A transistor is

The correct option is 'a current device'.

Key Points

• A junction transistor is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors or by sandwiching a thin layer of n-type semiconductor between two P-type semiconductors.
• NPN transistor: It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors

• PNP transistor: It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductor

Explanation:

The transistors are used as a current amplifier. 

• Current amplification factor (β): The ratio of the change in the value of the transistor's collector current to the change in the value of the transistor's base current in a transistor is called the current amplification factor.

\(β = \frac{{{\Delta I_c}}}{{{\Delta I_b}}}\)

Additional Information

Transistor consists of three main regions i.e. Emitter, Base and Collector.

Emitter (E) -

• It provides majority charge carriers by which current flows in the transistor.
• Therefore the emitter semiconductor is heavily doped. Hence option 2 is correct.

Base (B) –

• The based region is thin and lightly doped.
• It provides proper interaction between emitter and collector

Collector (C) –

• The size of the collector region is larger than the two other regions and it is moderately dopped.
• The main purpose of the collector is to collect the majority of charge carriers from the emitter.