The VDMOS transistor is a special form of the DMOS transistor [69,39] where two types of dopants, usually phosphorus and arsenic are co-implanted in the drain region of the device. During the high temperature annealing step the faster diffusing phosphorus is driving deeper into the device than the arsenic. This leads to a less abrupt concentration gradient in the drain region and therefore to a higher hot-carrier reliability than transistors with a single implanted drain.
![\includegraphics[width=0.55\linewidth]{graphics/appa_vdmos_structure.eps}](img200.gif)
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In the VDMOS transistor shown in Figure 5.1 the
drain contact is placed at the bottom of the device. These devices
have the advantage of a lower on-resistance and a smaller lateral size
compared to a lateral DMOS transistor.
Due to the high breakdown voltage of up to 
,
this type
of transistor is commonly used for power devices, for example, in
automotive electronics.