Rectifiers

RECTIFERS

• Thermionic diode valves
• Earlier rectifiers used
• Consists two electrodes and a glass envelope enclosing a vacuum
• Passes current through a vacuum in one direction and blocks any reverse flow
• Sold state rectifiers (semi conductors)
• Are modern rectifiers to thermionic diode valves

Passes electrons through sold material as opposite to vacuum.

SOLID-STATE RECTIFIER OR SEMI-CONDUCTORS

SOLID- STATE RECTIFIER: Is a solid – state device that serves as an electronic rectifying element.

It is made of a combination of P-Type layer and N-Type layer.

HOW P-Type Silicon is created

To make P-Type silicon, trivalent impurities such as Boron, Indium, Aluminum and Gallium are added or mixed with Silicon (Si).

Covalent bond occurs between Silicon and the trivalent atoms.

In process of bonding, a vacant hole is created within the covalent bond between one trivalent atom and a neighboring Silicon (Si) atom.

The holes are considered to be positive charge carriers.

Graphic Representation of P-Type silicon mixed with Boron.

N Type Semiconductors

If Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth is added to Silicon (Si) Valence electrons of phosphorus are locked up in covalent bond with valence electrons of four neighboring Silicon (Si) atoms The 5th valence electron of phosphorus atom does not find a binding electron and thus remains free to float.

Junction Barrier

When p-type and N-type are joined together, the electrons in the N material diffuse across the junction into the P material and fill some of the holes.

At the same time, the holes in the P material diffuse across the junction into the N material and are filled by N material electrons.

The loss of an electron from the N-type material created a positive ion in the N material, while the loss of a hole from the P material created a negative ion in that material.

FORWARD BIAS.

If an external voltage applied to a PN junction is call BIAS.

A battery is used to supply bias to a PN junction and is connected so that its voltage opposes the junction field, it will reduce the junction barrier and, therefore, aid current flow through the junction.

This type of bias is known as forward bias, and it causes the junction to offer only minimum resistance to the flow of current

The positive and negative terminals of the bias battery are connected to P-type and N-type respectively

The positive potential repels holes toward the junction where they neutralize some of the negative ions The negative potential repels electrons toward the junction where they neutralize some of the positive ions.

Neutralizing ions on both sides of the barrier, decreases its width.

This allows flow of electrons across the junction.

REVERSE BIAS

• The negative battery terminal is connected
• to the P-type material.

The positive battery terminal to the N-type material The negative potential attracts the holes away from the edge of the junction barrier on the P side.

The positive potential attracts the electrons away from the edge of the barrier on the N side.

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This action increases the barrier width because there are more negative ions on the P side of the junction, and more positive ions on the N side of the junction.

This increase in the number of ions prevents current flow across the junction

PNP JUNCTIONS

FORWARD BIAS

a) The collector is connected to high negative voltage with respect to base i.e. Vbc is very high.

• So c-b junction is reverse biased.
• The base is connected to low negative voltage with respect to emitter i.e. Veb is low.
• Also Vbc is always greater than Veb.
• When Veb is greater than or equal to potential barrier voltage of e-b junction, the transistor is forward biased.

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• Now large number of holes in emitter layer is repelled by positive terminal of Veb and they flow towards e-b junction.
• They cross the junction and enter into small base layer. Here some electrons combine with holes, some of them are attracted by negative terminal of Veb and remaining large number of holes flow into collector layer, crossing b-c junction.
• The resident holes of collector are repelled by these (guest) holes and thus, all the holes are attracted by negative terminal of Vbc

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Thus, all these holes complete their journey back into emitter layer and thus, produce currents in the transistor as shown in the above circuit.

Reverse bias

a)The collector is connected to high negative voltage with respect to base i.e. Vbc is very high.

• So b-c junction is reverse biased.
• The base is connected to low positive voltage with respect to emitter i.e. Vbe is low.
• Also Vbc is always greater than (–Veb)*.
• Since battery Vbe is connected in opposite direction, the emitter-base junction is now reverse biased.

So the holes in emitter layer are attracted by negative terminal of Vbe and thus, they cannot cross the junction.

• So there are no holes that can produce base current. Ib = 0.
• So collector current Ic = 0 and emitter current Ie = 0.

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j)Now if Vbe is increased further, nothing will happen as all the holes in emitter layer are already attracted by negative terminal of Vbe.

• Hence, we conclude that transistor is reversed biased.

NPN JUNCTIONS

Formed by sandwiching a p-layer with two n-layers

Note the following important points:-

• Area of its collector layer is largest.
• Area of its base layer is smallest. It is very thin.
• Area of its emitter layer is medium.
• Collector layer is moderately doped. It has medium number of charges (electrons).

Base layer is lightly doped. It has a very few number of charges (holes).

Emitter layer is heavily doped. It has largest number of charges (electrons).

• There are two junctions in this transistor.
• The junction between collector layer and base layer is called as collector-base* junction

or c-b junction.

The junction between base layer and emitter layer is called as base-emitter* or b-e junction

How NPN transistor is biased?

Forward Biasing

When Vbe is greater than or equal to potential barrier voltage of b-e junction, the transistor is forward biased.

Now large number of electrons in emitter layer is repelled by negative terminal of Vbe and they flow towards b-e junction.

They cross the junction and enter into small base layer. Here some electrons combine with holes, some of them are attracted by positive terminal of Vbe and remaining large number of electrons flow into collector layer, crossing c-b junction.

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The resident electrons of collector are repelled by these (guest) electrons and thus, all the electrons are attracted by positive terminal of Vcb.

Thus, all these electrons complete their journey back into emitter layer and thus, produce currents in the transistor as shown in the above circuit.

Now as Vbe increases, more electrons from base are attracted by positive terminal of Vbe. So base-collector junction is more and more forward biased and base current (Ib) increases.

Hence, we conclude that collector current (Ic) is the function of base current

REVERSED BIAS….Assignment

ADVANTAGE OF SOLD STATE HIGH TENSION RECTIFIERS OVER THERMIONIC DIODE VALVES

• They have longer life, for they have no filaments to burn out.
• They need no filament-heating transformers.
• They are more Robust.

They are small in size compared to vacuum diode.

Being smaller in size and absence of filament transformers becomes possible to produce mobile x ray sets which have full – wave rectification.

Gives less heat in the tank enclosing the rectifiers and high tension transformers due to absence of filament heating in the valves