What is the role of field plate technology in improving the breakdown voltage of GaN HEMTs?

The role of field plate technology in improving the breakdown voltage of GaN HEMTs is to reshape the electric field distribution at the drain edge of the gate, reducing the peak electric field and distributing it more uniformly across the gate-drain access region, which allows the device to withstand higher drain voltages before breakdown. Without a field plate: the electric field in a GaN HEMT peaks sharply at the drain-side edge of the gate metal, where the gate depletion region terminates. This peak can exceed the critical electric field of GaN (approximately 3.3 MV/cm) at drain voltages well below the material's theoretical limit, causing premature breakdown. With a field plate: a metallic plate (connected to either the gate or the source) extends over the gate-drain region on top of the passivation layer. This plate creates a second depletion region in the channel beneath it, which: adds a second electric field peak (under the field plate edge) that shares the voltage drop with the original gate-edge peak, reduces the peak electric field at the gate edge by 30-50%, distributes the total voltage drop over a longer channel region, and increases the breakdown voltage by 50-200% (depending on the field plate length and geometry). Common field plate configurations include: gate-connected field plate (the field plate is an extension of the gate metal, overhanging the gate-drain region by 0.5-2 um; this is the simplest implementation), source-connected field plate (the field plate is connected to the source potential via air bridges; provides better RF performance because the source field plate acts as an electrostatic shield between gate and drain, reducing C_gd and improving gain and stability), and multi-step field plates (two or three field plates of different lengths and heights, creating multiple electric field peaks for even more uniform field distribution).