L3 electronics pn junction


Instructor: Muhammad Bilal

The pn junction diode

When a pn junction is formed, electrons in the n-material diffuse across the junction and recombine with holes in the p-material. This action continues until the voltage of the barrier repels further diffusion. Further diffusion across the barrier requires the application of a voltage.

The pn junction is basically a diode, which is a device that allows current in only one direction. A few typical diodes are shown.



Forward bias

When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine.

The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V.

p-region n-region

p n

+

R

VBIAS

The forward-bias causes the depletion region to be narrow.



Reverse bias

When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented.

The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current.

p-region n-region

p n

+VBIAS

R

The reverse-bias causes the depletion region to widen.



Diode characteristics

The forward and reverse characteristics are shown on a V-I characteristic curve. In the forward bias region, current increases dramatically after the barrier potential (0.7 V for Si) is reached. The voltage across the diode remains approximately equal to the barrier potential.

VR VF

IF

IR

Reverse bias

Forward bias

0.7 V

Barrier potential

The reverse-biased diode effectively acts as an insulator until breakdown is reached.

VBR (breakdown)



Diode models

The characteristic curve for a diode can be approximated by various models of diode behavior. The model you will use depends on your requirements.

The ideal model assumes the diode is either an open or closed switch.

VR VF

IF

IR

Reverse bias

Forward bias

The complete model includes the forward resistance of the diode.

The practical model includes the barrier voltage in the approximation.

0.7 V



Half-wave Rectifier

Rectifiers are circuits that convert ac to dc. Special diodes, called rectifier diodes, are designed to handle the higher current requirements in these circuits.

The half-wave rectifier converts ac to pulsating dc by acting as a closed switch during the positive alteration.

The diode acts as an open switch during the negative alteration.

D

D

RL

RL

+

+



Full-wave Rectifier

The full-wave rectifier allows unidirectional current on both alterations of the input. The center-tapped full-wave rectifier uses two diodes and a center-tapped transformer.

F D1

D2RL

Vsec2

Vsec2

The ac on each side of the center-tap is ½ of the total secondary voltage. Only one diode will be biased on at a time.



Bridge Rectifier

The bridge rectifier is a type of full-wave circuit that uses four diodes. The bridge rectifier does not require a center-tapped transformer.

F

D1

D2

RL

At any instant, two of the diodes are conducting and two are off.

D3

D4



Peak inverse voltage

Diodes must be able to withstand a reverse voltage when they are reverse biased. This is called the peak inverse voltage (PIV). The PIV depends on the type of rectifier circuit and the maximum secondary voltage.For example, in a full-wave circuit, if one diode is conducting (assuming 0 V drop), the other diode has the secondary voltage across it as you can see from applying KVL around the green path.

0 V

Vsec

Notice that Vp(sec) = 2Vp(out) for the full-wave circuit because the output is referenced to the center tap.



Peak inverse voltage

For the bridge rectifier, KVL can be applied to a loop that includes two of the diodes. Assume the top diode is conducting (ideally, 0 V) and the lower diode is off. The secondary voltage will appear across the non-conducting diode in the loop.

0 V

Notice that Vp(sec) = Vp(out) for the bridge because the output is across the entire secondary.

Vsec



Power supplies

By adding a filter and regulator to the basic rectifier, a basic power supply is formed.

7805

FD1

D2C1

D3

D4C2

Typically, a large electrolytic capacitor is used as a filter before the regulator, with a smaller one following the regulator to complete filtering action.

1000 F 1 F

IC regulator



Special-purpose diodes

Special purpose diodes include Zener diodes – used for establishing a reference voltage

Varactor diodes – used as variable capacitors

Light-emitting diodes – used in displays

Photodiodes – used as light sensors



Troubleshooting power supplies

Begin troubleshooting by analyzing the symptoms and how it failed. Try to focus on the most likely causes of failure.

7805

FD1

D2C1

D3

D4C2

1000 F 1 F

IC regulator

A power supply has no output, but was working until a newly manufactured PC board was connected to it. (a) Analyze possible failures. (b) Form a plan for troubleshooting.




7805

FD1

D2C1

D3

D4C2

1000 F 1 F

IC regulator

The supply had been working, so the problem is not likely to be an incorrect part or wiring problem. The failure was linked to the fact that a new PC board was connected to it, which points to a possible overloading problem. If the load was too much for the supply, it is likely a fuse would have blown, or a part would likely have overheated, accounting for the lack of output.




1. Disconnect power and check the fuse. If it is bad, replace it. Before reapplying power, remove the load, open the power supply case, and look for evidence of overheating (such as discolored parts or boards). If no evidence of overheating proceed.2. Check the new pc board (the load) for a short or overloading of the power supply that would cause the fuse to blow. Look for evidence of overheating.

3. Verify operation of the supply with measurements (see next slide).

Based on the analysis, a sample plan is as follows. (It can be modified as circumstances warrant.)




Reapply power to the supply but with no load. If the output is okay, put a resistive test load on the power supply and measure the output to verify it is operational. If the output is correct, the problem is probably with the new pc board. If not, you will need to further refine the analysis and plan, looking for an internal problem.

The analysis showed that a likely cause of failure was due to an overload. For the measurement step, it may be as simple as replacing the fuse and confirming that the supply works. After replacing the fuse:



Majority carrier

Minority carrier

PN junction

Diode

The most numerous charge carrier in a doped semiconductor material (either free electrons or holes.

Selected Key Terms

The boundary between n-type and p-type semiconductive materials.

An electronic device that permits current in only one direction.

The least numerous charge carrier in a doped semiconductor material (either free electrons or holes.



Barrier potential

Forward bias

Reverse bias

Full-wave rectifier

A circuit that converts an alternating sine-wave into a pulsating dc consisting of both halves of a sine wave for each input cycle.

The condition in which a diode conducts current.

The inherent voltage across the depletion region of a pn junction diode.

Selected Key Terms

The condition in which a diode prevents current.



Bridge rectifier

Zener diode

Varactor

Photodiode A diode whose reverse resistance changes with incident light.

A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference.

A type of full-wave rectifier consisting of diodes arranged in a four corner configuration.

Selected Key Terms

A diode used as a voltage-variable capacitor.

L3 electronics pn junction

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