5.1.1 Structure of a VDMOS Transistor

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.

Figure 5.1: Cross section of the analyzed Vertical Double-diffused Metal Oxide Semiconductor transistor.

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 $100 \ {\rm V}$, this type of transistor is commonly used for power devices, for example, in automotive electronics.