Ham radio exam preparation: Semiconductor Diodes and Transistors

 We discussed conductors and insulators earlier. There is another group known as semiconductors like germanium and silicon which are bad conductors of electricity in their natural form. If some impurities like boron or antimony are added in minute quantities to these they can have either an increase in free electrons or a deficiency of electrons. Then they become semiconductors. The process of adding impurities is known as doping. When there is a deficit of electrons, it is called a P-type semiconductor. They have positively charged 'holes' as charge carriers. N-type semiconductors on the other hand have excess electrons as charge carriers. Silicon has four electrons in the outermost shell. When it is doped with boron having only three electrons in outer shell, it becomes a P-type semiconductor, with deficit of electrons or 'holes'. Doping with antimony which has five electrons in outer shell makes it an N-type semiconductor, with an excess of electrons.

PN junction is constituted by a combination of P-type and N-type semiconductors in a diode. Current flows across the PN junction only when the positive pole of the battery or voltage source is connected to the P-type semiconductor. This situation is known as forward bias. Positively charged holes are repelled by the positive terminal of the battery. Similarly electrons are repelled by the negative terminal of the battery. They cross the PN junction and combine with each other. For this crossing to occur, there should be at least 0.7V for silicon junctions and 0.3V for germanium junctions. At lower voltages, there is a depletion zone of carriers at the junction. This causes a current to flow across the PN junction when it is forward biased. In the reverse biased situation, no current flows across the PN junction. That is how a semiconductor diodes functions as a rectifier, converting alternating current into pulsating direct current, as it cuts off one half of the AC.