Overview

The super-linear (SL) MOSFET structure described in the previous chapter utilizes the charge-coupling concept to reduce the resistance in the drift region. The charge-coupling is achieved by forming a source connected electrode within a deep trench that surrounds the (mesa) drift region. In addition to allowing high doping concentrations in the drift region, the charge-coupling distributes the electric field below the P-base region resulting in a low electric field at the surface. This enables maintaining the channel in the linear mode of operation even at high drain bias voltages an essential feature to obtain the desired super-linear behavior.

In this chapter, a planar super-linear (SL) MOSFET structure is described which does not utilize the charge-coupling concept. The device fabrication process is simplified for this planar gate structure because of eliminating the deep trench regions. The super-linear mode of operation is obtained in the planar SL-MOSFET by utilizing a P ⁺ shielding region located below the P-base region. This shielding region depletes the transition region located under the gate at relatively low drain voltages. Once the transition region is pinched off by the depletion layers, the potential under the gate electrode no longer increases with increasing drain bias. This enables maintaining a low voltage across the channel even when the drain bias is increased up to the breakdown voltage of the structure. Thus, the super-linear mode of operation is achieved by keeping the channel operation in the linear regime while operating the drift region under velocity saturation mode.