The input and Base are grounded (0v)
Base-Emitter voltage VBE < 0.7V
Base-Emitter junction is reverse biased
Base-Collector junction is reverse biased
Transistor is "fully-OFF" (Cut-off region)
No Collector current flows ( IC = 0 )
VOUT = VCE = VCC = "1"
Transistor operates as an "open switch"
Then we can define the "cut-off region" or "OFF mode" when using a bipolar transistor as a
switch as being, both junctions reverse biased, IB < 0.7V and IC = 0. For a PNP transistor, the
Emitter potential must be negative with respect to the Base.
2. Saturation Region
Here the transistor will be biased so that the maximum amount of base current is applied,
resulting in maximum collector current resulting in the minimum collector emitter voltage
drop which results in the depletion layer being as small as possible and maximum current
flowing through the transistor. Therefore the transistor is switched "Fully-ON".
The input and Base are connected to VCC
Base-Emitter voltage VBE > 0.7V
Base-Emitter junction is forward biased
Base-Collector junction is forward biased
Transistor is "fully-ON" (saturation region)
Max Collector current flows (IC = Vcc/RL)
VCE = 0 (ideal saturation)
VOUT = VCE = "0"
Transistor operates as a "closed switch"
Then we can define the "saturation region" or "ON mode" when using a bipolar transistor as a
switch as being, both junctions forward biased, IB > 0.7V and IC = Maximum. For a PNP
transistor, the Emitter potential must be positive with respect to the Base.
Then the transistor operates as a "single-pole single-throw" (SPST) solid state switch. With a
zero signal applied to the Base of the transistor it turns "OFF" acting like an open switch and
zero collector current flows. With a positive signal applied to the Base of the transistor it
turns "ON" acting like a closed switch and maximum circuit current flows through the device.
An example of an NPN Transistor as a switch being used to operate a relay is given below.
With inductive loads such as relays or solenoids a flywheel diode is placed across the load to
dissipate the back EMF generated by the inductive load when the transistor switches "OFF"
and so protect the transistor from damage. If the load is of a very high current or voltage
nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay