Semiconductors are the substances whose conductivity lies between the conductors and insulators e.g. Germanium, Silicon, etc. These elements are members of the fourth group of the periodic table with valency 4. These elements have a partially filled conduction band and partially filled valence band. There are no free electrons for conduction in semiconductors at low temperature (absolute zero). Thus germanium crystal acts as an insulator at absolute zero. As the temperature increases, the width of the energy gap reduces and some electrons jump to the conduction band. Thus the conductivity of a semiconductor increases with the increase in the temperature.

Types of Semiconductors:

Depending upon the working semiconductors are classified into two types.
a) Intrinsic semiconductors and b) Extrinsic semiconductors

Intrinsic Semiconductors:

A semiconductor which is in extremely pure form is called an intrinsic semiconductor. e.g. Germanium, Silicon.

The crystal structure of these elements consists of regular repetition in three dimensions of a unit cell having the form of a tetrahedron, with one atom at each vertex. Consider a semiconductor like germanium having valency four. Germanium atom has four electrons in its outermost shell. Germanium has a crystalline structure in which each atom of germanium shares its valence electrons with four neighboring atoms forming four covalent bonds. The covalent bonds are strong bonds. Thus there is no free electron for conduction in germanium at low temperature (absolute zero). Thus germanium crystal acts as an insulator at absolute zero. A two-dimensional representation is as shown below.

At room temperature, the thermal energy of some electrons increases and they are set free. Thus the crystal shows a small conductivity.

Extrinsic Semiconductors:

The crystal of intrinsic semiconductors shows a small conductivity. The conductivity of semiconductors can be increased by adding a small quantity of some impurity in the pure crystal of the semiconductor. This process is called doping. The ratio of impurity is very low i.e. 1 atom of impurity for every 106 to 1010 atoms of semiconductors. These atoms of impurities are
so less that they do not affect the crystal structure of the semiconductor.

Generally, trivalent or tetravalent elements are added as impurities to semiconductor crystal. Depending upon the impurity the semiconductors are classified into two types a) p-type semiconductor and b) n-type semiconductor

Classification of Extrinsic Semiconductors:

Depending upon the impurity the semiconductors are classified into two types a) p-type semiconductor and b) n-type semiconductor

P-Type Semiconductors:

At absolute zero the conductivity of germanium crystal is zero. At room temperature, germanium shows a small conductivity. To increase the conductivity of germanium crystal small quantity of some impurity is added to it. This process is called doping.
Let us consider that the germanium is doped with an element from the third group say boron (trivalent impurity). Boron has three valency
electrons. Therefore, boron can form only three covalent bonds with neighboring germanium atoms. One of the covalent bonds around each boron atom has an electron missing. The absence of an electron is called a hole. This impurity is called acceptor impurity.
Under the action of an electric field, an electron from a neighboring completely filled covalent bond jumps into this hole creating a hole in the bond from which electron has moved. The process is repeated continuously. Thus the hole appears to move through the crystal from positive end to a negative end. Thus the conductivity of doped germanium increases.
The absence of an electron in the hole means the presence of a positive charge. Hence the doped material is called p-type semiconductor.

Characteristics of p-Type Semiconductors:

• In p-type semiconductors, doping is done with trivalent impurity i.e. impurity from the third group of the periodic table.
• The impurity in the p-type semiconductor is called the acceptor impurity.
• Each atom of impurity creates a hole in the crystal.
• The electrical conductivity is due to the hole.
• When a potential difference is applied across the p-type of semiconductor, the holes appear to move from positive end to a negative end.
• In p-type semiconductors holes are the major charge carriers.
• Example: Germanium crystal doped with Boron.

n-Type Semiconductors:

At absolute zero the conductivity of germanium crystal is zero. At room temperature, germanium shows a small conductivity. To increase the conductivity of germanium crystal small quantity of some impurity is added to it. This process is called doping.

Let us suppose the germanium is doped with an element from the fifth group say phosphorous (pentavalent impurity). Phosphorus has five Valency electrons. Therefore, phosphorous can form four covalent bonds leaving one free electron unbonded. Due to pentavalent doping the number of free electrons increases. This impurity is called the donor impurity.

Under the action of an electric field, free-electron around phosphorous moves through the crystal from the negative end to a positive end. Thus the conductivity of doped germanium increases.

The presence of an electron means the presence of a negative charge. Hence the doped material is called an n-type semiconductor.

Characteristics of n-Type Semiconductors:

• In n-type semiconductors, doping is done with pentavalent impurity i.e. impurity from the fifth group of the periodic table.
• The impurity in the n-type semiconductor is called the donor impurity.
• Each atom of impurity leaves one free electron in the crystal.
• The electrical conductivity is due to electron set free by the electron.
• When a potential difference is applied across n-type semiconductors, the electrons move from a negative end to a positive end.
• In n-type semiconductors, electrons are the major charge carriers.
• Example: Germanium crystal doped with Phosphorous.

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When a crystal of semiconductor is doped such that half the portion of the crystal is doped with a trivalent impurity and the other half is doped with a pentavalent impurity, then the junction (boundary) formed is called a PN junction.

