of pure silicon and diodes

1. Pure silicon is not a conductor because in its pure state its electrons are bonded tightly with the electrons of other neighbouring silicon atoms in such a way that it requires a relatively large increase in energy levels to cause electrons to break from the valence band into the conduction band.

2. When silicon is doped with phosphorus, the result would be that an excess of one electron is created on the valence band of the doped silicon atom. Since the valence band of an atom takes up to eight (8) electrons, there would exist one free moving electron which can easily jump into the conduction band, thus constituting flow of electrical current.
3. The main difference between a P-type silicon and an N-type silicon exists in what material was used to dope the silicon. Should the silicon undergo a Trivalent doping, using an element with a valence band consisting of 3 electrons (eg boron) there would exist one empty spot on the shell after the two elements are bonded thus leading to the formation of a P-type silicon. It is P-type (positive) because it lacks one more electron to complete its valence band.However should silicon be doped with a pentavalent element, (thus an element with 5 electrons on its valence band) there would exist an excess electron on the valence shell after the two elements bond. This excess electron, which is free moving, thus has the capacity to jump the gap into the conduction band.

4. A diode, being a device which allows electric current to flow in one direction is made up of a P-type and an N-type silicon arranged adjacent each other, leaving a thin film of space called the depletion region connected to an energy source. It is in the arrangement of the energy source in the circuit which dictates in which direction current flows.

   In a forward biased diode, there exists a positive voltage across the diode from the P to N type silicon. This enables the diode to act like a good conductor hence allowing current to flow in one direction.

   However, when the polarity in the energy source is changed so that there is almost no flow of current then one has a reverse-biased diode. This causes the diode to become almost non-conducting and measures a large amount of voltage across the devise.