Semiconductors Class 12 Notes | Term 2 | Physics |

Semiconductors Class 12 Notes | Term 2 | Physics |

Semiconductors 

Semiconductors have had a big impact on our society. You see them in microprocessor chips and transistors. Anything that is computerized or uses radio waves, like your phone, depends on semiconductors.
Silicon is the main thing used to make them now. Silicon Valley and the silicon economy are names for this because it’s what any electronic device needs to work.

1. Metals : They possess very low resistivity or high conductivity.
ρ ~ 10^-2.10^-8 Ωm, σ ~10². 108 Sm^-1
2. Semiconductors : They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10^-5. 10⁶ Ωm, σ ~ 10⁺⁵ .10^-6 Sm^-1

3. Insulators : They have a high resistivity or low conductivity.

ρ ~ 10^-11. 10^19 Ωm, σ ~ 10⁺¹¹ .10^-19 Sm^-1

Types of Semiconductors 

(i) Elements Semiconductors : These semiconductors are available in natural form, e.g. silicon and germanium.
(ii) Compound Semiconductors : These semiconductors are made by compounding the metals, e.g. CdS, GaAs, CdSe, InP, anthracene, polyaniline, etc.

Energy Band : In a crystal due to interatomic interaction, valence electrons of one atom are shared by more than one atom in the crystal. Now, splitting of energy level takes place. The collection of these closely spaced energy levels are called an energy band.

Valence Band : Valence band is the energy band which includes the energy levels of the valence electrons.

Conduction Band : Conduction band is the energy band above the valence band.

Energy Band Gap : The minimum energy required for shifting electrons from valence band to conduction band is called energy band gap (E_g ).

Differences between conductor, insulator and semiconductor on the basis of energy bands are given below:

Fermi Energy : It is the maximum possible energy possessed by free electrons of a material at absolute zero temperature (i.e. 0K)

On the basis of purity , semiconductors are of two types:

(i) Intrinsic Semiconductors : It is a pure semiconductor without any significant dopant species present
n_e  = n_h =n_i 
where , n_e and n_h are number densities of electrons and holes respectively and n_i  is called intrinsic carrier concentration.
An intrinsic semiconductor is also called an undoped semiconductor or i-type semiconductor

(ii) Extrinsic Semiconductors : Pure semiconductor when doped with the impurity, it is known as extrinsic semiconductor.

Extrinsic semiconductors are basically of two types: (a) n-type semiconductors
(b) p-type semiconductors

NOTE: Both the type of semiconductors are electrically neutral.

In n-type semiconductor, majority charge carriers are electrons and minority charge carriers are holes, i.e. n_e> n_h .
Here, we dope Si or Ge with a pentavalent element, then four of its electrons bond with the four silicon neighbours, while fifth remains very weakly bound to its parent atom.
Formation of n-type semiconductor is shown below:

In p-type semiconductor, majority charge carriers are holes and minority charge carriers are electron i.e. n_h  > n_e .
In a p-type semiconductor, doping is done with trivalent impurity atoms, i.e. those atoms which have three valence electrons in their valence shell.
Formation of p-type semiconductor is shown below:

At equilibrium condition, n_e n_h = n_i²
Minimum energy required to create a hole-electron pair, hv > E_g  where, E_g  is energy band gap

Electric current, I = eA(n_ev_e + n_hv_h) where, A is area of cross-section.
where, v_e and v_h are speed of electron and hole respectively.

 p-n Junction : A p-n junction is an arrangement made by a close contact of n-type semiconductor and p-type semiconductor.

Formation of Depletion Region in p-n Junction : During formation of p-n junction, due to the concentration gradient across p and n sides, holes diffuse from p-side to n-side (p —> n) and electrons diffuse from n-side to p-side (n —> p).
This space charge region on either side of the junction together is known as depletion region.
Depletion region is free from mobile charge carriers. Width of depletion region is of the order of 10^-6 m. The potential difference developed across the depletion region is called the potential barrier.

Semiconductor Diode/p-n Junction Diode : A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage.

The direction of arrow indicates the conventional direction of current (when the diode is under forward bias).

The graphical relations between voltage applied across p-n junction and current flowing through the junction are called I-V characteristics of junction diode.

