The recent evolution in the field of electronics is mainly due to a special kind of material through which lots of achievements have been accomplished. Today we are going to give you a brief about those precious material called Semiconductors. Almost 20 years before we had a different kind of vacuum tubes which were available in different shapes and size in electronics industries. Vacuum tubes were good for that time but the introduction of semiconductors have changed the scene completely. Semiconductors possess light weight, small size, simple in construction, reliable, effective, low cost and produce low heat.
A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors e.g. germanium and silicon etc. In other words that material which opposes the current with 102-ohm × cm is called semiconductor. Silicon material is used the most in industries but there are carbon, germanium, and gallium arsenide also available. According to the energy bands bases at room temperature, these materials possess empty conduction band and partially filled valence band. In between these bands, there is a small energy gap (equal to 1ev). These materials resistance decreases with increase in temperature because it squeezes the energy gap. This is why semiconductor material is known as negative temperature coefficient.
- Some important terms used while studying semiconductors.
Those electrons which revolve around the nucleus in the last orbit/shell are called valence electrons. The atom has become stable when valence electrons are completed in their last shell, there are only 8 electrons can be accommodated in the valence shell. Once it is completed not a single electron can emit or enter into the valence shell. There are 4 electrons in the valence shell of silicon and germanium. Silicon is discovered in 1823 while germanium discovered in 1886.
Those paths around a nucleus of an atom in which electrons revolve are called energy shells. The electron which is at a distance from the nucleus will have high energy compare to that electron which is in a shell closed to the nucleus. These electrons are strongly connected in force with nucleus which is close to the nucleus. When atom gain energy it jumps into the higher shell away from the nucleus but when atom loss energy it moves back to its original shell. Electrically an atom is neutral due to the same number of electrons and protons present in an atom.
Covalent bonds or Atomic bonding
As we know that a silicon atom contains 2 electrons in its first shell, 8 electrons in second and 4 in the valence or third shell. This is true for a single atom but when few atoms are combining together they make a solid automatic arrangement called crystalline structure. This arrangement happens due to covalent bonding. A covalent bond is that type of bonding in which two or more atoms share valence shell electrons with each other. As we know that an atom is electrically stable but unstable chemically. Both same atoms need to be stable so they share their electrons.
Free Electrons and Holes
When the ambient temperature is increased from absolute zero value, heat energy is been produce there which in result produce vibration in silicon/germanium crystal. Often as a result of thermal energy, the excited electron emit from their valence shell. This election is now called free electron because it is under no effect of a nucleus. The emitted electron leaves a vacancy which acts like positive charge. This process happens in every atom which collectively gives a large number of free electrons and holes.