Transistor is one of the most basic electronic devices, which is used when it is necessary to amplify voltage, current, or power. With a small signal applied to the transistor amplifier as input, the transistor and its associate circuit can produce an amplified version of the input signal. The output signal can be hundreds or even thousands of times larger than the input signal.

           Figure 1. A transistor              Figure 2. Transistor on electronic board

Every transistor has three regions, which is made of doped semiconductors. Due to different ways of construction, there are two types of transistors, npn transistor and pnp transistor. Following is the construction of npn transistor and pnp transistor, respectively.

                    npn Transistor                                    pnp Transistor

Figure 3. Transistor Construction

                            npn Transistor                         pnp Transistor

    Figure 4. Schematic symbols for transistors