The video about the simulation software
Experiment 1:Don't touch the adjustments. Simply press the button "Start".
Comment:By clicking the button "Start", the program starts calculating the forces acting on the extra electrons and holes. With every step, an electron can move to a nearby atom. Firstly, the force caused by all charged particles inside of the crystal layer is determined. Charges with the same sign cause repelling forces, while those with different signs cause attracting forces. A force with a random direction and an absolute value between zero and X, representing the thermal energy of the electron is added to the sum of the electrostatic forces. The electron is moving to those nearby atom which is closest to the angle of the resulting force. To keep the computing power low, the software doesn't calculate the forces acting on all electrons of all atoms. The force acting on a positive charge inside of the crystal lattice is calculated to get the direction of the next electron transfer between a hole and a nearby atom. (See the chapter doping for details about the mechanism p-type conduction).
If an electron and a hole are side by side, they recombine to a chemical bond between two neutral silicon atoms. Those recombination process reduces the number of mobile charge carriers inside of the crystal layer.
After some minutes, an equilibrium is established and no more recombination processes occur. Stop the animation and have a look at the distribution of charges.
Experiment 2:Increase the value "External Field" and have a look at the movement of the charge carriers.
Comment:The value "External field" virtually attaches the diode to a voltage source. If the device is forward biased, the positive terminal is attached to the p-doped half and the negative terminal is attached to the n-doped half. The voltage source generates an electric field which pushes electrons from the n-doped half to the p-doped half. Vice versa, the holes are pushed from the p-doped half to the n-doped half of the crystal. If the strength of the external electric field is high enough, moving charges are pushed into the area near the junction and once more recombination processes occur.
The number of electrons respectively holes inside of the crystal is kept constant by the voltage source, because electrons are injected into the n-doped half and holes are created by the extraction of electrons inside of the p-doped half.
In sum there is a movement of electrons through the diode from the negative to the positive terminal of the voltage source, hence an electric current is running through the device.
Caused by the different speed of movement of holes and electrons, the places of those recombination processes are predominantly inside of the lower half of the crystal. By lowering the value of "Electron mobility" respectively raising "Hole mobility", the movement of electrons can be slowed down and those of the holes fastened up inside of this virtual diode. Thus, the recombination processes are located inside of the n-doped half.
Experiment 3:Switch to "Reverse polarity" and observe the movement of the mobile charge carriers at different values of the external field.
Comment:Now the electrons are pushed to the upper, n-doped half of the crystal by the action of the external electric field. Additionally, electrons are injected into the p-doped half and they are removed from the n-doped half. The injected electrons at the p-doped half are annihilated by recombination processes with the holes inside of this half, by what the number of holes decreases. The number of moving charges inside the crystal decreases. The electric field near the junction is pointing into the same direction as the external field of the voltage source - both fields inhibit the migration of holes from the p-doped half respectively electrons from the n-doped half. There are no moving charges around the junction. No current is running through the device.
Experiment 4:Leave the adjustment "Reverse polarity" and adjust the value "External Field" to 70%. Adjust "New pairs" to a value of 5%.
Comment:The value "New pair" adjusts the probability of the creation of new electron-hole-pairs inside of the crystal. Because of the thermal energy, the atoms of the crystal lattice are oscillating around their position of rest. If those energy is high enough, a chemical bond can crack and an electron-hole-pair is formed. Many of those pairs recombine after a short span of time. Near the junction, the electric fields of the depletion layer and the voltage source are pointing into the same direction. The resulting field pulls electrons into the n-doped half by what the holes are pushed into the p-doped half. The electron-hole-pairs are separated from each other by the electric field, hence an electron is moving from the negative to the positive terminal of the voltage source. If those pairs are recombining inside of one half of the crystal, no charge transfer takes place. The higher the temperature of the crystal, the more charges are generated and the higher the current running through the diode becomes, even if it is reverse biased.