Component Testing - Transistor

Transistor is a three terminal semiconductor device that used to amplify electric current. This device is said to be one of the most greatest invention of man-kind in electronics because most of electronic gadgets and digital circuitry were composed of transistors. You can see transistor in a simple hand held radio, security system and even computers. There are two general types of transistor in used today - Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET).

fig1. Transistor in different packaging

Bipolar Junction Transistor (BJT)

A Bipolar Junction Transistor (BJT) is an active semiconductor device that is widely used as amplifier and a key component in most of high speed integrated circuit. It is composed of two coupled P-N junction. As shown in fig2, one P-N junction is in between collector and base while the other one is in between emitter and base. It is also a current controlled device because it can delivered a change in output voltage in response to a change in input current.

fig2. A typical representation of Bipolar Junction Transistor (BJT)

Classification of Bipolar Junction Transistor

There are two classification of Bipolar Junction Transistor, NPN and PNP. NPN transistor is formed by introducing a thin region of P-type material between two regions of N-type material. Same thing with PNP transistor, it is formed by introducing a thin region of N-type material between two regions P-type material.

fig3. NPN and PNP transistor

Determining NPN and PNP transistor using Multimeter

Note: We use Red probe as negative (-) and Black probe as positive (+)

1. Set multimeter to minimum resistance range.
2. Connect the positive (+) probe to base terminal and connect negative (-) probe to collector or emitter terminal.
   
3. If the multimeter needle deflects on either connection it is NPN transistor.

To determine PNP transistor reverse the polarity of multimeter probe connection.

Transistor Testing
Note: We use Red probe as negative (-) and Black probe as positive (+). Please set multimeter to minimum resistance. Remove transistor from its circuit.

There are three indications of bad transistor Open, Shorted and Leaky.

Open transistor will give you infinity resistance reading when measured all resistance in base-emitter terminal and base-collector terminal. The needle remains stationary and will not deflect at all.

fig4. base-collector resistance reading for open transistor

fig5. base-emitter resistance reading for open transistor

Shorted transistor will give you zero ohm resistance reading when measured all resistance in base-emitter terminal, base-collector terminal and collector-terminal terminal.

fig6. collector-emitter resistance reading for shorted transistor

fig7. base-collector resistance reading for shorted transistor

fig8. base-emitter resistance reading for shorted transistor

Leaky transistor will give you resistance reading when collector-emitter terminal resistance is measured. Please take note that resistance between emitter and collector should be infinity or needle should not deflect at all because transistor should not conduct at all.

fig9. collector-emitter resistance reading for leaky transistor

As shown in fig10, 11 and 12, good transistor should have the following resistance reading:

fig10. collector-emitter resistance reading for good transistor

fig11. base-emitter resistance reading for good transistor

fig12. base-collector resistance reading for good transistor

For PNP Transistors reverse the polarity of your multimeter test probe.