Lateral diffusion:

Although the doping is desired only in certain locations, the diffusion occurs in all directions. For example, the schematic below shows the desired doping in the ideal case and the doping that would occur in the real case.

Figure 6.8. Schematic illustrating the lateral diffusion effect

When the transistors are separated by relatively large distance (a micron or more), the lateral diffusion was not a significant issue. When the distance between the transistors are small, and when the distance between a source and the drain in the same transistor is small, then lateral diffusion causes short-circuiting. Hence methods that eliminate this problem must be used. Ion implantation is a method which can mitigate this issue to some extent and we will learn about that in the next section.