The BJT Transistor in Saturation

The BJT saturation region is usually defined as the region of operation in which the voltage at the collector of the transistor has fallen so low that current gain (Hfe) has fallen to 10% of its peek value.

The transistor is effectively acting as a low value resistor from the perspective of the collector load.  There is a small voltage across the collector emitter as there would be for a low value resistor and that is called the saturation voltage (eg. 0.2 V.) 

A transistor operating in the saturated region exhibits several distinct characteristics that significantly impact its behavior and applications. 

High Current Conduction: In saturation, the transistor acts like a closed switch, allowing a large current to flow from the collector to the emitter. 

Nonlinear Relationship between Currents: The collector current (IC) and the base current (IB) have a nonlinear relationship in saturation. While IC is still proportional to IB to some extent, the proportionality factor is significantly reduced compared to the active region.

Reduced Gain: (As above) The current gain (β) of a transistor is substantially diminished in saturation. This means that a smaller change in base current results in a smaller change in collector current compared to the active region.

Low Output Impedance: The input impedance of a saturated transistor is very low, almost like a short circuit. In the active region the output impedance is high, meaning a change in the voltage supplied to the collector does not have much impact on the current flowing through the transistor.

Slow Switching Speed: The switching speed of a transistor is significantly hampered in saturation due to the accumulation of charge carriers in the base region. This results in longer rise and fall times for the collector-emitter voltage.

Collector-base capacitance: The collector-base capacitance of a transistor in the saturation region is significantly increased because the collector base junction only has a small negative bias. 

Voltage Regulation: Intrinsic resistances can also contribute to voltage regulation in the saturation region. As the collector current increases, the voltage drop across the intrinsic resistances helps maintain a relatively constant collector-emitter voltage.

Power Dissipation: Saturated transistors dissipate the saturation voltage times the current flow watts. Since the saturation voltage is quite low the power dissipation is low. In high speed operation most of the power loss occurs during switching transitions.  

Applications: Saturated transistors are primarily used as digital switches, where their high current conduction and low input impedance make them suitable for controlling loads. They are also used in some analog circuits where nonlinearity is not a concern.