5-6-4-2 SiC High-Power Switching Transistors
Three terminal power switches that use small drive signals to control large voltages and currents (i.e., power transistors) are also critical building blocks of high-power conversion circuits. However, as of this writing, SiC high-power switching transistors are not yet commercially available for beneficial use in power system circuits. As well summarized in References 134, 135, 172, 180, and 186–188, a variety of improving three-terminal SiC power switches have been prototyped in recent years.
The present lack of commercial SiC power switching transistors is largely due to several technological difficulties discussed elsewhere in this chapter. For example, all high-power semiconductor transistors contain high-field junctions responsible for blocking current flow in the off-state. Therefore, performance limitations imposed by SiC crystal defects on diode rectifiers (Sections 5.4.5 and 22.214.171.124) also apply to SiC high-power transistors. Also, the performance and reliability of inversion channel SiC-based MOS fieldeffect gates (i.e., MOSFETs, IGBTs, etc.) has been limited by poor inversion channel mobilities and questionable gate-insulator reliability discussed in Section 5.5.5. To avoid these problems, SiC device structures that do not rely on high-quality gate insulators, such as the MESFET, JFET, BJT, and depletion-channel MOSFET, have been prototyped toward use as power switching transistors. However, these other device topologies impose non-standard requirements on power system circuit design that make them unattractive compared with the silicon-based inversion-channel MOSFETs and IGBTs. In particular, silicon power MOSFETs and IGBTs are extremely popular in power circuits largely because their MOS gate drives are well insulated from the conducting power channel, require little drive signal power, and the devices are “normally off” in that there is no current flow when the gate is unbiased at 0 V. The fact that the other device topologies lack one or more of these highly circuit-friendly aspects has contributed to the inability of SiC-based devices to beneficially replace silicon-based MOSFETs and IGBTs in power system applications.
As discussed in Section 5.5.5, continued substantial improvements in 4H-SiC MOSFET technology will hopefully soon lead to the commercialization of 4H-SiC MOSFETs. In the meantime, advantageous highvoltage switching by pairing a high-voltage SiC JFET with a lower-voltage silicon power MOSFETs into a single module package appears to be nearing practical commercialization . Numerous designs for SiC doped-channel FETs (with both lateral and vertical channels) have been prototyped, including depletionchannel (i.e., buried or doped channel) MOSFETs, JFETs, and MESFETs . Even though some of these have been designed to be “normally-off” at zero applied gate bias, the operational characteristics of these devices have not (as of this writing) offered sufficient benefits relative to cost to enable commercialization.
Substantial improvements to the gain of prototype 4H-SiC power BJTs have been achieved recently, in large part by changing device design to accommodate for undesired large minority carrier recombination occurring at p-implanted base contact regions . IGBTs, thyristors, Darlington pairs, and other bipolar power device derivatives from silicon have also been prototyped in SiC . Optical transistor triggering, a technique quite useful in previous high-power silicon device applications, has also been demonstrated for SiC bipolar devices . However, because all bipolar power transistors operate with at least one pn junction injecting minority carriers under forward bias, crystal defect-induced bipolar degradation discussed for pn junction rectifiers (Section 126.96.36.199.2) also applies to the performance of bipolar transistors. Therefore, the effective elimination of basal plane dislocations from 4H-SiC epilayers must be accomplished before any power SiC bipolar transistor devices can become sufficiently reliable for commercialization. SiC MOS oxide problems (Section 5.5.5) will also have to be solved to realize beneficial SiC high-voltage IGBTs. However, relatively poor p-type SiC substrate conductivity may force development of p-IGBTs instead of n-IGBT structures that presently dominate in silicon technology.
As various fundamental SiC power device technology challenges are overcome, a broader array of SiC power transistors tackling increasingly widening voltage, current, and switching speed specification will enable beneficial new power system circuits.