Transport, high-field physics, and breakdown

Low-field and RF transport

At low lateral field, current is set by 2DEG density, mobility, access resistance, and contact resistance. Mobility is limited by polar optical phonons, interface roughness, alloy disorder, dislocations, charged traps, remote surface charges, and background impurities.

For RF devices, short gate length, high transconductance, low capacitance, and high saturation velocity drive high f_T and f_max. But RF output power and linearity are often limited by trapping/current collapse and self-heating, not just intrinsic channel speed.

High-field region

The highest field usually occurs near the drain-side gate edge. That region controls hot-electron injection, gate leakage, field crowding, electroluminescence, dynamic R_ON, and off-state degradation. Field plates spread the electric field over a larger region, improving breakdown and reducing peak field, but they add capacitance and can slow RF/switching performance.

Breakdown is usually extrinsic

GaN’s intrinsic breakdown field is high, but real lateral HEMTs often fail through surface leakage, buffer leakage, punch-through, gate-edge field crowding, substrate leakage, trap-assisted conduction, thermal runaway, or imperfect passivation. The design tradeoff is straightforward: increasing gate-drain spacing and field-plate length improves voltage capability but increases device area, capacitance, and R_ON.

Design knobs

• Gate-drain spacing and field-plate geometry.
• Carbon/iron-doped buffers to suppress leakage, balanced against trap memory effects.
• Back barriers and superlattice buffers to improve confinement and isolation.
• Surface treatments and passivation to suppress surface leakage/trapping.
• Substrate choice and package design for heat removal.