Question

A N-P-N transistor conducts when collector is and emitter is with respect to base. (A) positive, negative (B) positive, positive (C) negative, negative (D) negative, positive

Short answer

An N-P-N transistor conducts when the collector is positive and the emitter is negative with respect to the base. Therefore, the correct answer is Option A (positive, negative).

Step-by-step solution

Understanding N-P-N Transistor

An N-P-N transistor consists of three layers: an N-type semiconductor layer sandwiched between a P-type layer (the base) and another N-type layer (the collector). When a positive voltage is applied to the base with respect to the emitter, the electrons move from the emitter (N-type) to the base (P-type), and then from the base to the collector (N-type). This establishes a current flow through the transistor, allowing it to conduct. Now, let's analyze each option and determine which one corresponds to the N-P-N transistor's correct conduction conditions.

Option A: Positive Collector, Negative Emitter

When the collector is positive and the emitter is negative with respect to the base, electrons move from the emitter (N-type) to the base (P-type) and then to the collector (N-type), resulting in conduction. This matches the behavior of an N-P-N transistor.

Option B: Positive Collector, Positive Emitter

When both the collector and emitter are positive with respect to the base, the electrons in the emitter (N-type) and collector (N-type) are repelled by the positive voltage and do not move towards the base (P-type). As a result, the transistor does not conduct in this condition.

Option C: Negative Collector, Negative Emitter

When both the collector and emitter are negative with respect to the base, the electrons in the base (P-type) are repelled by the negative voltage, preventing them from moving towards the collector (N-type) and emitter (N-type). This results in the transistor not conducting in this condition.

Option D: Negative Collector, Positive Emitter

When the collector is negative and the emitter is positive with respect to the base, the electrons in the emitter (N-type) move towards the base (P-type), but then, they are repelled by the negative voltage at the collector (N-type) and do not move to the collector. This doesn't allow for conduction.

Conclusion

Out of the given options, option A (collector positive and emitter negative with respect to the base) represents the correct conditions for an N-P-N transistor to conduct.

Key concepts

Semiconductors

Semiconductors form the backbone of modern electronics, bridging the gap between conductors and insulators. They are made from materials like silicon and germanium, which have an electrical conductivity between that of a conductor and an insulator. This unique property makes semiconductors ideal for controlling electrical current.
They enable the fabrication of various electronic components such as diodes, transistors, and integrated circuits.

• Intrinsic Semiconductors: Pure forms of semiconductor materials without any impurities.
• Extrinsic Semiconductors: Semiconductors that have been doped with impurities to enhance their conductivity.

In the case of the N-P-N transistor, it uses N-type and P-type semiconductor layers.
N-type semiconductors contain extra electrons, while P-type semiconductors have holes where electrons can move, facilitating the movement of current.

Transistor Conduction

Transistor conduction is the process by which a transistor controls the flow of electrical current. In an N-P-N transistor, this process is achieved by manipulating the voltage applied to the base, collector, and emitter terminals.
When the base-emitter junction is forward-biased (applying a positive voltage to the base relative to the emitter), electrons from the N-type emitter move into the P-type base.
This movement is crucial:

• Forward-bias Condition: Ensures that electrons flow easily from the emitter to the base.
• Reverse-bias Condition:** Stops the flow by creating a barrier for electrons.

The positive collector attracts these electrons from the base, thus completing the circuit and allowing current to flow through the transistor from the collector to the emitter, which is described as transistor conduction.

Electronic Components

Electronic components are the essential parts used to build electronic circuits. They are often classified according to their electrical functionality and purpose within a circuit.
These components include resistors, capacitors, diodes, and notably, transistors like the N-P-N transistor. The N-P-N transistor stands out as a main component because it can amplify signals or act as a switch depending on how it is configured within the circuit.
Transistors are fundamentally important because they have a wide range of applications:

• Amplification: Increasing the power of a signal.
• Switching: Turning electrical signals on and off.

Understanding how these components work together in a circuit is crucial for designing and troubleshooting electronic devices.

Current Flow in Transistors

Current flow in transistors is a fundamental concept that determines how these devices operate. In an N-P-N transistor, current flow can be understood through the interaction of electron movement from one terminal to another.
For conduction to occur in an N-P-N transistor, three main currents must be considered: the emitter current (I_e), the base current (I_b), and the collector current (I_c). These currents are interconnected by the relationship: \( I_e = I_b + I_c \).

• Emitter Current (\( I_e \)): Is primarily the total current created by electron flow from the emitter.
• Base Current (\( I_b \)): Is much smaller than \( I_e \), as it mainly controls the transistor's operation.
• Collector Current (\( I_c \)): Is the largest, as it involves most of the electrons that flow through the transistor.

The flow of these currents ensures that the N-P-N transistor can switch, amplify, and control electronic signals in various applications.