Basic Semiconductor Electronics PN Junction Diode Dr Pankaj

Basic Semiconductor Electronics P-N Junction Diode Dr. Pankaj Kumar Department of Physics, Bhavan’s Mehta Mahavidyalaya, (V. S. Mehta College of Science) Bharwari, Kaushambi (U. P)-212201, India

CONTENTS 1. Introduction 2. Unbiased P- N Junction Diode 3. Forward biased P- N Junction Diode 4. Reverse biased P- N Junction Diode 5. Mechanism of Breakdown 6. Characteristics of P- N Junction Diode 7. Ideal diode equation 8. Diode resistance: dc or static resistance 9. Diode resistance: ac or dynamic resistance 10. Diode capacitance: Transition capacitance 11. Diode capacitance: Diffusion capacitance 12. References

P-N Junction Diode: Introduction Intrinsic Semiconductor- The electrical characteristics of intrinsic semiconductor (Si or Ge) can be improved by adding impurity in a process called doping. External (doped) Semiconductor- Block Diagram N-type Semiconductor - Semiconductor doped with Pentavalent material (P, As, Sb etc. ) contains an excess of free electrons. P-type Semiconductor - Semiconductor doped with Trivalent material (Al, Ga etc. ) contains an excess of holes. Circuit Symbol P-N junction diode. One end of a semiconductor bar (Si or Ge) crystal is doped as a ptype material and the other end as a n-type material. Biasing of P-N junction diode- P-N Junction diode Unbiased - No external voltage is applied. Forward Biased- P-terminal of diode connected to positive terminal and N-terminal of diode connected to negative terminal of the battery. Reverse Biased - P-terminal of diode connected to negative terminal and N-terminal of diode connected to positive terminal of the battery. Forward Biased Reverse Biased

Unbiased P-N Junction Diode As soon as junction is formed, the electrons in the N-type material diffuse across the junction to the P-type material and the holes in the P-type material diffuse across the junction to the N-type material. In this process they recombine each other. After a few recombination of electrons and holes, there is depletion of mobile charge carriers in the region near the junction on the both sides. Such region is known as depletion or space charge region. Width of depletion region is inversely proportional to doping level and is directly proportional to reverse bias. Due to presence of opposite immobile impurity ions on both sides of the junction, a potential is developed which stopped further diffusion of electrons and holes. Such potential is known as barrier potential or built in potential. Barrier potential for Ge is 0. 3 V and that for Si is 0. 7 V. Formation of a depletion region around the junction .

Forward biased P-N Junction Diode In this case, two types of electric fields exit within the diode- External electric field across the diode terminals due to external bias voltage and Internal electric field across depletion region due to barrier potential. Due to external electric field, holes in P type semiconductor region are repelled from the positive terminal and electrons in N type semiconductor region are repelled from the negative terminal of the source. Therefore width of depletion region is reduced compared to unbiased P-N junction diode. The decreased barrier potential is helpful to the majority carriers in crossing the junction. Hence more majority carriers drift across the junction which results a current in forward biased diode. Such current is known as forward current and it is order of milli-ampere. The forward current increases with increasing the forward bias voltage.

Reverse biased P-N Junction Diode Due to external electric field, holes in P type semiconductor region are attracted toward the negative terminal and electrons in N type semiconductor region are attracted toward the positive terminal of the source. Therefore depletion region becomes wider compared to unbiased P-N junction diode. The increased barrier potential is helpful to the minority carriers in crossing the junction. Hence a current flows due to minority carries in reverse biased P-N junction diode. Such current is known as reverse saturation current. The reverse saturation current is order of nano-ampere in case of Si and micro-ampere in case of Ge. If the reverse bias voltage is made too high, the current through P-N junction diode increases abruptly. This phenomenon is known as breakdown of the diode and the voltage at which it occurs is known as breakdown voltage. There are two types of mechanism of breakdown- Avalanche breakdown and Zener breakdown.

Mechanism of Breakdown Avalanche Breakdown The increased electric field across the diode causes increase in the velocities of minority carriers. These high energy carriers break covalent bonds and therefore more electron-hole pairs are generated. Again these generated carriers are accelerated by the external electric field and they break more covalent bonds during their travel. This gives rise to a high reverse current through the diode. Such mechanism of breakdown is known as Avalanche Breakdown. Avalanche Effect ü Lightly doped semiconductor ü Carriers generated by collisions ü Breakdown voltage is high Zener Breakdown When diode is heavily doped, the depletion layer becomes extremely thin (order of micrometer). If reverse bias voltage is increased, the electric field across the depletion region becomes very high. This high electric field causes covalent bonds to break i. e. , to create electron-hole pairs within the depletion region. Hence a large no. of charge carriers are generated which results very high current. Such mechanism of breakdown is known as Zener Breakdown. Zener Effect ü Heavily doped semiconductor ü Carriers generated by high electric field ü Breakdown voltage is low

Characteristics of P-N Junction Diode Circuit forward bias Characteristics Circuit for Reverse bias Characteristics

Characteristics of P-N Junction Diode V-I Characteristics of a P-N Junction Diode

Ideal Diode Equation where I = Current flowing through diode V = Voltage cross diode Io = Reverse saturation current e = Charge on the electron k = Boltzmann’s constant T = Temperature in Kelvin k. T/e =Vth = Voltage equivalent to temperature At 300 K (room temperature), Vth = 26 m. V For forward bias, V >> Vth , therefore For reverse bias , , therefore

Diode Resistance: DC or Static Resistance The resistance offered by diode to a dc applied voltage is called dc or static resistance. At operating point Q, VD = Applied dc voltage to the diode ID = Current through the diode at Voltage, For forward bias, For reverse bias, DC Resistance of a Diode

Diode Resistance: AC or Dynamic Resistance The resistance offered by diode to an ac applied voltage is called ac or dynamic resistance. At operating point Q, VD = Applied dc voltage to the diode ID = Current through the diode at Voltage, By differentiating diode equation, forward bias, For reverse bias, AC Resistance of a Diode

Diode Capacitance: Transition Capacitance When diode is reverse biased, P and N regions act as the plate capacitor and the depletion region acts as the dielectric material. Thus the P-N Junction diode in reverse bias has an effective capacitance. Such capacitance is known as Transition Capacitance. ; where A is the cross sectional area of the region, is relative permittivity of the semiconductor (16 for Ge, 12 for Si) and w is the width of depletion region. Transition capacitance with Reverse voltage

Diode Capacitance: Diffusion Capacitance When diode is forward biased, a much larger capacitance than transition capacitance is offered by the diode. Such capacitance is known as Diffusion Capacitance. where τ mean life time for holes and other symbols have their usual meanings. Diffusion capacitance with Forward voltage

References 1. Electronics Fundamentals and Applications: J. D. Ryder 2. Electronics Devices and Circuits : J. Millman and C. Halkias 3. Solid State Electronic Devices: B. G. Streetman 4. Electronics Devices, Circuits and Applications: W. D. Stanley 5. Electrical Circuits and Introductory Electronics: Vinod Prakash 6. Principles of Electronics: V. K. Mehta 7. Solid State Electronics: J. P. Agarwal and A. Agarwal 8. Hand Book of Electronics: S. L. Gupta and V. Kumar

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