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Semiconductors and its Applications

There are several ways of defining a semiconductor. Historically, the term semiconductor has been used to denote materials with a much higher conductivity than insulators, but a much lower conductivity than metals measured at room temperature. Today there are two more types of conductors: superconductors and semimetals. This definition is not complete. Metals and semiconductors are distinguished by its conductivity. In the case of metals and semimetals, they regain its conductivity at low temperatures. But in the case of semiconductors, at low temperature it become insulators. At low temperature, both insulators and semiconductors are one class of materials. These are different from metals and semimetals. These classification is based on the energy gap of these materials.  


Basics of Semiconductors

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The materials such as copper, aluminium etc. are good conductors of electricity. While the materials such as wood, glass, mica etc. are very bad conductors of electricity and are called insulators. There is another class of materials, whose conductivity that is ability to carry electricity lies between that of conductors and insulators. Such materials are called semiconductors. Germanium and Silicon are two well known semiconductor materials. To understand how diodes, transistors, thyristors and integrated circuits work, it is necessary to know the basic physics behind the behavior of semiconductor materials.

Typical elemental semiconductors are germanium and silicon. An inspection of the periodic table of elements reveals that these materials belong to the fourth group while typical metals such as the alkalis are in the first group and typical nonmetals such as the halogens and the noble gases which crystallize at low temperatures are in the seventh and eighth group, respectively. Typical compound semiconductors are the III-V compounds such as galium arsenide, and indium antimonide and the II-VI compounds such as zinc sulfide. They crystallize in the zinc blende structure which can be obtained from the diamond structure by replacing the carbon atoms alternatively by zinc and sulfur atoms. 

Semiconductor Devices

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Semiconductor devices form the foundation of modern electronics, being used in applications extending from computers to satellite communication systems. A wide variety of devices are available, fabricated from a range of semiconductor materials. The most common active devices found in electronic systems include bipolar and field effect transistors, diodes, thyristors and triacs, although a large number of more specialized types, such as microwave optoelectronic devices are used in many applications. Silicon is the most commonly used semiconductor material for both discrete and integrated devices, although other materials such as gallium arsenide and indium phosphide are becoming more common for specific applications.

Applications of Semiconductor Devices

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Semiconductor devices have number of applications. Some of the important applications of semiconductor devices are described in this section.

i) P-N Junction Diode as a Rectifier
The function of a rectifier circuit is that to convert a.c signal to d.c signals using p-n junction diodes. The peculiarity of a p-n junction diode is that it conduct only in one direction. If a diode is in forward biased condition, it will conduct, otherwise it will not. If a pulsating a.c signal is applied to the diode, only during positive half cycle it will conduct. Thus p-n junction diode subjected to an a.c voltage acts as a rectifier converting alternating voltage to a pulsating d.c voltage.

ii) Transistor as an Amplifier
Transistor is a three terminal device: Base, emitter and collector, can be operated in three configurations common base, common emitter and common collector. According to configuration it can be used for voltage as well as current amplification. The input signal of a small amplitude is applied at the base to get the magnified output signal at the collector. Thus provides an amplification of the signal. The amplification in the transistor is achieved by passing input current signal from a region of low resistance to a region of high resistance. This concept of transfer of resistance has given the name TRANSfer- resISTOR (TRANSISTOR). 

Common Emitter Configuration:
As shown in figure, in this configuration input is applied between base and emitter, and output is taken from collector and emitter. Here, emitter of the transistor is common to both, input and output circuits and hence the name common emitter configuration. Common emitter configurations for both n-p-n and p-n-p transistors are shown in figure  and 1 and 2 respectively.
Common Emitter Configuration

iii) Transistor as a Switching Device
One of the very important applications of a BJT (bipolar junction transistor) is its use as a switching device for the computer logic circuits. The circuit shown in the figure can be used as a switch by proper selection of resistances. The transistor is made to operate in the two extreme modes, that is it operates either in the cut off or saturation mode. Consider an input signal shown in figure b applied at the input of the circuit in the figure a. When input Vin = -V1, transistor is designed to operate in cut off state by proper choice of RB, RC. Now IC is close to zero, therefore the output voltage V0 = VCC as shown in figure c. For Vin = V2, transistor goes into saturation or is driven 'ON'. When a transistor is biased in saturation region, we know that VCE = VCEsat. So that the output voltage V0 = VCEsat = 0.2 V. Thus, the output voltage is either high (= VCC for transistor OFF) or low (= 0 V for transistor ON). So, the circuit is indeed working as a switch.

