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A transistor state change is a shift in how the device conducts current. In a bipolar junction transistor (BJT) used as a switch, it usually means moving between cutoff (off, with negligible collector current) and saturation (on, conducting as strongly as the load circuit allows). The input changes the transistor’s bias; the circuit’s power supply provides the energy for the load.
What changes when a BJT switches?
A BJT has a control path from base to emitter and a load-current path from collector to emitter. In a switch circuit, a signal that supplies base current can allow current from the supply to flow through the collector-emitter path and the load. Removing that drive moves the transistor toward cutoff and reduces current through the load.
The base signal controls the path; it does not supply the load’s energy. The power supply connected to the load provides that energy. The exact current and voltages depend on the transistor, its bias, and the surrounding circuit.
Cutoff, saturation, and active mode
| Operating region | What the BJT does | Typical role |
|---|---|---|
| Cutoff | Collector current is negligible in the simplified switching model; real devices can have leakage. | Switch off |
| Active mode | Base current regulates collector current. | Amplification or controlled current |
| Saturation | The transistor is driven strongly on, so the load circuit largely determines the current. | Switch on |
These regions are not three unrelated device settings: they describe different bias conditions. A switching circuit aims to use cutoff and saturation as its off and on extremes. An amplifier commonly uses active mode, where changes in base drive regulate collector current rather than simply turning the load path fully off or on. See All About Circuits’ explanation of a BJT as a switch and its overview of active-mode operation.
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How transistor states relate to digital logic
A digital circuit can use transistor switching to help produce output voltages that another part of the circuit interprets as binary values. But the transistor’s operating state is not itself the logic value, and the output voltage is not necessarily the same as the input voltage. The circuit arrangement determines how an off or on transistor affects the output; for example, a switching action can cause an output to move in the opposite direction from the control signal.
In other words, distinguish three things: the input signal, the transistor’s bias and operating region, and the output voltage interpreted by the circuit. All About Circuits’ discussion of digital signals and gates explains how transistor operation can support binary logic levels.
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What “on” does not mean
There is no single voltage that defines “on” for every BJT in every circuit. A base-emitter voltage often quoted for a silicon transistor is an illustrative value, not a universal threshold; the relevant behavior depends on the device and operating conditions. Likewise, an example saturation voltage between collector and emitter is not a guaranteed value for all transistors or loads. Assess the bias and current in the circuit at hand rather than treating a rule-of-thumb voltage as the definition of a state change.
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“Transistor” includes different device families, but cutoff, active mode, and saturation here describe BJT behavior. These terms should not be assumed to explain every transistor type in the same way.
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