Does not mater how powerful a electrical devices can be, at the time when electric current does not flow throw it, it dies and gets its life back when electricity flows. Everything in the electrical world survives with the flow of electron. If we can somehow control the flow of electron it would be a great deal in terms of electronics. But unfortunately the copper wire or other conductive maters that we use, we can not possibly change its conductivity level according to our needs.
To make electricity flow we need suitable maters like Cu, Ag. These atoms has only one electron at its last shell which is not a unstable form (neither it has a pair nor it fulfills duplate/octave) and this electron acts as if it is not bound to any atom at all. This free electron moves around the mater freely, that makes the conductor really conductive as well as stable. But we may need a little amount of electrons to flow, but conductors atom is already in a stable form (full filled octave) so what can be done? We need a thing where we can change its conductivity what we want.
Semiconductors can be its solution! Semiconductors like Si, Ge, has 4 electrons at its last shell, usually Si stays in a lattice (so do Ge). Silicon atom has four electrons which it can share in covalent bonds with its neighbors.
You may argue that Semiconductor has no free electrons like Conductor does, so how can it be a solution? and your argument is true as it is intrinsic Si or Ge lattice, it can conduct very little amount of current. To make it a SOLUTION we need few alloy of impurity to add with it.
If we can replace a Si (of 4 electrons in outer shell) with a B (3 electron at its outer shell), so now both in Si and B’s outer shell we have 7 electrons, 1 more electron is needed for its stable form. So we can say it as a hole. This is called Positive-type or P-Type Semi conductor. So when ever current is passed through this, first of all it will fill up the holes and then rest will go. This way we can reduce conductivity. One more thing to be mentioned, usually B2H6 gas to diffuse boron into the silicon material.
If we can replace a Si (of 4 electrons in outer shell) with a Sb (5 electron in outer shell), so now both in Si and Si’s outer shell we have 8 electrons, which is stable but Sb has 9 electron, which is 1 more than the stable form and for the sake stability it can easily be free. So basically we are getting one more free electron. This is called Negative-type or n-type Semi conductor. So when ever current is passed through this, this free electron will flow. This way we can increase conductivity. One more thing to be mentioned, Phosphorous may be added by the diffusion of PH3 gas.
So adding a little percentage of foreign atoms in the crystal lattice of silicon/germanium produces a dramatic change in their electrical properties, producing n-type and p-type semiconductors and this is known as doping. Depending on the conductivity we want it to be it takes 6-12 hour and temperature of above 600° K.
By combining p and n type we, can make a p-n junction, which also illustrate new properties.
P-type has huge Holes (deficiency of electron), and on the other side N-type has huge electrons. So leaving a hole in N-type few of the electron gradually moves to P-type (which is known as diffusion). It makes P-Type negative and on the other hand due to that hole N-type become positive. But as long as electromagnetic coulomb’s force created through the movement of electron, it opposes the next electron to move. This newly observed electron and holes are considered as Depletion zone. After a certain time it reaches its Equilibrium. The more the size of this Depletion zone increases the less electron it flows. This is what happen when p-n junction is not connected with any voltage sources. It is also known as Zero-Bias.
Now when we connect a positive Voltage source with P-type and make N-type to the ground then electron and holes move toward the junction, the distance between depletion zone decreases and so do coulombs force. This applied voltage helps to overcome coulomb force and as electrons and so with a little resistance electron will be forwarded and this is also known as Forward Bias.
But if we connect a negative Voltage source with P-type and make N-type to the ground then electron and holes pulled away from the junction. As holes and electrons increases, the distance between depletion zone increases and so do coulombs force. So this increases the voltage barrier causing a high resistance to the flow of charge carriers, only minimal electric current can cross this p–n junction. This is called Backward Bias.
So after a certain Voltage, Forward Bias works like closed circuit and Backward Bias works like Open Circuit…