ADI KUMAR · POWER & DIGITAL INFRASTRUCTUREAUGUST 2026 · V1 · ~40 MIN READ
The AI Power Chain · Part II-A · Technical Companion
How wide-bandgap semiconductors actually work

The Wide-Bandgap Stack: Technical Companion

Where the market essay named the players, this piece explains the physics. What "wide bandgap" actually means, why SiC and GaN outperform silicon at the same voltage, how a boule of vapour-grown crystal becomes a switch that runs at 800V and 200kHz, and where the technology goes next. Cross-sections, process flows, and roadmaps for readers who want to understand the machine underneath the market.

By Adi Kumar Independent analysis Views are my own
How to read this piece. It is a technical companion to The Wide-Bandgap Stack (Part II of this series). The market piece named the vendors, sized the layers and mapped the M&A. This one explains the underlying physics and manufacturing, why SiC and GaN devices switch the way they do, and what the roadmap items on every analyst's list actually change. No prior device-physics background is assumed, but the reader is expected to know what a MOSFET does at a systems level.
1The physics · Why bandgap matters

What "wide bandgap" actually means

Every property that makes SiC and GaN interesting at rack scale flows from one number: the bandgap. Silicon's is 1.12 electronvolts. Silicon carbide's is 3.26 eV. Gallium nitride's is 3.39 eV. Those numbers sit inside almost every device advantage discussed in the market essay. The rest of this piece works out why.

Bandgap is the energy an electron in the valence band must gain to be promoted into the conduction band. Below that threshold, electrons stay put and the material insulates. Above it, they conduct. In a power switch, the point is not conductivity but blocking. When a MOSFET is off, the drain-to-source region must sustain the full bus voltage without letting current through. The parameter that governs this is the critical electric field, the field strength at which the material breaks down (avalanches). Critical field scales roughly as the square of the bandgap.

Silicon's critical field is about 0.3 MV/cm. SiC's is around 3 MV/cm, ten times higher. GaN's is comparable to SiC. What that means in practice: to block a given voltage, a SiC device needs roughly a tenth the drift-region thickness of a silicon device, doped roughly ten times more heavily. Thinner drift region plus higher doping equals dramatically lower on-resistance.

Bandgap and critical field: the three materials that matter
Bandgap energy (eV) and critical electric field (MV/cm) for Si, SiC (4H polytype) and GaN
Silicon: 1.12 eV, ~0.3 MV/cm. 4H-SiC: 3.26 eV, ~3 MV/cm. GaN: 3.39 eV, ~3.3 MV/cm. Standard semiconductor references (Sze, Baliga).

The Baliga figure of merit

The relationship between material properties and switch performance is captured in a single expression, published by B. Jayant Baliga in 1989: BFOM = ε·μ·Ec3. Permittivity times mobility times the cube of critical field. Higher BFOM means lower on-resistance per unit die area at a given breakdown voltage. Because critical field enters at the third power, a 10x advantage in Ec translates to a theoretical 1,000x reduction in the RDS(on) × Area product.

Silicon's BFOM is 1 by definition. 4H-SiC's is roughly 500. GaN's is around 900. Those numbers are the ceiling. Real devices leave most of it on the table because of contact resistance, channel resistance and packaging parasitics, but the numbers explain why a 1200V SiC MOSFET can be an order of magnitude smaller than a 1200V silicon superjunction MOSFET at the same on-resistance.

PropertySilicon4H-SiCGaNWhy it matters
Bandgap Eg (eV)1.123.263.39Higher Eg allows higher operating temperature and lower intrinsic leakage
Critical field Ec (MV/cm)0.3~3.0~3.3Sets the maximum voltage per unit thickness of drift region
Electron mobility (cm²/V·s)1,350950~2,000 (2DEG)Lower resistance per unit area for the same doping
Thermal conductivity (W/m·K)150370–490130 (bulk), higher on SiCAbility to move heat out of the die
Saturated drift velocity (cm/s)1×1072×1072.5×107How fast carriers respond, ceiling on switching speed
BFOM (Si = 1)1~500~900Theoretical Ron·A improvement at given breakdown voltage
Six-axis material comparison
Normalised to the best-in-class material on each axis. Larger area = better overall.
Values normalised: Eg, Ec, drift velocity and thermal conductivity divided by the highest value across the three. Mobility uses lateral 2DEG for GaN. BFOM is log-scaled then normalised to fit. The point of the radar is to show that no single material dominates every axis: GaN wins on mobility and BFOM, SiC on thermal conductivity, silicon on maturity (not shown).
What "wide bandgap" is not. A wider bandgap alone does not make a better device. Wide-bandgap materials tend to have lower bulk electron mobility than silicon (SiC channels move electrons more slowly than silicon channels). The improvement comes from being able to build the same voltage rating with a thinner, more heavily doped drift region, which more than compensates for the mobility penalty. GaN's edge is a different mechanism entirely (a 2D electron gas at a heterojunction interface, covered later).
2Crystal growth