Why do we need a doped substrate?

[uney]

From my textbook, I know why we use the P well instead of using the p-doped substrate directly, but why do we need a doped substrate?
I think this is a stupid question, and that's why I can't find out the ans from my testbook but considered that I am a new comer in semiconductor, please help me!

 

[dlgoff]

It's used in order to assure that none of the isolation junctions will be forward biased.

This substrate is connected to the most negative potential to accomplish this.

Welcome to PF

 

[Fish4Fun]

For an overview of the details on the process consult here:

http://en.wikipedia.org/wiki/Semiconductors
http://en.wikipedia.org/wiki/Doping_(semiconductor)

I think the problem you are having is the processes used to create components on a die. In general there are three distinctly different processes: 1) Removing substrate 2) Adding to the substrate 3) Altering the substrate. Think of the substrate like a sculpture; the sculptor begins with a block larger than the finished work. The sculpture emerges as the excess material is removed. This is the same basic process as semiconductor production. Parts of a silicon wafer are removed, doped or added to in a specific order in such a way as to leave a particular finished design with specific electrical properties.

If you have ever seen the process of PCB mfg, it is in some ways similar. In this case you start with a copper clad board. Using one of various methods you place an acid resistant layer where you want the copper to remain, then you submerge the entire board in copper-dissolving acid. When the process is complete, only the areas with acid resistance remain. You then remove the acid resist and you have a PCB.

With a silicon wafer, you choose the initial doping to best facilitate the construction of the rest of the process. If 60% of the silicone in the final die is doped in a particular way, then you might begin by doping the entire wafer to that level. In the remaining processes you mask, etch and re-dope or deposit material as required for the stage.

I hope this gives you a crude idea of the process, and the "why" of the initial doping. There is a lot of information about the various doping processes, silicon wafers and die production available on the net; some of it is fascinating reading.

Fish

 

[uney]

Thanks a lot