In our last discussion on semiconductors we discovered that there are 2 types of extrinsic materials, N type with abundance of free electrons and P type with an abundance of spaces or holes for electrons to fill.
We the discovered that when fused together a region of high resistance was created at the join or junction.
see www.youtube.com/watch?v=4SlfaocMfdA
Now we apply some voltage to see what happens.
2 things to remember 1. Nature loves balance and 2. opposites attract
OK I have a battery that I will hook up via a load resistor positive to P type (anode) and negative to N type (cathode), this condition is called forward bias.
As I gradually increase the voltage from zero to .1 of a volt a small current will flow due to electrons locked into the P material moving to the positive pole and electrons from the N material moving over the junction to the positive pole. The current will be an Ohms law current based on the applied voltage and junction resistance.
So the voltage increases to .7 of a volt, during this progress the junction resistance decreases due to electron migration etc. It is said that at around .7 of a volt for silicon (.3 for germanium) the junction is at its lowest resistance and current is free to flow depending upon applied voltage. IF there was no load resistor the device would burn out (the same applies for LEDs). In this condition there will be a .7 of a volt drop over the diode.
If I reverse bias the polarity ie negative on the anode (P type) and positive on the cathode (N type) the positive pole will attract electrons away from the junction thus increasing its resistance, therefore it becomes a better insulator. This continues until the device starts to conduct in the reverse direction (called the zener knee) and is set by the way the materials are constructed (this voltage can be set from 2 volts to 1000 volts for domestic diodes).
It doesnt take too much to figure that I now have a device that turns AC into pulsating DC or a rectifier. If I am blocking up to 1000 volts then there is no conduction in the reverse direction and a virtual short circuit in the forward direction.
OK thats it in its simplicity, do you want me to talk about power supplies?
Have a look at this video, bit long winded but explains all for you, even a section on opto.
