Fundamentals & Material System

Silicon has dominated power electronics for decades, but it runs into physical limits: at high blocking voltages (in high-voltage transistors), a Si device needs a thick, lightly doped drift region to safely support the electric field – which costs on-resistance and therefore losses. This relationship follows directly from the Poisson equation: the lower a material's critical breakdown field strength, the thicker the drift region must be designed for a given blocking voltage, and the on-resistance grows disproportionately as a result – roughly with the second to third power of the blocking voltage. In silicon this effect already becomes noticeable above a few hundred volts, and at several kilovolts the drift-region resistance dominates the total device resistance almost completely.

Compound semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) have a significantly higher breakdown field strength than silicon – a direct consequence of their larger bandgap, which binds electrons more strongly and thus raises the threshold for impact ionization. This allows thinner, more highly doped drift regions at the same blocking voltage – and thus noticeably lower conduction losses combined with a smaller chip area. In practice this means: at identical blocking voltage and identical on-resistance, a SiC or GaN device can be realized on a fraction of the chip area of a Si device, or conversely deliver a markedly lower on-resistance at the same area. This material advantage runs as a common thread through the entire development of power electronics over the last twenty years and is the real driver behind the commercial breakthrough of both material systems.

A direct comparison shows clear differences: silicon has a bandgap of 1.12 eV, 4H silicon carbide reaches 3.26 eV, and gallium nitride 3.4 eV – both compound semiconductors are clearly wider-gap materials and belong to the class of so-called wide-bandgap (WBG) semiconductors. This larger bandgap not only affects the breakdown field strength but also allows higher junction temperatures: while silicon devices are typically operated at junction temperatures around 150–175 °C, 200 °C and above are common practice for SiC devices, since the intrinsic carrier density at a given temperature remains many orders of magnitude lower.

The critical breakdown field strength rises from about 0.3 MV/cm for silicon to roughly 2.8 MV/cm for SiC and 3.3 MV/cm for GaN. SiC additionally stands out with high thermal conductivity (about 4.9 W/cmK versus 1.5 W/cmK for silicon), which makes heat dissipation at high power levels considerably easier and allows more compact cooling concepts – an advantage not available in the same way for GaN, due to its lower thermal conductivity (roughly 1.3 W/cmK) and the typical heteroepitaxy on foreign substrates. GaN, in turn, reaches a very high electron saturation velocity, enabling especially high switching frequencies; combined with the high mobility in the 2DEG channel (see the GaN chapter), this results in switching frequencies in the MHz range that are practically unattainable with Si or SiC devices. Taken together, this yields a clear division of labor: SiC where high blocking voltage, high power and good heat removal are required, GaN where switching frequency and form factor take priority.