Inside a diode
A diode lets electric current flow one way only, like a one-way street. Current may go in at the anode and out at the cathode, the end marked with a band. Push the other way and it stops.
Inside, two kinds of doped silicon (see Semiconductors and doping) touch. The p side is full of holes: places missing an electron, which act like + charges. The n side is full of spare electrons, which are - charges.
At the join, electrons cross over and fill nearby holes. That leaves a thin zone with no free carriers, called the depletion region. Its leftover charges push back, so it acts like a wall.
Forward and reverse
Connect a 3 V battery with its + side to the p side. This is forward bias. The push squeezes the wall thin, carriers stream across it, current flows round the loop and the lamp lights. The resistor limits the current.
The curve plots current against the voltage across the diode. Below about 0.6 V almost nothing flows. Then the current shoots up: a working silicon diode keeps about 0.6 to 0.7 V across itself.
Now flip the battery, so the p side is negative. This is reverse bias. The battery pulls the carriers away from the join, the wall grows wider, no current flows and the lamp goes dark.
Backwards, only a tiny leak gets through: at most 25 billionths of an amp for this diode. Push far harder, 100 V or more, and the wall breaks down and current rushes through. Normal diodes are kept well below that.
Using diodes
An LED is a light-emitting diode. When an electron drops into a hole at its junction, it gives off light. LEDs need more forward voltage than silicon: about 1.85 V for red up to 3.3 V for blue.
Mains electricity is AC, alternating current: its push swings forward and backward, 50 or 60 times a second. Here it is slowed right down. The lamp only lights on the forward swings.
The diode passes the forward half of each swing and blocks the other half. What comes out is a string of pulses that all go one way. Turning AC into one-way DC like this is called rectifying.
In detail
;; step 1/10 · one-way Diode symbol: anode ->|- cathode the triangle points the way current may flow the bar is the cathode, marked on the part by a band The part in the figure: a 1N4148, a small silicon diode (black cathode band).
The board circuit is a teaching model: the 3 V battery, the resistor and the lamp show the idea and are not taken from one datasheet. So is the LED row's 5 V supply (each LED has its own resistor, not drawn). The V-I curve is the 1N4148's typical forward curve at 25 °C (Vishay datasheet, Fig. 2): about 1 mA at 0.6 V and 10 mA at 0.72 V. Its reverse current, at most 25 nA at 20 V, is too small to draw and is shown as zero; breakdown starts at 100 V or more. The reverse axis is squeezed and broken, so it is not to scale. LED forward voltages are typical values at 20 mA from Kingbright's 5 mm LED datasheets: red 1.85 V, green 2.2 V, blue 3.3 V. Each part varies a little.