Intrinsic Semiconductors

Intrinsic Semiconductors

A sample of Semiconductors in its purest form is called an Intrinsic Semiconductor. The impurity content in Intrinsic Semiconductor is very very small, of the order of one part in 100 million parts of Semiconductor.

1. Crystal Structure of Intrinsic Semiconductor

• Consider an atomic structure of an Intrinsic Semiconductor material like silicon. An outermost shell of an atom is caple of holding eight electrons. It is said to be completely filled and stable, if it contains eight electrons. But the outermost shell of an Intrinsic Semiconductor like silicon has only four electrons. Each of these four electrons form a bond with another valence electron of the neghbouring atoms. This is nothing but sharing of electrons. Such bonds are called covalent bonds. The atoms align themselves to form a three dimensional uniform pattern called a crystal.

The crystal structure of germanium and silicon materials consists of repetitive accurrence in three dimensions of a unit cell. This unit cell is in the form of a tetrahedron with an atom at each vertex. But such a three dimensional structure is used to represent a three dimensional crystal form, as shown in the Fig. 0.4.1 (a).

• The Fig. 0.4.1    (a) shows two dimensional representation of a germanium crystal structure. Germanium has a total of 32 electrons. So its first orbit consists of 2 electrons, second consists of 8, third consists of 18 and the valence shell consists of 4 electrons. As there are 4 valence electrons, it is called tetravalent atom.

• The covalent bonds are represented by a pair of dotted lines encircling the two electrons forming the covalent bond. The more clear understanding of the covalent bonds can be obtained from the Fig. 0.4.1 (b) which shows the sharing of valence electrons. Both the electrons are shared by the two atoms. Hence the outermost shell of all the atoms is completely filled and the valence electrons are tightly bound to the parent atoms. No free electrons are available at absolute zero temperature. 

Such an intrinsic semiconductor behaves as a perfect insulator at absolute zero temperature.

2. Electrons and Holes in Intrinsic Semiconductors

• Let us see what happens at room temperature. At room temperature, the number of valence electrons absorb the thermal energy, due to which they break the covalent bond and drift to the conduction band. Such electrons become free to move in the crystal as shown in the Fig. 0.4.2 (a).

• Once the electrons are dislodged from the covalent bonds, then they become free. Such free electrons wander in a random fashion in a crystal. The energy required to break a covalent bond is 0.72 eV for germanium and 1.1 eV for silicon, at room temperature.

• When a valence electron drift from valence to conduction band breaking a covalent bond, a vacancy is created in the broken covalent bond. Such a vacancy is called a hole. Whenever an electron becomes free, the corresponding hole gets generated. So free electrons and holes get generated in pairs. The formation of electron-hole pair is shown in the Fig. 0.4.2 (b) while the corresponding energy band diagram is shown in the Fig. 0.4.2 (c). Such a generation of electron-hole pairs due to thermal energy is called thermal generation.

• The concentration of free electrons and holes is always equal in an intrinsic semiconductor. The hole also serves as a carrier of electricity similar to that of free electron.

An electron is negatively charged particle. Thus a hole getting created due to electron drift is said to be positively charged.

3. Conduction by Electrons and Holes

• When battery is applied to an intrinsic material, free electrons move towards positive of battery and responsible for the flow of current. This current is called an electron current. The free electron move in conduction band.

• Now consider three atoms of Ge in which one of the atom has a hole. Now hole is positively charged. Then valence electron of atom 2 tries to occupy hole of atom 3. When it does so, the hole now gets shifted to atom 2.

• Again valence electron of atom 1 fills the hole of atom 2 and hole further moves to atom 1.

• Thus valence electrons move to fill the holes and holes move in opposite direction to valence electrons. The holes keep on moving towards negative of battery. No free electrons are involved in this action.

• As holes are positively charged, the movement of holes cause the current called hole current. The hole current is in opposite direction to that of an electron current.

4. Conventional Current

• The current which flows from positive of battery to the negative of battery, external to the battery is called a conventional current.

• It is always the combined effect of electron and hole movement in the semiconductors.

• The conventional current direction is opposite to the direction of movement of free electrons while it is same as the direction of movement of holes, in the semiconductors.

• Most of the electronic appliances, devices and circuits are analysed using the flow of conventional current.

Why intrinsic materials are not used ?

• The conductivity of an intrinsic semiconductor at room temperature is very small. Very few electron-hole pairs get generated at room temperature. Similarly conductivity due to application of voltage also has very little practical significance. Hence the intrinsic semiconductors in the purest form is not used in practice for manufacturing of electronic devices.