Transistor as a Switching Device

Logic Gates and Their Realization

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Logic gates are the basic elements that make up a digital system. The electronic gaate is a circuit that is able to operate on a number of binary inputs in order to perform a particular logical function. The types of gates available are the AND, OR, NOT, NAND, NOR, XOR and XNOR. The gate is a digital circuit with one or more input voltages but only one output voltage. By connecting the different gates in different ways, we can build circuits that perform arithmetic and other functions associated with the human brain because they simulate mental processes. The operation of a logic gate can be easily understood with the help of truth table. A truth table is a table that shows all the input-output possibilities of a logic circuit; that is the truth table indicates the outputs for different possibilities of the inputs.

AND Gate:
The AND gate performs logical multiplication, more commonly known as the AND function. The AND gate may have two or more inputs and a single output, as indicated by the standard logic symbols shown in the figure.

AND Gate

The truth table for a two input AND gate is shown in the table. This table can be expanded for any number of inputs. For any AND gate, regardless of the number of inputs, the output is high only when all inputs are high.

 A              B  
 0               0         0
 0               1     0
 1               0     0
 1               1     1

OR Gate:
The OR gate performs logical addition, more commonly known as the OR function. An OR gate has two or more inputs and one output, as indicated by the standard logic symbol in the figure, where OR gate with two inputs are illustrated. An OR gate produces a high on the output when any of the inputs is high. The output is low only when all of the inputs are low. Hence, the purpose of an OR gate is to determine when one or more of its inputs are high and to produce a high on its output to indicate this condition. 

OR Gate

The truth table describes the logical operation of the two input OR gate. The truth table can be expanded for any number of inputs; however, regardless of the number of inputs, the output is high when any of the inputs is high.

 A              B 
 0              0         0           
 0              1     1
 1              0        1
 1              1     1

NOT Gate:
The NOT gate performs a basic logic function called inversion or complementation. The NOT gate changes one logic level to its opposite level. In terms of bits, it changes a logic 1 to a logic 0 and a logic 0 to a logic 1. The figure shows the symbol for the NOT gate.

NOT Gate

When a high level is applied to a NOT gate input, a low level will appear on its output. When a low level is applied to its input, a high level will appear on its output. This operation is summarized in the truth table, which indicates the output for each possible input in terms of bits.

Input  Output 

NAND Gate:
The term NAND is a contraction of NOT-AND and implies an AND function with a complemented (inverted) output. A standard logic symbol for a two input NAND gate and its equivalency to an AND gate followed by an inverter is shown in figure. 


The NAND gate is a universal gate as it can be used to construct an AND gate, an OR gate an inverter, or any combination of these functions. The logical operation of the NAND gate is such that a low output occurs only when all inputs are high. When any of the inputs is low, the output will be high. Note that this operation is opposite to that of the AND as far as output is concerned. The truth table summarizes the logical operation of the two input NAND gate.

 A            B    
 0             0     1
 0             1     1  
 1             0     1 
 1             1     0 

NOR Gate:
The term NOR is a contraction of NOT-OR and implies an OR function with an inverted output. A standard logic symbol for a two input NOR gate and its equivalent OR gate followed by an inverter is shown in the figure.

NOR Gate

Similar to NAND gate, the NOR gate is a universal gate, that is, NOR gate can be used to construct an AND gate, an OR gate, an inverter, or any combination of these functions. The logic operation of the NOR gate is that a low output occurs when any of its inputs is high. Only when all of its inputs are low, the output is high. The truth table for a two input NOR gate is shown in the table.

 A               B 
 0                0       1 
 0                1       0 
 1                0     0 
 1                1     0 

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