Placing P-type semiconductor between two N-type semiconductor is NPN transistor
It operates by a small current flow from the emitter to base.current will not flow from the emitter to the collector until a small voltage (at least 0.7 volts) is applied to the base.
Collector current is much larger than the base current.
Placing N-type semiconductor between two P-type semiconductor is PNP transistor.
Its operation is same as NPN but reversed.The voltage relations also reversed.So to turn on the device both the collector and base must be negative.
When the base voltage is about 0.85 volts, sufficient base current flows to turn the transistor fully on. The collector voltage drops to approximately half a volt because of the voltage drop across the collector resistor. A transistor which is conducting the maximum current is said to be in saturation.