Silicon Carbide Power Field-Effect Transistors
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Silicon Carbide Power Field-Effect Transistors
Jian H. Zhao Abstract Silicon carbide power field-effect transistors, including power vertical-junction FETs (VJFETs) and metal oxide semiconductor FETs (MOSFETs), are unipolar power switches that have been investigated for high-temperature and high-power-density applications. Recent progress and results will be reviewed for different device designs such as normally-OFF and normally-ON VJFETs, double-implanted MOSFETs, and U-shaped-channel MOSFETs. The advantages and disadvantages of SiC VJFETs and MOSFETs will be discussed. Remaining challenges will be identified. Keywords: field-effect transistors, metal oxide semiconductor field-effect transistors, MOSFETs, power switching, silicon carbide, vertical-junction field-effect transistors, VJFETs.
Introduction Several unique properties, including the high critical electric field at which avalanche breakdown occurs, a wide bandgap, and high thermal conductivity, make semiconductor silicon carbide (especially the 4H-SiC polytype, due to its high electron mobility) an excellent material for unipolar and bipolar electronic device applications under high-temperature and high-power-density conditions. While unipolar power devices such as field-effect transistors (FETs) have the advantage of high switching speeds independent of carrier lifetimes, their power handling capability cannot match those of bipolar devices. Recent commercial Si metal oxide semiconductor FETs (MOSFETs) cover the power range from 10 kW to 20 kW, with a frequency capability extending beyond 1 MHz. Insulated-gate bipolar transistors (IGBTs) dominate the power range from 10 kW to megawatt range, but with a lower-frequency capability. The difficulty in extending Si unipolar MOSFETs to a higher voltage range lies in their high conduction loss because of the absence of conductivity modulation. The 10 higher critical electric field of SiC, compared with Si, can substantially increase the power range covered by unipolar power FETs, because SiC power FETs can drastically lower conduction loss. For
MRS BULLETIN • VOLUME 30 • APRIL 2005
a given blocking or breakdown voltage design, the higher critical electric field reduces the required blocking or drift-layer thickness by about an order of magnitude, which in turn allows the doping concentration of the drift layer to be about 100 times higher. The result is a higher switching speed, with a nearly 1000 reduction in unipolar device specific ON-resistance, based on the following equation: Rdrift_SP 4VBr0Ec 3 ,
(1)
where Rdrift_SP is the drift-layer specific resistance, Ec is the critical electric field, VB is the blocking voltage, r is the relative dielectric constant, 0 is the free space permittivity, and is carrier mobility. Substantial improvements in power system weight, size, and efficiency, as well as new applications, are therefore expected from SiC power FETs. The major types of unipolar power FETs include metal semiconductor FETs (MESFETs), vertical-junction FETs (VJFETs), and MOSFETs. A
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