Silicon Carbide npnp Thyristors

John H. Glenn Research Center, Cleveland, Ohio

Thyristors (semiconductor controlled rectifiers) made from silicon carbide have been fabricated and tested as prototypes of power-switching devices capable of operating at temperatures up to 350 °C. The highest-voltage-rated of these thyristors are capable of blocking current at forward or reverse bias as large as 900 V, and can sustain forward current as large as 2 A with a forward potential drop of –3.9 V. The highest-power-rated of these thyristors (which are also the highest-power-rated SiC thyristors reported thus far) can block current at a forward or reverse bias of 700 V and can sustain an "on" current of 6 A at a forward potential drop of –3.67 V. The highest-current-rated of these thyristors can block current at a forward or reverse bias of 400 V and can sustain an "on" current of 10 A.

This Cross Section (not to scale) shows the npnp-layer structure of a representative thyristor of the present type. The n^+ -, p^- -, n-, and p^+ -doped 4H-SiC layers are formed by epitaxy on the n^+ 4H-SiC substrate, which is cut at an angle of 8° off axis.

These thyristors feature epitaxial n- and p-doped layers of 4H SiC in the sequence npnp starting on the substrate; this structure (see figure) stands in contrast to the pnpn structure of common silicon thyristors. The fabrication of the high-quality crystalline structures needed in these layers has been made possible by advances in growth of crystals, epitaxial growth of thin films, doping by both in situ and ion-implantation techniques, oxidation, formation of electrical contacts, and other techniques involved in the fabrication of electronic devices.

The reasons for choosing the npnp structure and for choosing the 4H polytype of SiC (instead of choosing the more common 6H polytype) are the following:

The npnp structure was adopted to avoid the very high resistances of typical p-doped SiC substrates. In the research that led to the development of the present thyristors, the resistances of p-doped substrates were found to dominate the characteristics of pnpn SiC thyristors.
It was found in this research that the electron mobilities along the electrical-current paths in 4H-SiC thyristors of the present 4-layer configuration are about 10× those of similar thyristors made from 6H-SiC. Thus, 4H-SiC offers the potential to achieve greater current densities.

It was also found in this research that the defect densities of the 4H-SiC layers (which are formed by epitaxy) are much smaller when substrates cut at large off-axis angles are used. 6H-SiC substrates are typically cut at 3.5° off axis. However, it was found that when 4H-SiC substrates are cut at 3.5° off axis, large numbers of 3C-SiC inclusions are observed in the epilayers. It was found that the 3C-SiC inclusions can be eliminated by growing on 4H-SiC substrates cut at 8° off axis. The highest-power-rated thyristors were found to be achievable only by use of 8°-off-axis-cut 4H-SiC substrates.

Some of these thyristors rated at voltages >400 V and currents >5 A have been characterized at temperatures up to 350 °C. The forward voltage drop at a current of 5 A was found to decrease monotonically from 3.91 V at 27 °C to 3.18 V at 350 °C. The leakage current density at a reverse bias of 400 V was found to increase from about 10–⁶ A/cm² at room temperature to 9 × 10^-3 A/cm² at 350 °C. Even at 350 °C, the ratio between the "on" current and the leakage ("off") current was found to be about 10⁵, which should be an acceptable ratio for a power device.

Some of the thyristors were packaged, then stored for 1,000 hours at 350 °C. While many of these thyristors failed, about 25 percent survived the 1,000 hours without significant degradation.

The npnp 4H-SiC thyristors were found be capable of switching at very high speeds. For example, a 600-V, 2-A device was tested to determine its maximum repetition rate for a peak current of 7 A pulsed at a 20-percent duty cycle. It was found that the gate pulse could be repeated after a period of only 4 μs, corresponding to a maximum pulse-repetition frequency of 250 kHz. This speed exceeds the speed of the fastest inverter-grade silicon thyristors.

Two other important parameters for a thyristor are (1) the maximum rate of increase of forward applied voltage that can be applied before the thyristor latches on and (2) the time taken to achieve a high forward current density. The 4H-SiC thyristors tested showed no turn-on even when forward bias was ramped up at a rate of 900 V/µs. Measurements in pulsed operation showed that it took between 3 and 5 nanoseconds for these devices to start carrying currents at densities of 2,800 A/cm².

This work was done by John Palmour of Cree Research, Inc., for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.

Inquiries concerning rights for the commercial use of this invention should be addressed to