DOUBLE-DIFFUSED MOS (DMOS)
The figure5.22 shows a double-diffused MOS (DMOS) structure. T he channel length, L, is controlled by the junction de pth produced by the n+ and p-type diffusions underneath the gate oxide. L is also the lateral distance between the n+ p junction and the p-n substrate junction. The channel length can be made to a smaller distance of about 0.5 micro meters. Thus, this process is similar to the situation with resspect to the base width of a double-diffused bipolar transistor. When a fairly large positive voltage is applied to the gate [>VTH], it will cause the inversion of the p-substrate region underneath thhe gate to n- type , and the n-type surface inversion layer that is produced will act as a conducting channel for the flow of electrons from sou rce to drain.
Figure 5.22 Double-Diffused MOS (DMOS) Structure
From the structure it is known that the n-type substrate is very lightly doped. This will help in making enough space for thee expansion of the depletion region between the p-type diffusion region and the n+ drain contact region. Due to this, the breakdown voltage w ill become higher between the drain and source.
The Power MOSFET is the three terminal (Gate, Drain and Source), four layer (n+pnn+),Unipolar ( only majority carriers in conduction) semiconductor device.