This is passive semiconductor device that allows (usually) the flowing of current only in one direction (this is called a rectifier). It doesn't obey Ohm law (i.e. it's not linear) and if you put it in a circuit, it won't have a Thevenin equivalent.
The main parameters for a diode are
- the forward voltage drop
- the leaking current
but you can consider also
- maximum forward current
- reverse recovery time
The schematics element is the following
with the arrow indicating the flow of current; usually there is a drop of 0.6V between anode and catode.
A diode in a circuit can be used as
- signal rectifier
- clamps and limiter
- non-linear element
see this video with a couple of examples.
A Zener diode behaves differently from a normal diode: has a reverse-breakdown current pre-determined and can be used as a voltage regulator. In other words, it allows flow in both directions, it's like having two diodes in parallel with different voltage drops.
Are another kind of diode with lower forward voltage drop and fast switching speed but with greater leakage current.
- video by Afrotechmods