Semiconductors can be considered as materials that have an electrical conductivity ranging between that of an insulator like glass and a good conductor like metal. These group of materials like Germanium and Silicon are neither good conductors nor good insulators. They have an important property in that the conductivity (or resistivity) of semiconductor materials can be changed to a very large extent by introducing impurities in the crystal structure. The addition of impurities for changing the electrical properties of a semiconductor is called ‘doping’. Semiconductors also have a negative temperature coefficient of resistance, i.e. if we raise the temperature of a semiconductor material, its resistivity decreases in opposition to the behaviour displayed by metals. To understand this property, we need to understand the concept of band gap. Let us look at the atomic structure of semiconductor materials as a precursor to that.
Electrons are filled in the atomic orbitals according to Aufbau’s principles in the order 1s,2s,2p,3s,3p,4s,3d,4p,5s etc.
Here, s,p,d,f etc. are the sub-shells of the orbitals K,L,M,N etc. We’ll use the shell numbers 1, 2, 3, 4,5,6,7.
The atomic structure of Silicon is given below.
There are 14 electrons in a Silicon atom. Two in the first shell, eight in the second, and four in the third. Since the ‘valence shell’ is the outermost shell of an atom, we can say Si has four ‘valence electrons’. Germanium has an atomic number 32, and the electron configuration is 2 electrons in the first shell, 8 in the second shell, eighteen in the third shell, and four in the outermost shell. Thus germanium also has four valence electrons and shares some properties with Si.