A working transistor with its three legs marked P, Q and R...

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 (negative) terminal of the multimeter to $R$ and the other (positive) terminal to $P$ or $Q$, some resistance is seen on the multimeter. Which of the following is true for the transistor ? (A) It is a pnp transistor with $R$ as emitter (B) It is an $n p n$ transistor with $R$ as collector (C) It is an $n p n$ transistor with $R$ as base (D) It is a pnp transistor with $R$ as collector

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 (negative) terminal of the multimeter to $R$ and the other (positive) terminal to $P$ or $Q$, some resistance is seen on the multimeter. Which of the following is true for the transistor ?
(A) It is a pnp transistor with $R$ as emitter
(B) It is an $n p n$ transistor with $R$ as collector
(C) It is an $n p n$ transistor with $R$ as base
(D) It is a pnp transistor with $R$ as collector

Key Concepts

Identification of Transistor Type (npn versus pnp)

The direction in which a diode junction conducts depends on the doping: in an npn transistor the base is p–type and the emitter and collector are n–type, while in a pnp transistor the emitter and collector are p–type and the base is n–type. When a multimeter’s negative (common) terminal is connected to the junction that must serve as the cathode for both diode junctions, the polarity of the other terminals (showing conduction when made positive) can be used to deduce whether the transistor is npn or pnp. This approach allows one to determine which terminal is common – and thus is the base – and to decide the transistor’s actual type.

Biasing of the Transistor Junctions

In a transistor the two p–n junctions (base–emitter and base–collector) will conduct only when they are forward–biased. This means that the role of the base, which is shared between the two junctions, can be deduced by observing that conduction occurs only when the applied polarity makes the p–regions (whether in the emitter or collector) positive relative to the terminal that is common to both junctions. Testing for conduction from one terminal to each of the other two helps identify that common (base) terminal.

Bipolar Junction Transistor (BJT) Structure

A bipolar transistor has three terminals – emitter, base and collector – whose internal structure forms two pn?junctions (emitter–base and collector–base). In a properly functioning device these junctions act like diodes with a distinct forward?bias drop, and the base terminal is common to both junctions. Understanding this three?terminal configuration is essential for interpreting tests and behavior of the device.

Diode Behavior and Multimeter Testing

Diodes conduct current only when they are forward?biased – that is, when the p–side is at a higher potential than the n–side. A multimeter set to measure diode drops (or resistance) applies a small voltage so that if a pn junction is forward–biased, a characteristic drop (often about 0.6–0.7 V for silicon devices) is observed. This property is used to test the internal junctions of a transistor by checking conduction between various pairs of terminals while changing which lead is positive and which is negative.

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