Electrical Revolution

The full form of the SCR is silicon controlled rectifier.
It is a three junction, three terminal and four layer type PNPN semiconductor device. The terminals are anode (A), cathode (K) and gate (G).
It allows current flow only one direction so it is called as unidirectional device.
The structure and symbol of the SCR is shown in the Figure A.

Structure

The fabrication of the particular SCR depends upon service expected from that type.
The SCR consists of multilayered ‘pellet’ of alternate P and N type semiconductor material.
The semiconductor material is always silicon ( Intrinsic semiconductor ) in which proper impurities are added.
The junctions are either planer diffused ( all diffused ) or alloy diffused.

All diffused ( Planer diffused )

The cross section of the all diffused type SCR is shown in the Figure B.
It consists of four layered pellet of P and N type semiconductor materials.
This technique is used for making number of units from single wafer.
The planer construction is best suitable for low power SCRs.
The main disadvantage of the planer structure is that it requires more silicon per ampere current capacity.

Alloy diffused

The cross section of the alloy diffused is shown in the Figure C.
The inner junction J2 is obtained by diffusion whereas the two outer layered are alloyed to it.
As the PNPN structure is required to handle large current, it is properly braced molybdeneum or tungsten plates in order to provide greater mechanical strength.
One of these plates is hard soldered to an aluminium or copper stud which is further threaded for attachment of heat sink.
The heat sink provides efficient thermal path for dissipating the internal losses to the surrounding medium.

Operation

Forward blocking mode

When anode is made positive with respect to cathode terminal and gate is open circuited, two outer junctions J1 and J3 are forward biased but the middle junction becomes reverse biased.
The current does not flow through the device due to formation of depletion layer at the junction J2.
The leakage current flows through the device due to drift of the mobile charges.
As the SCR does not conduct in the forward biased condition, it is called as forward blocking or off state of the device.

Reverse blocking mode

When anode is made negative with respect to cathode and gate is open, two outer junction J1 and J3 becomes reverse biased whereas the middle junction becomes forward biased.
The current does not flows through the device because of two outer junction J1 and J3 are reverse biased.
The small amount of leakage current through the device is not sufficient to turn on the device. This is called as reverse blocking mode of the device.

Forward conduction mode

If the anode to cathode voltage increases in the forward blocking mode, the width of the depletion layer at the junction J2 decreases. ( the width of the depletion layer is inversely proportional to the applied voltage ).
If the anode to cathode voltage increases progressively, the depletion layer width becomes zero at certain level and junction J2 breakdown.
It is known as avalanche breakdown because of junction J1 and J3 are always forward biased.
Now the electrons from N type material moves toward junction J3 while holes from P type material moves toward cathode therefore the gate current starts to flow.
Due to gate current, anode current increases so more number of electrons available at junction J2. The junction J2 break down in very small time and SCR conducts heavily resulting gate loses all the controls.
The SCR does not turn off in spite of removing gate signal.

The following points to be noted from above theory.

The SCR may be turned on by making anode to cathode voltage is equal to break over voltage with gate circuit is kept open circuited or increases anode to cathode voltage less than the break over voltage with small gate voltage is applied.
The SCR behaves like a switch. ( conducting or non – conducting )
When the anode is made negative with respect to cathode and supply voltage increases to certain level say threshold, a zener breakdown occurs and SCR may be destroyed.
The anode to cathode voltage is reduced to zero in order to make the SCR non conducting.

Two transistor analogy

The equivalent circuit of the SCR is shown in the Figure E.
The SCR is made up of two transistors: One PNP and the other NPN.
These two transistors are connected such that the collector of the one transistor is connected to base of the other transistor whereas the base of the first transistor is connected to collector of the other transistor.

When no gate signal is applied and anode to cathode voltage less than forward break over voltage, two outer junctions J1 and J3 are forward biased while inner junction J2 becomes reverse biased.
The junction J1J2 and J2J3 can be considered to constitute a PNP and NPN transistor respectively where the junction J2 is collector – base junction to both transistors.
The collector current of the transistor is given by

I_C = aI_E + I_CO

I_C1 = a₁I_E1 + I_CO1

I_C2 = a₂I_E2 + I_CO2

I_A = I_C1+ I_C2

= ( a₁ + a₂ ) I_A + I_CO1+ I_CO2

( 1 – a₁– a₂ ) I_A = I_CO1+ I_CO2

I_A = [ I_CO1+ I_CO2 ] / ( 1 – a₁– a₂ )

The value of a₁ + a₂ is always less than the unity.
When the anode to cathode voltage V_AK is increases, the leakage current also increases. It means that the value of a₁ and a₂ also increases.
As the anode voltage reaches towards forward break over voltage, carrier multiplication factor ( hole multiplication factor and electron multiplication factor ) is large enough to breakdown junction J2.
As the value of a₁ + a₂ approaches towards unity, the current becomes large enough that both transistors are saturated.
As the voltage drop across SCR falls nearly one voltage, the anode current is limited by external resistance R during turned on of the SCR.
The SCR can be turned off only when the forward current falls below the holding current at which value of a₁ + a₂ less than unity.
When the gate signal is applied, gate current start to flow.
Higher the gate current, low the anode to cathode voltage at which the SCR turns on.
The current amplification factor a₂increases with increase in gate current and current amplification factor a₁ depends upon overall forward current.
It means that a₁ and a₂ depends upon gate current and overall forward current.

Working of SCR

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