Introduction of Semiconductor

Electron Devices

Chapter 1: Semiconductor

Introduction

• The smallest particle of matter is an atom which is made up of three fundamental particles,

i) Neutron

ii) Proton

iii) Electron

• The proton is positively charged, an electron is negatively charged while the neutron is uncharged, i.e. electrically neutral.

• According to to Bohr's theory, the atoms planetary type structure. All protons and neutrons are bound together at the centre of an atom which is nucleus.

• The electrons revolve around the nucleus in different orbits or shells. Each shell contains maximum number of 2n² electrons where n is number of shell. The exception to this is that the last shell contains maximum 8 electrons irrespective of its number.

• The electrons in the first orbit are closest to the nucleus and under tremendous force of attraction by the nucleus.

• The last shell electrons are loosely bound to the nucleus. Such shell is called valence shell and electrons in this shell are called valence electrons.

• Each shell has energy level associated with it. The energy level of first shell is lowest and that of last shell is highest.

• When atom absorbs energy from heat or light, the electrons in valence shell absorb this energy and become free from the attraction of nucleus. Such electrons are called free electrons and are responsible for the conduction. More the number of free electrons, better is the conductivity.

Energy Bands

• The energy levels of electrons in each orbit merge into each other to form an energy band.

• The energy levels of valence electrons merge into each other to form a valence band.

• When a valence electron absorbs energy, it becomes free electron. The energy levels of all the free electrons merge into each other conduction band.

• An energy band which separates the conduction band and the valence band is called forbidden band or forbidden gap.

• The electrons in valence band are still under the force of attraction of nucleus. The electrons in the conduction band are free electrons which are free from the force of attraction of nucleus.

• No electron exists in the forbidden band. The energy difference between energy of conduction band and valence band is called forbidden gap energy denoted as E_G.

• This E_G represents amount of energy required to be given to electrons of valence band to transfer them to the conduction band.

• The graphical representation of energy bands is called energy band diagram which is shown in Fig. 1.1.1.

• The energy gap E_G is measured in the unit electron‒volt (eV).

Classification based on Energy Band Diagram

• Based on energy gap E_G the materials are classified as conductors, insulators and semiconductors.

• In conductors, large number of free electrons exist at normal room temperature. So E_G does not exist. The valence and conduction bands are overlapped. This is shown in Fig. 1.1.2 (a). The examples are copper, aluminium, silver etc.

• In insulators, the E_G is very high about 7 eV. Hence at very high voltage or temperature also, the electrons cannot move from valence to conduction band. Hence these materials can not conduct at all and called insulators. This is shown in Fig. 1.1.2 (b). The examples are wood, mica, paper etc.

• In semiconductors, at absolute zero temperature. (‒273 °C), the conduction band is empty and they behave like perfect insulators. The energy gap E_G is about 1 eV. At normal temperature, some electrons move from valence to conduction band hence few free electrons are available. Hence materials can conduct partially. At room temperature E_G = 1.12 eV for silicon and 0.78 eV for germanium. As temperature increases the E_G decreases and large number of free electrons are available. This is shown in Fig. 1.1.2 (c). The examples are silicon and germanium.

Structure of Silicon and Germanium

• The atomic number of silicon is 14 while that of germanium is 32.

• Fig. 1.1.3 (a) shows the structure of silicon while Fig. 1.1.3 (b) shows the structure of germanium.

• In both the materials, there are 4 valence electrons.

Review Question

1. Explain the classification of materials based on energy band diagram.