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How to Check Semiconductors

Semiconductors are circuit components made from semiconducting materials such as silicon or germanium. They are useful because of their characteristic of conducting electricity only when certain conditions are met, such as receiving a minimum current or voltage. The diode is the most basic type of semiconductor. It has many functions, and may be optimized to perform specific tasks. LEDs, for example, are specifically constructed to provide illumination to other circuits and devices. Transistors are semiconductors that perform amplifying or switching functions, and thyristors are semiconductors constructed to perform as switches. To check semiconductors, first learn how to test a diode with a multimeter, then use the multimeter to check the diode-like properties of the component of interest.

Things You'll Need

Digital multimeter
NPN or PNP diode
NPN transistor

Instructions

Checking Diodes
- 1
  Switch on the multimeter and put it on the diode setting. The diode setting has an arrow-like symbol on the casing.
- 2
  Measure the diode's forward-biased voltage. Do this by placing the positive or red probe of the multimeter on the anode side of the diode, which is unmarked. Place the multimeter's negative or black probe on the cathode side of the diode, which has a stripe.
- 3
  Record the voltage. It will will read about 0.5 to 0.7 volts for a normal silicon diode. For a normal germanium diode, it will typically read around 0.1 - 0.3 volts. These measurements show that the diode conducts when it is forward-biased.
- 4
  Measure the reverse-biased voltage. Do this by placing the multimeter's red lead on the cathode side of the diode, while placing its black lead on the anode side of the diode. The reading will show infinite resistance, or no current flow, which means that the diode is non-conducting in this direction.
Checking Transistors
- 5
  Identify the base, emitter and collector leads on the NPN transistor. Do this by studying the data sheet or the back of the component's package.
- 6
  Study the transistor's diode properties. An NPN is modeled as two back-to-back diodes, where the base-emitter behaves as a forward-biased diode, and the base-collector behaves as a reverse-biased diode.
- 7
  Test the forward and reverse-biased behavior of the base and emitter transistor leads. The multimeter must be on the diode setting, and the transistor should be face up. Place the multimeter's red probe on the base and its black probe on the emitter, and then reverse them. The base-emitter will conduct, but the emitter-base will not.
- 8
  Test the reverse and forward-biased behavior of the base and collector transistor leads by first placing the multimeter's red probe on the base and the black probe on the collector, and then reversing the probes. Do the same for the collector-emitter leads. The base-collector will conduct, which means that it is reverse-biased or against easy current flow, but the collector-base will not. The collector-emitter will not conduct regardless of how the probes are placed.

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