Chapter 22.Semiconductor Devices

 

Unit-5 Modern Physics

Chapter 22.Semiconductor Devices Notes 

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Unit-5 Modern Physics

Chapter 22.Semiconductor Devices Notes 

PN junction

When a P-type semiconductor is joined to an N-type semiconductor, the junction is formed which is called the PN junction. The PN junction leads to the invention of diode transistors and Integrated circuits (I.C).

Depletion region and potential barrier

When the p-type semiconductor is joined with an N-type semiconductor charge carrier diffuses as spreads from the high concentration region to the low concentration region. so the free electrons from N-region diffuse into P-region and combine with the hole to get neutralized. Similarly, the holes from the P-region diffuse into the N-region and combine with electrons to get neutralized

The P-region near the junction is left with an immobile negative charge and the N-region near the junction with an immobile positive charge due to the electron-hole recombination. so, a thin layer around the junction becomes depleted of the free charge carrier. This region is called the depletion region.

Due to the presence of different charges near the junction, a potential difference is established with percent further diffusion of charge this potential is called barrier potential.

Semiconductor diode (P-N junction diode)

A device forward by connecting P-type semiconductor with N-type semiconductor called P-N junction diode. It has two terminals namely an anode and cathode. Anode refers to the p-type region and cathode refers to the N-type region the symbolic representation of the P-N junction diode is given below

Biasing of P-N junction diode

The process of applying a potential difference across the P-N junction diode for its operation is called biasing of the P-N junction. Depending upon the connection of the P-N junction diode with an external battery, there are two types of biasing.

1. Forward biasing
2. Reverse biasing

1. Forward biasing

A diode is said to be forward biasing if its p-side is connected to the positive terminal and N-side is connected with the negative terminal of the battery.

Fig: Forward biasing

In forward biasing the positive and negative terminal of the battery repels the hole towards the free electron towards the Junction these free electrons and holes at the junction combine with each other which reduces the width of the depletion region hence the potential barrier, also decreases so in forwarding biasing current will flow.

Its Characteristics

Characteristics of a junction diode are:

The graph shows the relation between bias voltage and circuit current of a Junction diode called Characteristics of a Junction diode. Its type is;

1. Forward bias characteristics
2. Reverse bias characteristics

1. Forward bias characteristics

It is a graph showing the variation of circuit current with forwarding biasing voltage. The forward voltage is gradually increased. In steps and corresponding millimeter reading is noted. A graph is plotted then between voltage and current. Practically current doesn’t flow until the forward voltage across the barrier potential. When a forward voltage is greater than barrier potential the current increases rapidly known as knee voltage.

2. Reverse bias characteristics

It is a graph showing the variation of circuit current with negative biasing voltage is gradually increased in steps and corresponding microammeter readings are noted. If the graph is plotted between voltage and current then at one point current is rapidly increased. This voltage when the current rapidly increases called the breakdown voltage.

Rectifier

A device that is used to convert ac into dc is called a rectifier and the process of conversion of ac into dc is called rectification.

When the P-N junction is forward biased it offers low resistance hence current will flow through it. But reverse biasing have high resistance and no current will flow. This property of the P-N junction is used for the rectifier.

There are two types of rectifiers

1) Half wave rectifier
2) Full wave rectifier

Full-wave rectifier

It is a device that is used to convert that full cycle of AC into DC. There are two types of full-wave rectifiers. They are

a) Central tapped full wave rectifier
b) Bridge Rectifier

a) Central tapped full wave rectifier

It is a rectifier that converts the full cycle of AC into DC. The circuit diagram of the full-wave rectifier is shown in the figure below in which the AC is applied across the primary coil of the transformer and the secondary coil is connected to the diodes D1 and D2. The load resistance R2 is centrally tapped from the middle of the secondary coil.

Fig: Center tapped full wave rectifier

During the positive half cycle, A is positive & B is negative so is the diode.

D1 ---> Reverse ( Does not conduct current)

D2 ---> Forward (Conduct current)

The current flow is D2

the waveform of input and output voltage is

Fig: Output and Input voltage in a central tapped full-wave rectifier

b) Bridge Rectifier

A Bridge rectifier is a rectifier that converts the full cycle of ac into DC. It consists of 4 diodes D1, D2, D3, and D4 in form of a Wheatstone bridge. The AC input is applied across the primary coil of the transformer and the ends of the secondary coil are connected to the two opposite ends A and C of the Wheat stone bridge network and load resistance R is connected across the two remaining ends B and D is shown in the figure below:

Fig: Circuit diagram of the bridge rectifier

During the positive half cycle: A becomes positive & B becomes negative so diodes

D₁ and D ---> forward (conductor current)

D₂ and D₄ ----> Reverse (not conductor current)

The current flow through AD₁BR_LDD₃CA

During the negative half-cycle: A becomes negative B becomes positive then diodes

D₁ D₃---> Reverse (conductor current)

D₂ D₄---> forward (not conductor current)

The current flow through CD₂BR_LDD₄AC

In this way in both cycle unidirectional current flow through load resistance so the bridge rectifier acts as a full-wave rectifier.

The waveform of input and output voltage in the case of the bridge rectifier is shown in the figure below,

Logic gate

An electronic circuit that gives a logical decision as an output signal due to the result of one or more inputs is called a logic gate. A logic gate has one or more inputs but only one output. The input and output states are on and off i.e. on for 1 and off for 0.

1) OR gate

A logic gate is said to be an OR gate if the high (1) output obtained for one or more of its inputs is high (1). If A and B are two inputs for the OR gate then a logic symbol of the OR gate is given by,

Symbol of OR gate 

input		output
A	B	Y = A+B
0	0	0
0	1	1
1	0	1
1	1	1

2) AND gate

A logic gate whose output is high (1) only if all of its input is high is called AND gate. If A and B are two inputs for AND gate then a logic symbol of AND gate is given by

Symbol of AND gate 

input		output
A	B
0	0	0
0	1	0
1	0	0
1	1	1

3) NOT gate

It is a single input and single output gate whose output is high if its input is low and vice-versa. Its symbol is 

Symbol of NOT gate 

input (A)	output (y = A)
0	1
1	0

4) NAND gate

When the output of AND gate is connected to the input of the NOT gate then such obtained gate is called the NAND gate. It gives high output if any one or all of the inputs are low the symbol of it is

Symbol of NAND gate 

input		output
A	B	Y = A.B
0	0	1
0	1	1
1	0	1
1	1	0

5) NOR gate

When the output of the OR gate is connected with the NOT gate then such obtained gate is called the NOR gate. NOT hate to give high output if all of the inputs are low its symbol is.

Symbol of NOR gate 

input		output
A	B	Y = A+B
0	0	1
0	1	0
1	0	0
1	1	0

NAND gates as universal gates

The NAND gate is a universal gate because from if all other gates are formed.