The silicon power MOSFET has been the most important device for changing low- and medium-voltage power for more than forty years. It was the best choice for everything from consumer electronics and telecom infrastructure to industrial power supplies and automotive systems because it was easy to use, had a well-established manufacturing ecosystem, and its performance was easy to predict. Architectural improvements that have been made over time, such as moving from planar to trench and superjunction designs, have steadily lowered on-resistance while keeping voltage capability.

But silicon MOSFET technology is getting close to its basic physical limits. Wide-bandgap semiconductors, especially gallium nitride (GaN), are becoming the natural successor in the low-voltage range as modern electronics need more efficiency, more power density, and faster dynamic performance.  GaN changes the way power devices work at the atomic level, which opens up new power conversion architectures and changes the way systems work as a whole.

The Physical Limits of Silicon MOSFETs

Silicon MOSFET performance is constrained by a well-known trade-off between conduction losses and switching losses. Achieving low on-resistance (R_DS(on)) typically requires increasing the device area, which in turn increases parasitic capacitances such as gate charge (Q_g) and output capacitance (C_oss). The trade-off between higher capacitance and slower transition speeds is due to the properties of silicon, especially its relatively low critical electric field. Silicon can't handle high electric fields, so devices need to be bigger to block voltage, which directly increases capacitance. Because of this, MOSFETs become less efficient as the switching frequency goes up, which means that designers have to run converters at lower frequencies. For decades, the need to balance conduction and switching losses while keeping switching frequencies within practical limits has shaped the design of power electronics.

On-Resistance vs. Breakdown Voltage

In unipolar devices such as MOSFETs, the relationship between on-resistance and breakdown voltage is fundamentally determined by the properties of the semiconductor material. The specific on-resistance of the drift region can be approximated as:

R_DS(on) = 4*V_BR² / (ε₀ ε_r μ_n E_crit³)

where V_BR is the breakdown voltage, μ_n is electron mobility, ε_r is relative permittivity, and E_crit is the material’s critical electric field.

This relationship reveals the dominant role of the critical electric field. Since R_DS(on) scales with the inverse cube of E_crit, materials capable of sustaining higher electric fields allow dramatically lower resistance at the same voltage rating.

Silicon has a relatively low critical electric field (~0.23 MV/cm). Consequently, devices designed to block higher voltages require thick, lightly doped drift regions, which dramatically increase resistance and chip area. This is the fundamental trade-off that has shaped silicon power electronics for decades.

The 2DEG: The Heart of the GaN HEMT

GaN devices use a special device structure that is based on a heterojunction between aluminum gallium nitride (AlGaN) and GaN.

When a thin layer of AlGaN is grown on GaN, the difference in lattice structure and polarization create strong electric fields inside the material. At the interface between the two materials, these fields make a two-dimensional electron gas (2DEG).

This 2DEG creates a very thin conduction channel with a very high carrier density and electron mobility that is much higher than that of bulk GaN. The outcome is a conduction path with very low resistance that doesn't require heavy doping, which would otherwise make mobility worse.

This mechanism allows GaN high-electron-mobility transistors (HEMTs) to achieve exceptionally low R_DS(on) while maintaining high voltage capability.

Maurizio Di Paolo Emilio, Director of Global Marketing Communications at EPC