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Note for Basic Electronics - BE By JAYAPRASAD KM

  • Basic Electronics - BE
  • Note
  • N.M.A.M. Institute of Technology - NMAMIT
  • Electronics and Communication Engineering
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MODULE 1 NOTES 18ELN14/24 BASIC ELECTRONICS  Similarly some holes from the p-type are attracted across the junction to PART 1 combine with electrons on n-type and form the positive ions. SEMICONDUCTOR DIODES AND APPLICATIONS 1.1 P-N Junction  The movement of majority charge carriers across the junction is called as diffusion i.e. from a region of high carrier concentration to a region of low  The pn junction is formed by joining p-type semiconductor and n-type carrier concentration.  The p-type semiconductor and n-type semiconductor are electrically neutral semiconductor.  The holes are the majority charge carriers in p-type semiconductor and the electrons are the majority charge carriers in n-type semiconductor.  The majority charge carriers are uniformly distributed in the semiconductor.  Since holes and electrons are close together at the junction, some free electrons from the n-type are attracted across the junction to fill the adjacent holes on p-type and form the negative ions. before the majority charge carriers diffuse across the junction.  When the negative ions are created on the p-type, the portion close to the junction on p-type acquires a negative voltage.  Similarly when the positive ions are created on the n-type, the portion close to the junction on n-type acquires a positive voltage.  The negative voltage on p-type repels the additional electrons crossing the junction and the positive voltage on n-type repels the additional holes crossing the junction.  This initial diffusion of charge carries creates a voltage at the junction known as barrier voltage.  The magnitude of barrier voltage depends on doping densities, electronic charge and junction temperature.  The barrier voltage opposes the flow of majority carriers across the junction but assists the flow of minority carriers across the junction.  The movement of majority charge carriers across the junction leaves a layer on each side that is depleted of charge carriers. This region is known as depletion region. Fig.1.1: pn junction Prepared by: Prof. Jayaprasad K M, Prof. Vinay H S, Prof. Dayanand G K. Page 3

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MODULE 1 NOTES 18ELN14/24 BASIC ELECTRONICS  On n-type the depletion region consists of donor impurity and on p-type the depletion region consists of acceptor impurity.  If the semiconductors have equal doping densities, then the depletion layers on each side have same widths.  Only a small amount of reverse voltage is sufficient to sweep all the available minority carriers across the junction.  Increase in the bias voltage further does not increase the current level and this current is known as reverse saturation current.  If the p-type semiconductor is highly doped than n-type, then the depletion layer penetrates more deeply into n-type. Similarly if the n-type semiconductor is highly doped than p-type, then the depletion layer penetrates more deeply into p-type. 1.1.1 Biased junction  When an external voltage is applied to a pn junction, the pn junction is said to be biased. 1.1.2 Reverse-biased junction Fig.1.2: Reverse-biased  When the positive terminal is connected to n-type and negative terminal is connected to p-type, the electrons from the n-type are attracted to the positive 1.1.3 Forward-biased junction terminal and holes from the p-type are attracted to the negative terminal.  When the positive terminal is connected to p-type and negative terminal is  This widens the depletion region and the barrier voltage increases. connected to n-type, the electrons from the n-type are repelled from the  When barrier voltage increases there is no possibilities of majority carrier negative terminal and holes from the p-type are repelled from the positive current flow across the junction and the junction is said to be reverse biased and have a high resistance.  During the reversed biased condition the minority carriers on each side can still cross the junction. terminal.  This reduces the width of the depletion region and the barrier potential.  When the applied bias voltage increased the barrier voltage decreases and it disappears. The majority charge carriers can easily flow across the junction.  The movement minority carriers across the junction results in small reverse current. Prepared by: Prof. Jayaprasad K M, Prof. Vinay H S, Prof. Dayanand G K. Page 4

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BASIC ELECTRONICS MODULE 1 NOTES 18ELN14/24 Fig.1.3: Forward -biased  A majority charge carrier current flows and the junction is said to be forward biased.  Increase in the bias voltage further from zero towards knee of the characteristics the barrier potential is overcome and allows more majority charge carriers to flow across the junction. 1.1.4 pn-junction diode  A pn-junction can be used as a switch, i.e. on when forward biased and off when reverse biased.  A pn-junction provided with copper wire connecting leads becomes an electronic device known as diode.  The circuit symbol for a diode is an arrowhead and a bar. The arrowhead indicates the direction of current flow when the diode is forward biased.  The p-type of the diode is always the positive terminal for forward bias and is termed as anode. The n-type of the diode is always the negative terminal for forward bias and is termed as cathode. Prepared by: Prof. Jayaprasad K M, Prof. Vinay H S, Prof. Dayanand G K. Fig.1.5: pn junction diode characteristics Page 5

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MODULE 1 NOTES 18ELN14/24 BASIC ELECTRONICS 1.1.5 Diode Relationship Problem 2 1. Calculate the forward and reverse resistances offered by a silicon diode with IF = 100mA, VF = 0.75V and VR = 50V, IR = 1µA. Forward resistance Reverse resistance Problem 3 Calculate the diode current for the given circuit having a silicon diode Apply KVL for the circuit we get Therefore diode current is given by Prepared by: Prof. Jayaprasad K M, Prof. Vinay H S, Prof. Dayanand G K. Page 6

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