carrier concentration in the pn jun
You already know these things but I guess it doesnt hurt to review.
The PN junction diode has three regions of importance. The P doped side, the N doped side and the PN junction. Since the mass of the proton is 1000X than that of the electron, the electron will in almost all situations be the carrier of charge or transmitter of charge. This isnt actually the case because protons dont move, but rather holes or free spaces in the outer most covalent bond arrange themselves to shuffle along a charge or the opposite which is the absence of one( or a collection of charge)
Anyways if a positive charge is applied to the P terminal, ionization or polarization will occur. Meaning electrons within the P type will want to diffuse to the positive charge and positive charges or holes will want to diffuse the other way(that is toward the PN junction). With a collection of holes or carriers as we shall call them along the PN junction on the P type side, we now have a greater concentration of carriers present in this region. than the originally doped concentration. Similarily, the same type of mechanism is occuring on the N type side. That is electrons will accumulate along the PN junction on the N type side. This is called the accumulation region.
If you havent already noticed + one side, - other side = capacitance, or specifically the PN junction capacitance in this example.
Well as charges accumulate in this region, the capacitance increases. And if the voltage is increased along the P type terminal an inversion layer is created along the PN junction. When the voltage reaches a high enough potential to give the electrons the energy to cross the PN junction. This energy or voltage is related to alot of things like bandgap energy, junction capacitance, doping concentrations, temperature, etc. But usually it is around 0.6 - 0.7 volts. At this voltage the diode is considered on, forward biased, or conducting.
When reversed biased a normal diode does not conduct, unless it is a breakdown diode which operates in the breakdown region, which is a high enough negative voltage or bias that will allow reverse current to flow.
As the voltage is increased, carrier concentrations and will allow conduction in a similar manner as forward bias for conduction to occur. Usually you dont see the same i-v (current-voltage) characteristics in reverse bias as you do forward bias. Normally there is a small reverse leakage current called Irs reverse saturation current.
Reverse bias diodes are known as Zener diodes.
Most of what I just typed seems right but I havent done this stuff for years so theres a good chance something is wrong here. I may have even flip-flopped the P and N regions. But in general this is a good ideal how diodes work, and since transistors work on the same principles of the PN junction diode, this should give you an idea of how transistors work, which are somewhat back to back PN junctions.
Good luck and if any corrections need to be made someone please do so.