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Difficulty: HardTransistors

A technician is testing an unlabeled three-terminal semiconductor component using a digital multimeter in diode-check mode. The meter indicates a forward voltage drop of approximately 0.7 V0.7\text{ V} when the positive (red) meter lead is placed on Terminal 1 and the negative (black) lead is placed on either Terminal 2 or Terminal 3. All reverse probe combinations, as well as direct measurements between Terminal 2 and Terminal 3, show an open-circuit (over-limit) reading. Which device type and lead configuration does this component represent?

  1. A
    PNP bipolar junction transistor with Terminal 1 as the base
  2. NPN bipolar junction transistor with Terminal 1 as the baseAnswer
  3. C
    NPN bipolar junction transistor with Terminal 1 as the collector
  4. D
    PNP bipolar junction transistor with Terminal 1 as the emitter

Answer

An NPN bipolar junction transistor with Terminal 1 as the base.
In a bipolar junction transistor (BJT), the base-emitter and base-collector regions behave as two separate PN junction diodes sharing a common base material. For an NPN transistor, the base is composed of P-type semiconductor material, while the collector and emitter are N-type. Applying a positive test lead to the P-type base (Terminal 1) and a negative lead to either N-type terminal (Terminals 2 and 3) forward-biases each junction, yielding the standard silicon forward drop of approximately 0.7 V0.7\text{ V}.

Step-by-Step Solution

1
Analyze the multimeter diode test behavior
Conduction occurs only when Terminal 1 is connected to the positive meter lead and Terminals 2 or 3 are connected to the negative lead.
A digital multimeter diode test applies a small forward voltage from the red (+) probe to the black (-) probe to measure the junction threshold drop.
2
Identify the semiconductor layer structure
Terminal 1 acts as a common P-type anode, while Terminals 2 and 3 act as separate N-type cathodes.
Silicon PN junctions drop approximately 0.7 V0.7\text{ V} when forward-biased (anode positive relative to cathode).
3
Determine the transistor type and lead identity
The component is an NPN transistor where Terminal 1 is the Base (P-region), Terminal 2 is the Collector/Emitter (N-region), and Terminal 3 is the Emitter/Collector (N-region).
An NPN transistor electrically models as two back-to-back diodes with anodes joined at the base.

Key Concept

BJT internal PN junction polarity and diode-test verification
Estimated Time:1m 15s
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Topics

Transistors

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