Transistors - electron/hole mobility

[wolfric]

Transistors

I was searching for an electronics forum and found this place but even the elementary questions don't seem that elementary to me so bare with me if this is rather simplistic.

I was looking over a video on how transistors are made. When you dope semiconductors with impurities, you create either a surplus or shortage of electrons which depending on the type, attract electrons or (allow them to be attracted to other elements?). However if you had a shortage, When electrons flow through the material, wouldn't they simply just fill up all the wholes and then the material would become stable and not conduct electricity? and vice versa with the material loosing the additional electrons.

Or is it that with impurities, you're providing elements with an unstable shell that the electrons are more free to move?

 

[keith1200rs]

Transistors

I will give my take on this in the hope that someone more experienced answers it properly later!

If impurities added extra electrons and those extra electrons could move to fill holes in a nearby material then the original impurity would not be the same anymore - it has permanently lost it's electrons.

Or, another way I would look at it is the impurities add extra (or missing) electrons which allow things to move around, but the total number must stay the same in each place.

Time for a physicist to jump in now!

Keith

 

[Mad I.D.]

Re: Transistors

/hi

Hi wolfric. The most important thing to learn is the PN junction. By studying it you will be able to understand more about what happens to electrons & holes. Forget about transistors for now.

I will answer your question shortly. For example, in a NPN bipolar transistor the base is made so short that electrons manage to "fly" over it. Of course some of them gets trapped by the holes (that's why we have base current in bipolar transistor), but most of them get pass the base into the collector.

There is always some recombination being done, but again, study PN junction and you will understand, hopefully, everything.