Transistors have four main operating regions: reverse saturation, saturation, active, and cut off. In the saturation region, the transistor behaves like a closed switch with maximal collector and emitter currents. In the active region, the transistor performs well as an amplifier with the collector current multiplied by the base current. In the cut off region, the transistor behaves like an open switch with zero collector, emitter, and base currents. The reverse active region has the collector-base junction forward biased and base-emitter junction reverse biased.

1. Industrial Electronics N1
Transistor operating regions

2. Transistor Operating
Regions
Transistors are three-terminal semiconductor devices used to control current or boost
an input signal. Transistors are also used to switch electronic signals. The addition of
electrons is utilized to regulate the flow of electrical current in all types of transistors.
This technique causes voltage differences that result in proportionately larger
variations in output current, effectively amplifying the signal.
Transistors have four main operating regions:
reverse saturation, saturation, active and cut off

3. Transistor
Regions
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4. Saturation region
This is the region where the transistor behaves most like a closed
switch. The transistor's collector and emitter are effectively shorted. In
this mode of operation, the collector and emitter currents are maximal.
When both the emitter and collector junctions are forward biased, the
transistor functions in the saturation region. Like is well known, the
transistor tends to operate as a closed switch in the saturation region.
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5. Forward Region
This is the region where transistors find a wide variety of applications.
Additionally, this is referred to as a linear region. While in this region, a
transistor performs better as an amplifier.
Active Area between saturation and cutoff is this region. When the
emitter junction is forward biased and the collector junction is reverse
biased, the transistor works in the active region. Collector current is
multiplied by the base current in the active state.
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6. Cut Off
This is the region where the transistor behaves most like an open
switch. The transistor has the effect of opening its collector and base.
In this mode of operation, the collector, emitter, and base currents are
all zero.
When both the emitter and collector junctions are reverse biased, the
transistor operates in the cutoff region. As in the cutoff region, the
collector, emitter, and base currents are all zero.
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7. Reverse Active
The base-emitter junction is forward biased in the forward-active area,
while the collector-base junction is reverse biased. We might just as
easily operate the transistor by forward biasing the collector-base
junction and reverse biasing the base-emitter junction.
Notably, the collector-base junction is forward biased in the reverse-
active area, whereas the base-emitter junction is reverse biased.
Transistors are optimized for forward active region operation. As
expected, when operated in the reverse active region, transistor
metrics such as speed and? F deteriorate.
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8. Summary
Now that you have completed this segment, you should be able to:
• Explain the operation of a transistor in the saturation region
• Explain the operation of a transistor in the active region
• Explain the operation of a transistor in the cut off region
• Explain the operation of a transistor in the reverse active region
For additional information please refer to the reading list
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9. References
Cirovic, M., 1979. Basic Electronics: Devices, Circuits and Systems. 1 ed.
Virginia: Prentice-Hall .
Gobind, M. & Heerden, R. v., 2019. N1 Industrial Electronics. 1 ed.
Northcliff: Macmillan .
Kraft, J., 2016. N1 Industrial Electronics. 3 ed. Mowbray: Future
Managers.
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