Semiconductors have valence shells that are exacly half full and can conduct electricity under some conditions but not others. This property makes them useful for the control of electrical current.

A semiconductor is a material that has a limited ability to conduct electrical current with conductivity between that of an insulator and that of a conductor. Silicon, a cheap and abundant material, is the most used semiconductor material although other materials are used in the electronics components made from semiconductors. The primary advantages of a semiconductor over a conductor is that the conductivity of a semiconductor can be varied under an external electrical field giving engineers precise control over complex circuits and, unlike conductors like metals, a semiconductor's conductivity increases with increased temperatures.

All conductors have resistance and the amount of resistance varies with the element. In general, metals make the best conductors of electricity and non-metals make the worst conductors of electricity.

The purpose of a capacitor is to retain electric charge and it will do so even after its input voltage is removed. After the input voltage is removed, the capacitor will slowly discharge but, depending on the size and characteristics of the capacitor, discharging could take from a few minutes to a few years. So, it's never safe to touch a capacitor with your hands and you should make sure to discharge any capacitors in a circuit before working on it using an appropriately high-wattage resistor or a capacitor discharge tool.

Semiconductors have valence shells that are exacly half full and can conduct electricity under some conditions but not others. This property makes them useful for the control of electrical current.