What happens when a transistor is in a saturation region ?

With NPN or PNP transistors operation is very similar, except that the polarities are reversed. This discussion is about bipolar transistors. In the active area of operation the current in the emitter base circuit determines the current in the emitter collector circuit according to beta, which is the current gain. The voltage across the emitter base circuit needs to exceed the barrier potential before significant current flows, so it will be 700mV or more for silicon transistors. This base emitter voltage versus current behaves in a non linear way like the forward current in a diode, and a series resistor is used with a higher voltage source to define the base current in a practical circuit. As the base current is increased the collector current also increases. Eventually the increases in base current do not result in a corresponding increase in collector current. The transistor is said to be saturated, as the collector current does not change even when base current is increased. Further explanation: See the first link below for a transistor Current-Voltage (I-V) characteristic and an explanation of saturation. These are sometimes called saturation curves. This shows a family of curves with collector voltage versus the corresponding collector current. There are separate curves for a number of different base currents. A load line (in red) is equivalent to the external resistance in the collector circuit. It is a straight line from the collector supply voltage to the current that flows with that supply voltage and resistance. It just represents a resistor, following ohms law. It shows the voltage and current relationship for that resistance. If the collector current doesn't change with changes in collector voltage then the collector is a constant current circuit, or at least approaching that. That is the horizontal part of the blue lines. These lines represent a very high equivalent resistance, megohms. In each curve there is a low collector voltage where the current no longer follows this constant current slope. At some low voltage the current drops away into an almost vertical line on the left. This is because the collector can no longer maintain constant current. This represents a saturation condition, as the constant current behaviour is not maintained beyond some point as the collector emitter voltage approaches zero, becoming a low collector emitter resistance instead. This is simple enough to follow. In the example the external resistor is 3K (load line) the supply is 15V and the maximum external current is therefore about 5mA from ohms law. The saturation resistance of the collector circuit is represented by the slope of the almost vertical part where all the blue lines join, a couple of hundred ohms in this case. What this is saying is that a transistor is a programmable constant current device in the active region, where the collector current is set by the base current within reason. With external circuit resistance, the base current cannot increase the collector current beyond that externally limited current, so the collector voltage drops and the device is saturated with a low "on" resistance. Application and comparison: A transistor can be used as a switch to turn a suitable lamp or LED on and off. The collector supply is positive (for NPN) and passes through a current limiting resistor for an LED, through the led itself, to the collector, through the transistor to the emitter, and back to the other pole of the supply. When there is no base current the transistor is in the cutoff state, meaning no current flows, and is said to be "off". The full collector supply voltage appears across the collector emitter terminals of the transistor. When biased in the active region (not used here, except during the time for the switching transition) the collector current is proportional to the base current. When biased into the saturated region the transistor is drawing as much current through the LED as the circuit allows. It is said to be "on". There is a small voltage across the collector emitter, usually tens of millivolts. The current through the led might be limited to 10mA whn it is on (by the series ballast resistor). If the transistor has a beta of thirty, the base current only needs to be 10mA/30 to approach saturation.

1) When the base of a transmitter is biased ON (about 0.6 Volts) the transmitter will then either conduct (NPN) or cease conducting (PNP) through its collector/emitter connections. 2) A switch. This can be turned off by removing the base saturation voltage, turning it either on or off, depending if the transistor is NPN, or PNP.

when transistor satuartions state occur means produces more losses