFETs and JFETs

High temperature characteristics of 4H-SiC power JFETs and DMOSFETs are presented in this paper. Both devices are based on pn junctions in 4H-SiC, and are capable of 300oC operation. The 4H-SiC JFET showed very predictable, well understood temperature dependent characteristics, because the current conduction depends on the drift of electrons in the bulk region, which is not restricted by traps in the MOS interface or at the pn junctions. On the other hand, in a 4H-SiC DMOSFET, electrons must flow through the MOS inversion layer with a very high interface state density. At high temperatures, the transconductance of the device improves and threshold voltage shifts negative because less electrons are trapped in the interface states, resulting in a much lower MOS channel resistance. This cancels out the increase in drift layer resistance, and as a result, a temperature insensitive on-resistance can be demonstrated. The performance of the two devices are compared, and a discussion of issues for their high temperature application is presented.

Ultra High Power 10 kV, 50 A SiC PiN Diodes

Ultra high power 10 kV, 50 A SiC PiN diodes have been developed with a low forward voltage drop (VF) of 3.75 V and a fast reverse recovery time of 150 nsec. This is the highest reported power rating for a single SiC chip (dimensions: 8.7 mm times 8.7 mm). Furthermore, all of the historical problems with the SiC PiN diode technology such as ineffective edge termination (VBD increased to > 70% of ideal breakdown voltage), poor ohmic contacts to p-type SiC (rhoc reduced to < 10-4 Omegacm2), and forward voltage drift (DeltaVF reduced to < 0.1 V) have been solved with design, material, and process improvements which have resulted in high overall device yields. A combination of performance, reliability, and yield has the SiC PiN diode technology poised for a revolutionary impact in the world of power semiconductor devices

9 kV 4H-SiC IGBTs with 88 mΩ·cm2 of R diff, on

SiC IGBTs are suitable for high power, high temperature applications. For the first time, the design and fabrication of 9 kV planar p-IGBTs on 4H-SiC are reported in this paper. A differential on-resistance of ~ 88 m(cm2 at a gate bias of –20 V is achieved at 25°C, and decreases to ~24.8 m(cm2 at 200°C. The device exhibits a blocking voltage of 9 kV with a leakage current density of 0.1 mA/cm2. The hole channel mobility is 6.5 cm2/V-s at room temperature with a threshold voltage of –6.5 V resulting in enhanced conduction capability. Inductive switching tests have shown that IGBTs feature fast switching capability at both room and elevated temperatures.

A Study on the Reliability and Stability of High Voltage 4H-SiC MOSFET Devices

Gate oxide reliability measurements of 4H-SiC DMOSFETs were performed using the Time Dependent Dielectric Breakdown (TDDB) technique at 175°C. The oxide lifetime is then plotted as a function of the electric field. The results show the projected oxide lifetime to be > 100 years at an operating field of ~3 MV/cm. Device reliability of 2.0 kV DMOSFETs was studied by stressing the gate with a constant gate voltage of +15 V at a temperature of 175°C, and monitoring the forward I-V characteristics and threshold voltage for device stability. Our very first measurements show very little variation between the pre-stress and post-stress conditions up to 1000 hrs of operation at 175°C. In addition, forward on-current stressing of the MOSFETs show the devices to be stable up to 1000 hrs of operation.

Development of 8 mΩ-cm2, 1.8 kV 4H-SiC DMOSFETs

8 mΩ-cm2, 1.8 kV power DMOSFETs in 4H-SiC are presented in this paper. A 0.5 μm long MOS gate length was used to minimize the MOS channel resistance. The DMOSFETs were able to block 1.8 kV with the gate shorted to the source. At room temperature, a specific onresistance of 8 mΩ-cm2 was measured with a gate bias of 15 V. At 150 oC, the specific onresistance increased to 9.6 mΩ-cm2. The increase in drift layer resistance due to a decrease in bulk electron mobility was partly cancelled out by the negative shift in MOS threshold voltage at elevated temperatures. The device demonstrated extremely fast, low loss switching characteristics. A significant improvement in converter efficiency was observed when the 4H-SiC DMOSFET was used instead of an 800 V silicon superjunction MOSFET in a simple boost converter configuration.

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

High Current 6 kV 4H-SiC PiN Diodes for Power Module Switching Applications

Forward voltage (VF) drift, in which a 4H-SiC PiN diode suffers from an irreversible increase in VF under forward current flow, continues to inhibit commercialization of 4H-SiC PiN diodes. We present our latest efforts at fabricating high blocking voltage (6 kV), high current (up to 50 A) 4H-SiC PiN diodes with the best combination of reverse leakage current (IR), forward voltage at rated current (VF), and VF drift yields. We have achieved greater than 60% total die yield onwafer for 50 A diodes with a chip size greater than 0.7 cm2. A comparison of the temperature dependent conduction and switching characteristics between a 50 A/6 kV 4H-SiC PiN diode and a commercially available 60 A/4.5 kV Si PiN diode is also presented.

10.3 m/spl Omega/-cm/sup 2/, 2 kV Power DMOSFETs in 4H-SiC

High voltage power DMOSFETs in 4H-SiC are presented in this paper. A 0.5 μm long MOS gate length was used to minimize the MOS channel resistance. The DMOSFETs were able to block 2 kV with gate shorted to the source. At room temperature, a specific on-resistance of 10.3 mΩ-cm was measured with a gate bias of 12 V. The specific on-resistance was reduced to 8 mΩ-cm with 17 V on the gate. At 150 C, the specific on-resistance increased to 14 mΩ-cm with a VGS of 12 V. The increase in drift layer resistance due to a decrease in bulk electron mobility was partly cancelled out by the negative shift in MOS threshold voltage. The device showed substantially lower parasitic capacitance values compared to a typical silicon power MOSFET with a comparable blocking voltage rating, which suggest that this device can offer significant improvement in switching performance over commercially available silicon power MOSFETs.