A diode is the simplest
semiconductor component you can make. Take some
P material. Fuse it with some
N material. In the result,
current will flow from the P to N, but not backwards.
Most diodes always a voltage drop of about .7V across the component. If you run the voltage high enough in the reverse direction, you will eventually hit the breakdown voltage, above which the diode will act as a resistanceless voltage drop. It will also probably blow out at that voltage, unless you take the proper precautions. This is what zener diodes are designed to do.