# How do semiconductors affect a circuit?

I understand the difference between p-type and n-type semiconductors but I just don't understand how adding a
p-n junction to a circuit affects it.
Can someone please explain how semiconductors work?
For example, if I had a circuit with just a battery and a light bulb, what would change if I added a p-n junction right before the bulb?

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what would change if I added a p-n junction right before the bulb?
The bulb would be on or off depending upon the direction of the p-n junction relative to the polarity of the battery.

An electronic part consisting of a simple pn junction is the DIODE. This part allows a current in one direction only - of course, caused by a corresponding voltage of (0.6...0.7) volts (forward operation). If you change the polarity of the voltage there will be no current (backward operation, off condition). Under this condition, the diode behaves like a very large resistance (nearly infinite).

Really I think you need to ask this as 2 questions....1) How does a PN junction work? 2) How does this affect a circuit - how is it used? I say this because you do not need to completely understand one to understand the other.

When a voltage is applied on a PN junction you are said to "bias" the junction - and sets up an electric field in the semiconductor. In the forward mode the "holes" in the P side and Electrons in the N side are pushed together by the electric field in the diode - when an electron - combines with a hole this in essence allows current flow. It is a lot more complicated but that is as simple as I can state it. In the reverse bias, the Holes and Electrons are pulled away from each other, they can not combine - and they can not conduct current.

As for use - think of the device as a one-way valve.

BTW introduction to semiconductors is a full semester EE, or Solid State Physics class - the HOW they work.... granted it covers much more than diodes. I am not saying you need a semester class, but you probably can not figure it all out in a forum without a lot of research on your part. But note the Use side, how Diodes and other basic semiconductors are applied, is typically only a part of a circuit analysis course. - Using them is easier than understanding them.

Devguy101