Transistor: Combination of two p-n junctions

Introduction:
A transistor is a three-layer semiconductor device that is widely used in electronic circuits. It is made up of two p-n junctions, which are formed by doping the semiconductor material with impurities. The combination of these two p-n junctions allows the transistor to function as an amplifier or a switch.

Explanation:
The correct answer to the given question is option 'D', which states that a transistor is a combination of two p-n junctions with their p region connected together and n region connected together. Let's understand this in detail:

P-N Junction:
A p-n junction is formed by joining a p-type semiconductor (which has an excess of positive charge carriers, or holes) with an n-type semiconductor (which has an excess of negative charge carriers, or electrons). This creates a depletion region at the junction, which acts as a barrier for the flow of current.

Transistor Structure:
A transistor is typically made up of three layers of semiconductor material: two layers of one type (either p or n) sandwiched between a layer of the opposite type.

Emitter-Base Junction:
The first p-n junction in a transistor is called the emitter-base junction. It is formed between the heavily doped p-type region (emitter) and the lightly doped n-type region (base). The emitter-base junction is forward-biased, meaning that the p-side is connected to the positive terminal of the power supply and the n-side is connected to the negative terminal.

Base-Collector Junction:
The second p-n junction in a transistor is called the base-collector junction. It is formed between the lightly doped p-type region (base) and the heavily doped n-type region (collector). The base-collector junction is reverse-biased, meaning that the p-side is connected to the negative terminal of the power supply and the n-side is connected to the positive terminal.

Operation:
When a voltage is applied across the emitter-base junction, it causes a current to flow from the emitter to the base. This current consists of both electrons (from the emitter) and holes (from the base).

The base-collector junction, being reverse-biased, prevents the majority of the current from flowing through it. However, a small current (known as the collector current) can flow from the base to the collector.

The transistor operates in three different regions: cutoff, active, and saturation. In the active region, the transistor acts as an amplifier, while in the cutoff and saturation regions, it acts as a switch.

Conclusion:
In conclusion, a transistor is a combination of two p-n junctions, with their p region connected together and n region connected together. This arrangement allows the transistor to function as an amplifier or a switch, depending on the applied voltage and the configuration of the circuit.