![Zener diode shown with typical packages.Reverse current is
shown.
Zener diodes are widely used as voltage references and
as shunt regulators to regulate the voltage across small circuits. When
connected in parallel with a variable voltage source so that it is reverse
biased, a Zener diode conducts when the voltage reaches the diode's
reverse breakdown voltage. From that point on, the relatively low
impedance of the diode keeps the voltage across the diode at that
value.[8]
In this circuit, a typical voltage reference or regulator, an input voltage,
UIN, is regulated down to a stable output voltage UOUT. The breakdown
voltage of diode D is stable over a wide current range and holds
UOUT relatively constant even though the input voltage may fluctuate
over a fairly wide range. Because of the low impedance of the diode
when operated like this, resistor R is used to limit current through the
circuit.
In the case of this simple reference, the current flowing in the diode is
determined using Ohm's law and the known voltage drop across the
resistor R; The value of R must satisfy two conditions :
1.R must be small enough that the current through D keeps D in
reverse breakdown. The value of this current is given in the data sheet
for D. For example, the common BZX79C5V6[9] device, a 5.6 V 0.5 W
Zener diode, has a recommended reverse current of 5 mA. If
insufficient current exists through D, then UOUT is unregulated and less
than the nominal breakdown voltage (this differs to voltage-regulator
tubes where the output voltage will be higher than nominal and could
rise as high as UIN). When calculating R, allowance must be made for
any current through the external load, not shown in this diagram,
connected across UOUT.
2.R must be large enough that the current through D does not destroy
the device.](https://image.slidesharecdn.com/shivam110918010624-phpapp01-160910132459-180727072624/85/Physics-Project-for-class-12-isc-Junction-and-diode-20-320.jpg)
Physics Project for class 12 isc Junction and diode
- 1. in DEPARTMENT OF PHYICS MOTHER TERESA SR. SEC. CO-ED SCHOOL BHOPAL MOTHER TERESA SR. SEC. CO-ED SCHOOL KOLAR ROAD BHOAPL DEPARTMENT OF PHYSICS PROJECT REPORT INVESTIGATORYPROJECT ON PN JUNCTION AND DIODES PROJECT REPORT Submitted by Govind Patel Submitted to Mrs Ratna Hajela RegisterationNo:
- 2. In the accomplishment of this project successfully, many people have best owned upon me their blessings and the heart pledged support, this time I am utilizing to thank all the people who have been concerned with project. Primarily I would thank god for being able to complete this project with success. Then I would like to thank my principal Mr. James MJ and physics teacher Mrs Ratna hajela, whose valuable guidance has been the ones that helped me patch this project and make it full proof success her suggestions and her instructions has served as the major contributor towards the completion of the project. Then I would like to thank my parents and friends who have helped me with their valuable suggestions and guidance has been helpful in various phases of the completion of the project. Last but not the least I would like to thank my classmates who have helped me a lot. ACKNOWLEDGEMENT
- 3. CERTIFICATE This is to certify that project work titled A STUDY ON PN JUNCTIONS AND DIODES being submitted by Govind Patel a student of class XII-A has successfully completed the research on the below mentioned project under the guidance of Mrs Ratna Hajela ( Subject Teacher ) during the year 2016-17 in partial fulfillment of physics practical examination conducted by AISSCE Signature of external examiner Signature of physics teacher
- 4. INDEX • PN Junction • Diode Equation • Zener Diodes
- 5. Semiconductor: An Introduction • Conductors: Allow Electric current to flow through them • Insulators: Do not Allow Electric current to flow through them • Semiconductors: Materials whose conductivity lies in between of Conductors and Semiconductor
- 6. Insulators, Semiconductors, and Metals: Comparison This separation of the valence and conduction bands determines the electrical properties of the material Insulators have a large energy gap electrons can’t jump from valence to conduction bands no current flows Conductors (metals) have a very small (or nonexistent) energy gap electrons easily jump to conduction bands due to thermal excitation current flows easily Semiconductors have a moderate energy gap only a few electrons can jump to the conduction band leaving “holes” only a little current can flow
- 7. P-N JUNCTION Also known as a diode One of the basics of semiconductor technology - Created by placing n-type and p- type material in close contact Diffusion - mobile charges (holes) in p-type combine with mobile charges (electrons) in n-type
- 8. P-N JUNCTION Region of charges left behind (dopants fixed in crystal lattice) Group III in p-type (one less proton than Si- negative charge) Group IV in n-type (one more proton than Si - positive charge) Region is totally depleted of mobile charges - “depletion region” Electric field forms due to fixed charges in the depletion region Depletion region has high resistance due to lack of mobile charges
- 9. THE P-N JUNCTION Direction of Current
- 10. DEPLETION LAYER FORMATION The “potential” or voltage across the silicon changes in the depletion region and goes from + in the n region to – in the p region
- 11. Biasing the P-N Diode Forward Bias Applies - voltage to the n region and + voltage to the p region CURRENT! Reverse Bias Applies + voltage to n region and – voltage to p region NO CURRENT DIODES CAN BE CONSIDERED AS SWITCH
