- Semiconductors have intermediate energy gape. In their pure (intrinsic) state, semiconductors are neither good conductors nor good insulators.
- Semiconductors do not used in their pure state because at room temperature very few electrons can jump the energy gap to the conduction band and become free electrons that causing conduction current.
- To effectively increase the conductivity of semiconductors we have to add impurities to intrinsic semiconductors to increase the free electron in conduction band.
- In N-type semiconductors, penta-valent impurity atoms (atoms with five electrons in the valence band) are added to intrinsic semiconductors. Four electrons of these five are bonding with different four atoms of semiconductors substance leaving one free electron.
- While for P-type semiconductors, tri-valent impurity atoms (atoms with three electrons in the valence band) are added to intrinsic semiconductor. These three electrons are bonding with three semiconductor’s atoms. The fourth semiconductor adjacent atom find no valence electron to share but instead imaginary hole (same but positively charge as electron).
- When the N and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the junction so some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.
- However, because the electrons have moved across the junction from the N-type silicon to the P-type silicon, they leave behind positively charged donor ions (ND) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region were there are large numbers of free electrons.
- As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions (NA), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.
- The significance of this built-in potential across the junction, is that it opposes both the flow of holes and electrons across the junction and is why it is called the potential barrier.
- In practice, a PN junction is formed within a single crystal of material rather than just simply joining or fusing together two separate pieces.
- Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be made to an external circuit.
- Then the resulting device that has been made is called a PN junction Diode or Signal Diode.
- If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
- By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased.
There are two types of semiconductors; N-type and P-type.
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