A diode is a device that allows current to flow in only one direction.  When a so-called forward bias (a voltage in the "forward" direction) is applied, current flows freely through the device.  When the voltage is applied in the opposite direction (called a reverse bias), no current flows.  A p-n junction exhibits this very behavior, so it is sometimes called a diode.

All of the action shown in the two animations on the previoius page occurs as soon as the silicon is doped.  The diode you buy at an electronics store therefore begins in the state illustrated by the band diagram below.  Consider first what happens if you apply a voltage to the junction by connecting the positive terminal of a battery to the p-side of the junction and the negative terminal to the n-side.  The positive terminal attracts the conduction electrons from the n-side, over the potential barrier at the depletion zone.  This effect is represented in a band diagram by a lowering of the energy barrier, or by a by a tipping of the bands.  As the electrons start flowing, a few more electron-hole pairs may combine, but eventually a steady-state current is reached.  Click on the image below to see an animation of this effect.

****Now animation starting at equilibrium and showing a forward bias applied - first, by lowering the energy barrier, then by tipping the figure.

Now consider what would occur if the terminals of the battery had been attached in the opposite direction.  Starting again at equilibrium, we connect the negative battery terminal to the p-side and the positive terminal to the n-side.  The positive terminal attracts the conduction electrons in the n-side, but no stable current can be produced.  There are no conduction electrons in the p-side to replace the electrons that leave the n-side for the battery.  As the animation below shows, this reverse bias increases the energy barrier at the junction and inhibits current flow.

***animation starting at equilibrium and showing a reverse bias applied
 

****Picture of Tom's wheel with two diodes on it as interesting application of LED?******
 
 
 

That's it for this Module! The next module will explore some of the applications of pn junctions!