Transistor is symmetrical to a vacuum triode and relatively very small in size. Transistor is a composition of two words Transfer and Varistor. A transistor consists of three layers of semi conductor material and each layer is having the capability of transferring current to the other layers. This three layer semiconductor device consisting of either two n-type and one p-type layers of material or two p-type and one n-type layers of material. First type is called an npn transistor, while the other is called a pnp transistor respectively.

　　Germanium and silicon are most preferable semiconductor materials which conducts electricity in semi energetic way. By the process of doping to the semi conductor material, the result adds additional electrons to the material or produce holes in the material. The outer layers have widths much greater than the inserted p-type or n-type material that is typically in 10:1 ratio or less. Lower doping level decreases the conductivity and increases the resistance of this material, by limiting the number of free carriers.

　　The difference between the diode and the transistor is: A diode is made up of two layers and one junction. Transistor is made of three layers with two junctions. A transistor can acts as an on/off switch or an amplifier.

　　Operation Modes of the Transistor

　　There are four modes of operations they are saturation, cutoff, active and reverse active. Saturation Mode: In this mode transistor acts as a switch .From collector to emitter the current will flow unconditionally (short circuit). Both diodes are in state of forward biased.

　　Cut-off Mode: In this mode also transistor acts like a switch but there is no current flow from collector to emitter (open circuit). There is no current flow through both emitter and collector terminals.

　　Active Mode: In this mode the transistor acts like an amplifier that is the current from the collector terminal to emitter terminal is corresponding to the current through the base terminal. Base will amplify the current moving into the collector terminal and outgoing from the emitter terminal.

　　Reverse active Mode: The current from the collector terminal to emitter terminal is corresponding to the current through the base terminal but this flow is in reverse direction.