GaN HEMT Channel: 2DEG Drift & Pinch-Off

Bias Controls

Gate voltage V_GS −1.0 V

Drain voltage V_DS 5.0 V

Lattice temperature T 300 K

Channel State

2DEG OPEN — channel conducting

Low-field mobility μ₀(T)

— cm²/V·s

Avg. drift velocity

— cm/s

Drain current density I_DS

— mA/mm

How it works

An AlGaN/GaN heterojunction spontaneously forms a two-dimensional electron gas (2DEG) right at the interface, thanks to the large spontaneous and piezoelectric polarization mismatch between the two nitride layers — no doping is needed. That sheet of electrons is the HEMT's channel, and it is what this scene renders as a swarm of drifting particles between the source and drain contacts.

Electron drift velocity does not scale linearly with field forever. It follows a Caughey–Thomas velocity–field relation that bends into velocity saturation at high field, set by polar-optical-phonon scattering:

v(E) = μ₀(T)·E / [1 + (μ₀(T)·E / v_sat)ⁿ]^(1/n)
μ₀(T) ≈ μ₀(300K)·(300/T)^1.5   (phonon-limited mobility falls as T rises)
v_sat ≈ 2.2 × 10⁷ cm/s for GaN

Because the source-to-gate spacing is short and the gate-to-drain access region is long (a real HEMT layout choice for high breakdown voltage), the electric field is not uniform — it crowds toward the drain-side edge of the gate at high V_DS. The "Drain field crowding" toggle switches this local field enhancement on and off so you can compare it against a uniform-field approximation.

The gate is a Schottky/MIS capacitor sitting on top of the barrier: raising it toward pinch-off (V_GS → V_th, here ≈ −3.5 V) depletes the 2DEG directly beneath it, narrowing — and eventually closing — the channel, while the ungated access regions on either side stay fully populated. That's the visual "necking" you see under the gate electrode as you lower V_GS.

V_GS — sets how much of the 2DEG survives under the gate (capacitive depletion, pinch-off below V_th).
V_DS — sets the lateral channel field, hence drift velocity and how strongly it saturates.
T — phonon scattering raises with temperature, lowering low-field mobility μ₀ (electrons jitter more, drift slower for the same field).

This is the exact transport mechanism that makes GaN HEMTs the device of choice for RF power amplifiers and fast chargers: high 2DEG mobility plus a saturation velocity roughly double silicon's lets them switch fast at high voltage.

2DEG electrons Gate / source / drain metal AlGaN barrier

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