Electronic Devices Test -8

In an insulator, the forbidden energy gap is very large. In general, the forbidden energy gap is more than 5eV and almost no electrons are available for conduction. Therefore, a very large amount of energy must be supplied to a valence electron to enable it to move to the conduction band.

In the case of materials like glass, the valence band is completely filled at 0 K. The energy gap between valence band and conduction band is of the order of 10 eV. Even in the presence of high electric field, the electrons cannot move from valence band to conduction band.

A pure semiconductor is called intrinsic semiconductor, e.g., silicon, germanium. The presence of the mobile charge carriers is the intrinsic property of the material. At room temperature, some covalent bonds are broken and electrons are made free. The absence of an electron in the covalent bond form hole.

The electrical conduction is by means of mobile electrons and holes. Hole act as a positive charge, because it can attract an electron. If some other bond is broken and the electron thus freed fills this hole(vacancy), it seems as though the hole is moving.

Actually, an electron is travelling in the opposite direction. In a pure(intrinsic) semiconductor, the number of holes is equal to the number of free electrons.

In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbours. Each ionic core, consisting of the nucleus and non-valent electrons, has a net charge of +4 and is surrounded by 4 valence electrons. Since there are no excess electrons or holes, In this case, the number of electrons and holes present at any given time will always be equal.

Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole balance will be changed. This impurity will only be able to contribute three valence electrons to the lattice, therefore leaving one excess hole. Since holes will "accept" free electrons, a Group 3 impurity is also called an acceptor.

p-type germanium semiconductor is formed when it is doped with a trivalent impurity atom. So it should be doped with Indium.

In a pure (intrinsic) Si or Ge semiconductor, each nucleus uses its four valence electrons to form four covalent bonds with its neighbours. Each ionic core, consisting of the nucleus and non-valent electrons, has a net charge of +4, and is surrounded by 4 valence electrons. Since there are no excess electrons or holes, In this case, the number of electrons and holes present at any given time will always be equal. Note each +4 ion is surrounded by four electrons.

Now, if one of the atoms in the semiconductor lattice is replaced by an element with three valence electrons, such as a Group 3 element like Boron (B) or Gallium (Ga), the electron-hole balance will be changed.

This impurity will only be able to contribute three valence electrons to the lattice, therefore leaving one excess hole. Since holes will "accept" free electrons, a Group 3 impurity is also called an acceptor.

A semiconductor doped with an acceptor. An excess hole is now present. Because an acceptor donates excess holes, which are considered to be positively charged, a semiconductor that has been doped with an acceptor is called a p-type semiconductor.

Doping is the process of intentionally introducing impurities into an extremely pure (i.e., intrinsic) semiconductor in order to change its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic semiconductor. A semiconductor which is doped to such high levels that it acts more like a conductor than a semiconductor is called degenerate.

Dopant atoms can either be “donors” or “acceptors.” Donors increase the electron concentration in the silicon, whereas acceptors increase the hole concentration.

When Arsenic crystal is doped with silicon atoms, then it becomes N-type.

Doping is the process of intentionally introducing impurities into an extremely pure (i.e., intrinsic) semiconductor in order to change its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic semiconductor. A semiconductor which is doped to such high levels that it acts more like a conductor than a semiconductor is called degenerate.

Dopant atoms can either be “donors” or “acceptors.” Donors increase the electron concentration in the silicon, whereas acceptors increase the hole concentration.

When Germanium crystal is doped with phosphorus atoms, then it becomes N-type.

In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have been diffused away, or have been forced away by an electric field. The only elements left in the depletion region are the ionized donor or acceptor impurities.

When p-n junction is formed, the electrons from n-region diffuse through the junction into p-region provide positive ions in n-region similarly holes from p-region diffuse into n-region provide negative ions in p-region. Due to these ions, an electric field is set up across the junction from positive ions to negative ions. This electric field sets a potential barrier at the junction.

The width of depletion layer depends on the nature of semiconductor and doping concentration of the two sides of p-n junction. If the doping concentration is small, the width of p-n junction is large and vice-versa. Thus the width of p-n junction will be of the order of10⁻⁴cm to ​10⁻⁶cm.