The crystal itself is called a semiconductor diode or a solid-state diode. Due to such doping, one side acts as a p-type semiconductor and the other side acts as an n-type semiconductor.

Depletion Layer and Barrier Potential in PN Junction:

At the ordinary condition, the holes from the p region diffuse into n-region and electron from n- region diffuses into p-region. Thus a thin layer of the positive ion is formed in n-region near the junction and a thin layer of negative ions is formed in p-region near the junction. This thin layer formed near the junction is called the depletion layer or potential barrier

The negative layer on the p-side opposes the further flow of electrons and the positive layer opposes the further flow of holes. The depletion layer has negative potential on the p-side and a positive potential on the n-side which acts as a battery.

Biasing of PN Junction:

Forward Biasing:

When the p-region of a semiconductor diode is connected to the positive terminal of a battery and the n-region of the semiconductor diode to the negative terminal of a battery then the junction is said to be forward-biased. In this case, the electrons will be pulled towards the positive terminal of the battery and holes towards the negative terminal of the battery and thus electronic current flows in the circuit as shown in the figure.

Reverse Biasing:

Let us consider that the p-region of a semiconductor diode is connected to the negative terminal of a battery and the n-region of semiconductor diode to the positive terminal of a battery then the junction is said to be reverse-biased. In this case, the electrons will be pulled towards the positive terminal of the battery and holes towards the negative terminal of the battery and thus electrons and holes move in the opposite direction away from the junction. Due to the nonavailability of charge carriers, electronic current does not flow in the circuit.

The Use of PN Junction as Diode:

The action of the P-N junction is similar to that of a vacuum diode. It allows the flow of electric current through it when it is forward biased and does not allow the current to flow through it when it is reverse biased. Thus p-n junction allows electric current to flow in one direction only. Thus p-n junction can be used as a rectifier.

A rectifier is electronic device which converts electrical alternating quantity (A.C.) into electrical direct quantity (D.C.)

Use of P-N Junction as Half Wave Rectifier:

A rectifier is electronic device which converts electrical alternating quantity (A.C.) into electrical direct quantity (D.C.) flows in the direction from A to B through load RL.

Circuit Diagram:

The A.C. voltage which is to be rectified is applied across the terminals P1 and P2 of primary of the transformer. The secondary terminals S1 and S2 of the transformer are connected in series with semiconductor diode D and load resistance RL.

Working :

The alternating signal voltage to be rectified is applied across the junction. The alternating signal provides opposite kinds of bias voltage at the junction in each half cycle i.e. if the diode is forward biased in the first half, it is reversed biased in the second half of the cycle.

During the positive half of the input a.c. signal the terminal S1 is positive and terminal S2 is negative, the diode gets forward biased so that the current flows in the direction from A to B through load RL. During the negative half of the input a.c. signal the terminal S1 is negative and terminal S2is positive, the diode gets reverse biased so that there is no flow of current through the load resistance.

Thus, there is a flow of current through RL from A to B during the positive half of a.c. signal only and no current flows during the negative half cycle thus the signal gets rectified. A unidirectional voltage is developed across the output load. As half the input cycle of a.c. signal is used and rectified the arrangement is called a half-wave rectifier.

Graphical Representation:

Variation of A.C. input voltage and D.C. output voltage with time is as shown in the following graphs.

Use of PN Junction as Full Wave Rectifier:

A rectifier is an electronic device which converts electrical alternating quantity (A.C.) into electrical direct quantity (D.C.)

Circuit Diagram:

The A.C. voltage which is to be rectified is applied across the terminals P1 and P2 of primary of a center-tapped transformer.
The secondary terminals S1 and S2 of the transformer are connected to P side of semiconductors diode D1and D2respectively. The n side of the semiconductor diodes is connected together and their common point is connected to center tap through the load resistance RL.

Working :

The alternating signal voltage to be rectified is applied as shown in the circuit diagram. The alternating signal provides opposite kinds of bias voltage at the junction in each half cycle i.e. if the diode is forward biased in the first half, it is reversed biased in the second half of the cycle.
During the positive half of the input a.c. signal the terminal S1 is positive and terminal S2 is negative, the diode D1 gets forward biased and diode D2 gets reverse biased. so that the diode D1 conducts current and the current flows in the direction from A to B through the load RL. During this time the diode D2 remains non conducting.
During the negative half of the input a.c. signal the terminal S1 is negative and terminal S2 is positive, the diode D1gets reverse biased and diode D2 gets forward biased. so that the diode D2 conducts current and the current flows in the direction from A to B through the load RL. During this time the diode D1 remains non-conducting.
Thus, there is a flow of current through RL from A to B during both the half cycles of a.c. signal, thus the signal gets rectified. A unidirectional voltage is developed across the output load. As full input cycle of a.c. signal is used and rectified the arrangement is called a full-wave rectifier.

Graphical Representation:

Variation of A.C. input voltage and D.C. output voltage with time is as shown in the following graphs:

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