Polarization physics and 2DEG formation

Spontaneous plus piezoelectric polarization

Wurtzite III-nitrides lack inversion symmetry, so they have strong polarization along the c-axis. Two terms matter:

• Spontaneous polarization: built into the crystal even without strain.
• Piezoelectric polarization: caused by strain, especially tensile-strained AlGaN grown on GaN.

The interface sheet charge is approximately the discontinuity in total polarization:

σ_pol = P_total(barrier) − P_total(channel)

For common Ga-face AlGaN/GaN HEMTs, this creates positive fixed charge at the interface and attracts electrons into the GaN side.

How the 2DEG forms

The positive polarization charge bends the bands and pulls the GaN conduction band below the Fermi level near the interface. Electrons accumulate in a triangular quantum well, forming a high-density 2DEG. Typical sheet density is often around 10¹³ cm⁻², although the exact value depends on Al fraction, barrier thickness, strain, surface states, gate work function, and back-barrier design.

The channel can be undoped, so ionized-impurity scattering is reduced compared with doped channels. But because the electron gas is close to the surface and interface, it remains sensitive to interface roughness, remote charges, surface traps, and passivation quality.

Band-diagram intuition

At zero gate bias, a depletion-mode AlGaN/GaN HEMT already has an occupied 2DEG. A negative gate bias raises the channel potential and depletes the 2DEG. A positive gate bias can increase channel charge, but gate leakage and reliability limit how far this can go.

Enhancement-mode devices modify the band diagram under the gate so that the 2DEG is depleted at zero bias. The main approaches are p-GaN gates, recessed MIS gates, fluorine treatment, and cascodes.