Overview

The vertical double-diffused power MOSFET structure discussed in the previous chapter operates by blocking voltage across a P/N junction where the depletion occurs primarily on the lightly doped N-type drift region. This produces a triangular shaped electric field profile in the drift region. Based up on this one-dimensional shape, it is possible to develop a relationship between the specific resistance of the drift region and the breakdown voltage ^{[1] , [2]} that has allowed creating the Baliga's Figure of Merit for power devices. An equation for the ideal specific on-resistance for the drift region was provided in Chapter 3. This resistance was believed to be lowest attainable value for power MOSFETs with a specified breakdown voltage until the development of the two-dimensional charge-coupling concept. The two-dimensional charge-coupling concept ^[3] was first proposed for enhancing the performance of lateral power MOSFETs.

Two methods to extend the two-dimensional charge-coupling concept to vertical devices were independently proposed ^{[4] , [5]}. In the first method, the two-dimensional charge-coupling is created between the N-drift region and an electrode embedded within an oxide filled trench located adjacent to the drift region. This method is especially suitable for devices with breakdown voltages ranging from 30 to 200 volts. At lower breakdown voltages, the benefits of the two-dimensional charge-coupling are not significant because the doping concentration of the drift region becomes high even for the conventional device structure. The approach is also difficult to implement at breakdown voltages above 200 volts because of...