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Sketch a basic AND circuit with twodiodes and explain its operation.Explain how doping increases the conductivity in semiconductors ? |
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Answer» Solution :AND gate `:` It has two input terminals and one out terminal. If both the inputs are low ( or ) one of the inputs is low. `implies ` The out is low in an AND gate. If both the inputs are high `implies` The output of the gate is high. Note `:` If A and B are the inputs of the gate and the output is 'Q' then 'Q' is a logical function of A and B.  The logicalfunction AND is represented by the symbol dot so thatthe output , Q= A.B andthe circuit symbol used for the logic gate AND is shown in Figure. The logic function ANDis similart o the multiplication.  Implementaion of AND gate using diodes `:` Let `D_(1)` and `D_(2)` represents two diodes. A potential of 5V represents the logical value 1 and a potential of 0V represents the logical value zero ( 0 ). When A = 0 , B = 0 both the diodes `D_(1)` and `D_(2)`are forward -biased and they behavelike closed switches. Hence, the output Q is same as that A or B ( equal to zero ) . When A = 0 or B =0, `D_(1)` or `D_2)` is forward-biased and Q is zero. When A=1 and B =1 both the diodes are reverse -biasedandthey behave like open switches. There is no current through the resistance Rmaking the potential at Q equal to 5V i.e., Q= 1 . The output is same as that of an AND gate. Doping INCREASES the conductivity in Semiconductors `:` If a pentavalent impurity ( Arsenic )is added to a pure tetravalent semiconductorit is called n-type semiconductor . Arsenic has 5 valence electrons, but only 4 electrons are needed to form covalent bonds with the adjacent Germanium atoms. The fifth electron isvery loosely BOUND and become a free electron. Therefore excess electronsare available for conduction and conductivity of semiconductor increases. Similarly when a trivalent impurity Indium is added topute Germanium it is called p-typesemi-conductor. In this excess holes in ADDITION to those formed due to THERMAL energy are available for condcution in the valenece band and the conductivity of semicondcutor increases. |
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