Intrinsic Semiconductors

INTRINSIC SEMICONDUCTORS

The pure form of semiconductor material is known as intrinsic semiconductors, common examples of such semiconductors are pure germanium and silicon which have forbidden energy gaps of 0.72 ev and 1.1 ev respectively. The energy gap is so small that even at ordinary room temperature, there are many electrons which possess sufficient energy to jump across the small energy gap form the valence to the conduction band.

The fig 3.11 shows the two dimensional representation of crystal structure of silicon. The silicon has a total of 14 electrons in its atomic structure. Each atom in a silicon crystal contributes 4 valence electrons. The four valence electrons are shown by the dots in the figure 3.11.

The circle with +4 designation, represents the central portion of the atom, including the nucleus and the electrons except those in the outermost orbit. This portion is often known as core of the atom. These atoms are bonded together by a force, which results from the sharing of the neighbouring atoms. This bonding is called a covalent bond and is shown by a pair of curved lines in the figure 3.12

At low temperature, an intrinsic semiconductor will have exactly the ideal structure as shown in fig. In that structure, all the valence electrons are tightly held by the parent atoms and through covalent bonds by other atoms. Since the electrons are not free to move, through the crystal structure. Thus an intrinsic semiconductor at absolute zero temperature behave like insulator.

Now, let us assume that the temperature of the semiconductor is increased to room temperature. At this temperature, some of the covalent bonds are broken. This situation is shown in fig 3.13.

Here an electron, which was a part of covalent bond earlier, is now shown as a dislodged electron. This electron is free to move any where within the crystal. The energy required to break such a covalent bond is equal to the band gap energy (EG). The value of EG at room temperature is about 0.72 ev for germanium and 1.1 ev for silicon.

The vacancy of an incomplete covalent band, left behind the dislodged electron is called a hole. A combination of such a free electron and a hole is known as electron- hole pair. The process of an electron will give up its energy and fall into one of the holes in the covalent band is known as charge carrier recombination. The time between the generation of an electron hole pair and recombination is called the lifetime of the electron hole pair.

In an intrinsic semiconductor the number of holes and electrons are equal because thermal energy produces free electrons and holes in pairs. Thus there is flow of electrons and holes occur inside the semiconductor in opposite direction hence the total current in the semiconductor is equal to the sum of hole current and electron current.