Most of the material here was excerpted from this page, this page, and this page. Go there for further reading, or check out the Wikipedia article. Finally, a highly recommended paper discussing a little more detail about diode physics was recently published in the American Journal of Physics.
A diode is a two-terminal nonlinear device, with the (+) terminal called the anode (a), and the (–) terminal called the cathode (k).
anode (A) cathode (K)
In an ideal diode, current flows in only one direction (from anode to cathode internally). When a forward voltage is applied, the diode conducts; and when a reverse voltage is applied, there is no conduction. Diodes are the electrical version of a valve and early diodes were actually called valves.
An ideal diode has the following i-v characteristic:
Forward Voltage Drop
Electrons use up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for most diodes which are made from silicon. This forward V drop can vary for various other types of diodes, especially LEDs. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic. So, our idealized i-v characteristic is slightly modified (note that even in the forward conducting region, the diode i-v curve has a very steep but finite slope, which means that there is a small resistance in forward bias):
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few μA or less, which is nearly constant over a large range of reverse voltage. This can be ignored in most circuits because it will be much much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown. We don't need to worry about breakdown in 20.309, but you should know that it exists.
Taking these various effects into account, a more real diode i-v characteristic looks like this:
A circuit model that takes into all of the above effects into account, but still uses idealized circuit components takes the following form :
The top branch (in which current can only flow to the right) represents the forward regime, which has a finite small resistance Rf and the forward voltage drop Vpn. The lower branch is the reverse regime, with the large reverse resistance represented by Rr, and only allowing current to flow to the left.
Light Emitting Diodes (LEDs)
These devices work very similarly to standard semiconductor diodes, except that forward current passing through them causes light emission. LEDs that are packaged individually look similar to the figure below. The cathode (–) lead is typically shorter, and the LED package has a small flat next to that lead.
Photodiodes are p-n junctions specifically designed to optimize their inherent photosensitivity. Photodiodes can be used two ways -- in a photovoltaic (here it becomes a current source when illuminated -- see solar cell), or photoconductive role. To use a photodiode in its photoconductive mode, the photodiode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated.