industry SiC electronics
a switch. When the transistor is in its ‘off’ state it blocks high voltages, and in its ‘on’ state it delivers high currents with low power losses.
The SiC community has devoted significant effort to the development of SiC power MOSFETs. However, the improvement in high temperature performance of SiC MOSFETs is marginal over a silicon equivalent. That’s because the high-temperature limit for operation is determined by the interface between SiO2
than the level of current leakage from the pn junction. The conduction band offset between SiO2
and SiC is
quite small, resulting in the degradation of the MOS interface at relatively low temperatures. As a result, as of today, the junction temperature of the only commercially available SiC MOSFE is rated up to a just 125 °C.
To address this weakness and allow this class of transistors to fulfil its full potential, we have developed a high-temperature SiC switch: The ‘Super’ Junction Transistor (SJT). The hallmark of this gate-oxide free, normally-off, majority carrier device is its incredibly high current gain, which can exceed 88. It also has many other virtues, including a ‘square’ reverse-biased safe operating area, which allows extremely rugged operation in typical inductive motor and actuator drive applications.
What’s more, the temperature co-efficient of on- resistance for this transistor is slightly positive, which is desirable for paralleling multiple devices for high-current configurations; ‘turn-on’ and ‘turn-off’ times are less than 20 ns; operating temperature can exceed 250 °C; and the SJT features a low ‘on-state’ voltage drop and high- current operation, thanks to the absence of a channel region and a near-zero drain-source offset voltage.
Our SJTs combine a near-theoretical breakdown voltage with a temperature-independent, low-reverse-leakage current up to 325 °C. In addition, our SJT displays a distinct lack of a quasi-saturation region, and is notable for the merging of the different gate current I-V curves in the saturation region. The implication of these two features is a lack of charge storage in the drift region of the transistors – this distinguishes it from a ‘bipolar’ silicon BJT. The resulting benefit is temperature- independent, fast-switching transients. The low, on-state voltage drop stems from appropriate metallization schemes and an optimised epilayer design.
The switching performance of our 4 mm2 and 16 mm2
SiC SJTs have been evaluated by pairing them with an inductive load and our ‘free-wheeling’ 1200 V/ 7 A and 1200 V/30 A SiC Schottky diodes. To drive the SJTs, we used a commercially available IGBT gate driver with an output voltage swing from -8 V to 15 V. For the 4 mm2 SJTs, a 100 nF dynamic capacitor connected in parallel with the gate resistor can generate high initial dynamic gate currents of 4.5 A and -1 A during turn-on and turn- off switching, respectively. During the turn-on pulse, a constant gate current of 0.52 A was maintained. These
A realistic assessment of the performance of our SiC transistors and diodes demands a comparison with state-of-the-art silicon devices deployed in real circuits. To do this, we procured three best-in-class 1200 V silicon IGBTs: A NPT1 ‘non-punch-through’ IGBT, which is rated up to 125 °C and 1200 V; a ‘non-punch-through’ variant rated up to 150 °C and 1200 V, the NPT2; and a trench field stop (TFS) IGBT rated up to 150 °C and 1200 V. All three IGBTs were pre-packaged with silicon fast recovery diodes in anti-parallel configuration. These efforts revealed
March 2012
www.compoundsemiconductor.net 35
Figure 3: (Left,a) Turn-On and (Right,b) Turn-off drain current and voltage transients recorded for switching 800 V and 8 A through a 4 mm2
Figure
2.GeneSiC’s expertise includes – from left to right (a) two- dimensional device design through computer intensive finite element simulations; (b) Precise sub-micron controlled SiC reactive ion etching; (c) Complex integration of designs to fabrication tools and techniques; (d) high temperature/high voltage on-wafer testing and characterization; (e) die attach and wire bonding; and (f) final high temperature optimized packaged products
and SiC, rather
SiC
SJT.There is no difference in switching speed between 25 °C and 250 °C,due to the unipolar nature of the SJT device design
large initial dynamic gate currents rapidly charge and discharge the device input capacitance, yielding faster switching performance. We recorded temperature- independent, ultra-low drain current rise and fall times of just 12 ns and 13 ns, respectively, for switching 8 A and 800 V by the 4 mm2
SJT (see Figure 3).
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