(i) Junction diode is said to be forward bias when the positive terminal of the external
battery is connected less to the p-side and negative terminal to the n-side of the diode. The circuit diagram and I-V characteristics of a forward biased diode is shown below:

The circuit diagram and I-V characteristics of a reverse biased diode is shown below.

p-n Junction : A p-n junction is an arrangement made by a close contact of n-type semiconductor and p-type semiconductor.
 Formation of Depletion Region in p-n Junction : During formation of p-n junction, due to the concentration gradient across p and n sides, holes diffuse from p-side to n-side (p —> n) and electrons diffuse from n-side to p-side (n —> p).
This space charge region on either side of the junction together is known as depletion region.
Depletion region is free from mobile charge carriers. Width of depletion region is of the order of 10^-6 m. The potential difference developed across the depletion region is called the potential barrier.

Semiconductor Diode/p-n Junction Diode : A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage.

The direction of arrow indicates the conventional direction of current (when the diode is under forward bias).
The graphical relations between voltage applied across p-n junction and current flowing through the junction are called I-V characteristics of junction diode.
(i) Junction diode is said to be forward bias when the positive terminal of the external
battery is connected less to the p-side and negative terminal to the n-side of the diode. The circuit diagram and I-V characteristics of a forward biased diode is shown below:

The circuit diagram and I-V characteristics of a reverse biased diode is shown below.

The DC resistance of a junction diode,
r_DC = V/I
The dynamic resistance of junction diode,
r_AC = ∆V/∆I
Diode as Rectifier : The process of converting alternating voltage/current into direct voltage/current is called rectification. Diode is used as a rectifier for converting alternating current/voltage into direct current/voltage.
There are two ways of using a diode as a rectifier i.e.

Diode as a Half-Wave Rectifier : Diode conducts corresponding to positive half cycle and does not conduct during negative half cycle. Hence, AC is converted by diode into unidirectional pulsating DC. This action is known as half-wave rectification.
Circuit diagram of p-n junction diode as half-wave rectifier is shown below:
The input and output wave forms have been given below:

Diode as a Full-Wave Rectifier : In the full-wave rectifier, two p-n junction diodes, D₁ and D₂ are used. The circuit diagram or full-wave rectifier is shown below:

The input and output wave forms have been given below:

Its working based on the principle that junction diode offer very low resistance in forward bias and very high resistance in reverse bias.
The average value or DC value obtained from a half-wave rectifier,

The average value or DC value obtained from a full-wave rectifier,

The pulse frequency of a half-wave rectifier is equal to frequency of AC.
The pulse frequency of a full-wave rectifier is double to that of AC.

 Optoelectronic Devices : Semiconductor diodes in which carriers are generated by photons, i.e. photo-excitation, such devices are known as optoelectronic devices.
These are as follows:
(i) Light Emitting Diode (LED) : It is a heavily doped p-n junction diode which converts electrical energy into light energy.

LEDs has the following advantages over conventional incandescent low power lamps
(a) Fast action and no warm up time required
(b) It is nearly monochromatic
(c) Low operational voltage and less power consumed
(d) Fast ON-OFF switching capability.

(ii) Photodiode : A photodiode is a special type of junction diode used for detecting optical signals. It is a reverse biased p-n junction made from a photosensitive material. Its symbol is

Its V-I characteristics of photodiode are shown below:

We observe from the figure that current in photodiode changes with the change in light intensity (I) when reverse bias is applied.
(iii) Solar : Cell Solar cell is a p-n junction diode which converts solar energy into electrical energy. Its symbol is

V-I characteristics of solar cell are shown below:

Zener Diode : Zener diode is a reverse biased heavily doped p-n junction diode. It is operated in breakdown region.

 Zener Diode as a Voltage Regulator : When the applied reverse voltage (V) reaches the breakdown voltage (V_z) of the Zener diode there is a large change in the current. So, after the breakdown voltage V_z, a large change in the current can be produced by almost insignificant change in the reverse bias voltage i.e. Zener voltage remains constant even though the current through the Zener diode varies over a wide range. The circuital arrangement is shown as follows.

Types of Basic Logic Gates

There are several basic logic gates used in performing operations in digital systems. The common ones are;

• OR Gate
• AND Gate
• NOT Gate
• XOR Gate

Additionally, these gates can also be found in a combination of one or two. Therefore we get other gates such as NAND Gate, NOR Gate, EXOR Gate, EXNOR Gate.

Also Read: Transistor

OR Gate

In OR gate the output of an OR gate attains the state 1 if one or more inputs attain the state 1.

The Boolean expression of OR gate is Y = A + B, read as Y equals A ‘OR’ B.

The truth table of a two-input OR basic gate is given as;

AND Gate

In AND gate the output of an AND gate attains the state 1 if and only if all the inputs are in state 1.

The Boolean expression of AND gate is Y = A.B

The truth table of a two-input AND basic gate is given as;

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