The problem of breakdown in MODFETs

Understanding the breakdown mechanisms in FETs is a key factor in designing high power MMICs. The breakdown can be improved by reducing the electric field at the drain, and the gate leakage current. For MESFETs the drain field can be reduced by increasing the distances between the drain and gate contact. This leads to asymmetric source-gate and gate-drain spacings. For MODFETs with recessed gates, asymmetric designs are not successful in increasing the breakdown voltage because there is a low potential drop in the highly doped cap. Therefore wide etched gate recesses are required, but because of the symmetric etch processes, the source resistance is enhanced leading to a degradation in extrinsic transconductance. An other possibility for improving MODFET breakdown behaviour, is the use of surface depleted caps. However these MODFETs result in source current saturation in the 2DEG leading to an increase of the source and drain resistances at high currents.