# Semiconductor Electronics - V

Logic Gate:
A logic gate is a digital circuit that has one or more inputs but only one output. It follows a logical relationship between input and output voltage.

Truth Table:
This table shows all possible input combination and the corresponding output for a logic gate.

Boolean Expression:

It is a shorthand method of describing the function of a logic gate in the form of an equation or an expression. It also relates all possible combination of the inputs of a logic gate to the corresponding outputs.

Positive and Negative Logic:
If in a system, the higher voltage level represents 1 and the lower voltage level represent 0, the system is called a positive logic. If the higher voltage represents 0 and the lower voltage level represents 1, then the system is called a negative logic.

OR Gate:
An OR gate can have any number of inputs but only one output. It gives higher output (1) if either input A or B or both are high (1), otherwise the output is low (0).
A + B = Y
Which is read as ‘A or B equals Y’.

AND gate:

An AND gate can have any number of inputs but only one output. It gives a high output
(1) if inputs A and B are both high (1), or else the output is low (0). It is described by the Boolean expression.
A . B = Y
Which is read as ‘A and B equals Y’.

NOT Gate:
A NOT gate is the simplest gate, with one input and one output. It gives as high output
(1) if the input A is low (0), and vice versa.
Whatever the input is, the NOT gate inverts it. It is described by the Boolean expression:

Which is read as ‘not A equal Y’.

NAND (NOT+AND) gate:

It is obtained by connecting the output of an AND gate to the input of a NOT gate. Its output is high if both inputs A and B are not high. If is described by the Boolean expression.
Which is read as ‘A and B negated equals Y’.

NOR (NOT+OR) Gate:
It is obtained by connecting the output of an OR gate to the input of a NOT gate. Its output is high if neither input A nor input B is high. It is described by the Boolean expression.
Which is read as ‘A and B negated equals Y’.

XOR or Exclusive OR gate. The XOR gate gives a high output if either input A or B is high but not when both A and B are high or low. It can be obtained by using a combination of two NOT gates, two AND gates and one OR gate. It is described by Boolean expression:

The XOR gate is also known as difference gate because its output is high when the inputs are different.

Integrated Circuits:
The concept of fabricating an entire circuit (consisting of many passive components like R and C and active devices like diode and transistor) on a small single block (or chip) of a semiconductor has revolutionized the electronics technology. Such a circuit is known as Integrated Circuit (IC).

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