Semiconductor Fundamentals :-

→ The materials whose electrical properties lie between those of conductors and Insulators are known as semiconductors.

→ The exampels ofsuch materials are germanium (Ge), Silicon (Si), gallium arsenide (Ga As), Cadium Sulfide (Cds), lead telluride etc,.

→ At absolute zero temperature (ie at 0K) there are no elctrons in the conduction band of semiconductors and the valence band is completely filled. Thus the semiconductor behave like an insulator at 0K.

→ If the temperature is increased the width of the energy gap reduces, consequently, some of the electrons jump into condition band and semiconductors show some conductivity.

→ It is thus obvious, that the conductivity of semiconductors increases with the increases in temperture.

Co-Valent bonds :-

→ The atoms of every element are held together by the bonding action of valence electrons.

→ In semiconductors bonds are formed by sharing of valence electrons such bonds are called co-valent bonds.

→ In the formation of co-valent bond, each atom continue equal number of valence electrons and the contributed electrons are shared by the atoms engaged in the formation of the bond.

Commonly used semiconductors :-

→ The most frequency used materials are germanium (Ge), and Silicon (Si).

→ It is because then energy required to break their co-valent bonds is every small;being 0.7 eV for Ge and 1.1 eV for Silicon.

→ Germanium is an each element. It is recovered from the ash pd certain coals or from the flue dust of Zinc smelters.

→ The atomic numbers of germanium is 32. It has four valence electrons (ie) it is tetravalent element.

→ Figure 2 shows how the various germanium atoms are held through co-valent bonds.

→ As the atoms are arranged in an orderly pattern, therfore germanium has crystalline structure.

Note :-

A sunstance in which the atoms or molecules are arrenged in an orderly pattern is known as crystal. All semiconductors have crystalline structure.

Silicon :-

→ Silicon is an element in most of the common rocks. Actually, sand is silicon dioxide. The silicon compounds are chemically reduced to silicon which is 100% pure for use as a semiconductors.

→ The atomic number of silicon is 14. Silicon atom has four valence elements (ie) it is a tetravalent element.

→ Like germanium, Silicon atoms are also arranged in an orderly manner. Therfore , silicon has crystalline structure.

Silicon atom

→ Consider a pure germanium crystal. Germanium atom has four valency electron. When a small amount of pentavalent impurity is added germanium crystal, the four valence electrons wil form co-valent bonds four valence electrons of germanium atom. The fifth electron of pentavalent impurity finds no place for co-valent bond and is thus free electron.→ Thus each arsenic atom introduced on free electron. Thus a samll amount of arsenic provides enough atoms to suply millions of free electrons.→ Thus many new free electrons are produced by the addition of pentavalent impurity.→ Because of thermal energy a few hole-electron pairs are created. But the number of free electrons exceeds the number of holes.

n – type conductivity :-

The current conduction in an n –type semiconductor material is by free electrons (ie) negative changes and is called n – type or electron type conductivity.→ When p.d is applied across the n – type semiconductor, the free electrons are directed towards the positive terminal, conssitutes electric current. As the current flow through the crystal is by free electrons which are carriers of negative charge, therfore this type of conductivity is called negative (or) n – type conductivity.

p – type Semiconductor :-

→ When a small amount of trivalent impurity is added to a pure semiconductor, it is called p – type semiconductor.→ The addition of trivalent impurity provides a large number of hloes in the semiconductor material.→ Typical examples of trivalent impurities are gallium and indium.

→ The impurities which produce p – type semiconductor are known as acceptor impurities. Because the holes created can accept the electrons.→ To explain the formation of p – type semiconductor, consider a pure germanium crystal, when a small amount of trivalent impurity is added, it forms only there co-valent bonds with germanium atom.→ But the fouth co-valent bond is incompelete, for this bond germanium atom only provides one valence electron.→ Hence the missing electron is treated as a hole. Thus each gallium atom introduced will give one hole. Thus a small amount of gallium provides millions of holes ;

p – type Conductivity :-

→ The current conduction in a p – type semiconductor material is mainly because of holes (ie) posotive changes and is called p – type (or) hole type conductivity.