Problem 64 - Master Resource Book in JEE Main Physics

Question

A A working transistor with its three legs marked \(P, Q\) and \(R\) is tested using a multimeter. No conduction is found between \(P\) and \(Q .\) By connecting the common (- ve) terminal of the multimeter to \(R\) and the other (positive) terminal to \(P\) or \(Q\), some conduction is seen on the multimeter. Which of the following is true for the transistor? (a) It is an \(n-p-n\) transistor with \(R\) as base (b) It is an \(p-n-p\) transistor with \(R\) as collection (c) It is a \(p-n-p\) transistor with \(R\) as emitter (d) It is an \(n-p-n\) transistor with \(R\) as collector

Step-by-Step Solution

Answer

It is an npn transistor with R as base.

Step 1: Understanding Transistor Configuration

Transistors have three regions called Emitter, Base, and Collector. The connectivity between these regions helps determine the type of transistor (npn or pnp). In a npn transistor, the base is p-type, and in a pnp transistor, the base is n-type.

Step 2: Analyzing Multimeter Readings

The multimeter shows no conduction between terminals P and Q, which indicates no direct connection between those two points without reference to a third terminal, which suggests a possibility of the base terminal being different from P and Q.

Step 3: Understanding Terminal Polarity Based on Readings

When the multimeter's negative terminal is connected to R and the positive to either P or Q, conduction occurs. This suggests that R must be a positively doped region (p-type) in a npn transistor or a negatively doped region (n-type) in a pnp transistor.

Step 4: Identifying the Transistor Type

The conduction occurs with the negative terminal on R, which confirms R is acting as a p-type base. Therefore, R is the base in an npn transistor, as the base must be positive relative to the emitter or collector for conduction to occur in an npn type.

Step 5: Determining Terminal Roles

As R is identified as the base in an npn transistor, P or Q must be the collector or emitter. The specific conduction setup implies P and Q have different characteristics compared to R. Further testing could reveal stronger conduction with either P or Q, indicating the emitter position among them.

Key Concepts

NPN Transistor

An NPN transistor is a type of bipolar junction transistor. It consists of three distinct regions: the emitter, the base, and the collector. In an NPN transistor, the outer regions, known as the emitter and collector, are made of n-type semiconductor material. The middle region, called the base, is composed of p-type material. This configuration allows it to control the flow of current when a positive voltage is applied to the base, relative to the emitter.

When in use, the base acts as a gate for electrons to transition from the emitter to the collector. The base is thin and lightly doped, allowing a small current to control a larger current flow from the emitter to the collector. The NPN transistor is generally used in amplification and switching applications due to its efficient control of electron flow in the circuit.

• Emitter: Heavily doped with a large number of free electrons.
• Base: Thin and lightly doped, facilitates the control of electron flow from emitter to collector.
• Collector: Moderately doped and large to dissipate heat efficiently.

PNP Transistor

A PNP transistor operates oppositely compared to an NPN transistor. It also consists of three regions: emitter, base, and collector. However, in a PNP transistor, the outer regions (emitter and collector) are made of p-type material, while the base is made of n-type material.

The PNP transistor is characterized by the movement of holes from the emitter to the collector, controlled by a small current flowing out of the base. The base, being n-type, requires a lower voltage than the emitter for the transistor to conduct.

• Emitter: Supplied with enough holes to support charge flow.
• Base: Thin and n-doped, controls the flow of holes from emitter to collector.
• Collector: Captures holes transmitted from the emitter.

This type of transistor is typically used in similar applications as NPN transistors but with different polarity requirements.

Multimeter Usage

A multimeter is a versatile tool essential in electronics for measuring electrical characteristics such as voltage, current, and resistance. When testing a transistor, a multimeter can be used to diagnose the conduction properties of the transistor's terminals.

To test a transistor, first identify the terminals, usually marked as P, Q, and R. Set the multimeter to the resistance measurement mode, and check for conduction between these terminals by following these steps:

• Connect the multimeter's negative lead to one terminal, say R.
• Connect the positive lead to another terminal, say P or Q.
• Observe the readings; a reading indicates conduction.

The negative lead of the multimeter should be connected in such a way that it reflects the inherent material type of the transistor segment, allowing you to determine the type of transistor and its configuration.

Transistor Terminals

Transistors have three key terminals – the emitter, base, and collector. Each terminal plays a critical role in the transistor's operation. Understanding these roles is essential to effectively diagnose and work with transistors through experiments and diagnostic tools such as a multimeter.

Here's a closer look at each terminal and its function:

• Emitter: This terminal emits charge carriers (electrons or holes) into the base. In an NPN transistor, the emitter emits electrons.
  This terminal is often connected to a current source for the transistor’s operation.
• Base: Serves as the control terminal of the transistor. It is responsible for modulating the flow of carriers between the emitter and the collector.
  Due to its narrow width and light doping, it can effectively control larger currents with a small controlling current.
• Collector: This terminal collects carriers from the base and transmits them to the circuit. It is designed to handle the major current flow.
  The collector region is usually larger, allowing it to dissipate heat effectively.

Identifying and testing these terminals correctly is fundamental for determining the type and configuration of the transistor.