- 12. P-N Junction – Reverse Bias positive voltage placed on n-type material electrons in n-type move closer to positive terminal, holes in p-type move closer to negative terminal width of depletion region increases allowed current is essentially zero (small “drift” current) No current Flow Depletion layer width Increses
- 13. P-N Junction – Forward Bias positive voltage placed on p-type material holes in p-type move away from positive terminal, electrons in n-type move further from negative terminal depletion region becomes smaller - resistance of device decreases voltage increased until critical voltage is reached, depletion region disappears, current can flow freely
- 14. P-N Junction - V-I characteristics Voltage-Current relationship for a p-n junction (diode)
- 15. Current-Voltage Characteristics THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE
- 16. Ideal diode equation where: I = the net current flowing through the diode; I0 = "dark saturation current", the diode leakage current density in the absence of light; V = applied voltage across the terminals of the diode; q = absolute value of electron charge; k = Boltzmann's constant; and T = absolute temperature (K). The diode equation gives an expression for the current through a diode as a function of voltage. The Ideal Diode Law, expressed as:
- 17. Semiconductor diode - opened region The p-side is the cathode, the n-side is the anode The dropped voltage, VD is measured from the cathode to the anode Opened: VD VF: VD = VF ID = circuit limited, in our model the VD cannot exceed VF
- 18. Semiconductor diode - closed region • Closed: VF < VD 0: – VD is determined by the circuit, ID = 0 mA • Typical values of VF: 0.5 ¸ 0.7 V Cut-off: 0 < VD < VF: ID 0 mA Semiconductor diode - cut-off region
- 19. A Zener diode allows current to flow from its anode to its cathode like a normal semiconductor diode, but it also permits current to flow in the reverse direction when its "Zener voltage" is reached. Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage and sharpness of the knee are not as well defined as for a Zener diode. Also normal diodes are not designed to operate in the breakdown region, but Zener diodes can reliably operate in this region. The device was named after Clarance Melvin Zener, who discovered the Zener effect. Zener reverse breakdown is due to electron quantum tunnelling caused by a high strength electric field. However, many diodes described as "Zener" diodes rely instead on avalanche breakdown. Both breakdown types are used in Zener diodes with the Zener effect predominating under 5.6 V andavalanche breakdown above. Zener diodes are widely used in electronic equipment of all kinds and are one of the basic building blocks of electronic circuits. They are used to generate low power stabilized supply rails from a higher voltage and to provide reference voltages for circuits, especially stabilized power supplies. They are also used to protect circuits from over-voltage, especiallyelectrostatic discharge Zenerdiode
- 20. Zener diode shown with typical packages.Reverse current is shown. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.[8] In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated down to a stable output voltage UOUT. The breakdown voltage of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, resistor R is used to limit current through the circuit. In the case of this simple reference, the current flowing in the diode is determined using Ohm's law and the known voltage drop across the resistor R; The value of R must satisfy two conditions : 1.R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6[9] device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUT is unregulated and less than the nominal breakdown voltage (this differs to voltage-regulator tubes where the output voltage will be higher than nominal and could rise as high as UIN). When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across UOUT. 2.R must be large enough that the current through D does not destroy the device.
- 21. 2.If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX correlate as such: . A load may be placed across the diode in this reference circuit, and as long as the Zener stays in reverse breakdown, the diode provides a stable voltage source to the load. Zener diodes in this configuration are often used as stable references for more advanced voltage regulator circuits. Shunt regulators are simple, but the requirements that the ballast resistor be small enough to avoid excessive voltage drop during worst-case operation (low input voltage concurrent with high load current) tends to leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller loads. These devices are also encountered, typically in series with a base-emitter junction, in transistor stages where selective choice of a device centered around the avalanche or Zener point can be used to introduce compensating temperature co-efficient balancing of the transistor p–n junction. An example of this kind of use would be a DC error amplifier used in aregulated power supply circuit feedback loop system. Zener diodes are also used in surge protectors to limit transient voltage spikes. Another application of the Zener diode is the use of noise caused by its avalan che breakdown in a random number generator.
- 22. NCERT textbook class 12 NCERT physics lab Manuel INTERNET www.yahoo.com www.scribd.com www.google.com