TRANSISTOR DESIGN How to overcome transistor failures?

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Whether manufacturing defects or overuse, a transistor may incur failures during its life cycle. Transistors do not come with expiration dates, but transistor faults are not a new concept in the semiconductor industry. Here is a guide to understanding transistor failures, the reason behind the faults, and possible solutions.

What is a transistor failure?

A failure (or fault) occurs in a transistor when it does not operate effectively. A transistor fault model describes the conditions of transistor failure during logic operations.

A transistor failure can be classified into two failure modes:

Struck-short: The transistor behaves like a short circuit. The resistance of the junction(s) becomes very low or zero. Such a low value makes the transistor operate in a conducting mode.

Struck-open: The transistor behaves like an open circuit. The resistance of the junction(s) becomes extremely high or infinite. Such a high value makes the transistor act as a resistor.

What are the causes of transistor failure?

There are many reasons for a transistor to fail on a component level. The failures occur in the early stages and ending periods of the transistor life cycle. These 7 transistor failures may occur:

• 1. Manufacturing faults

• 2. Hot electrons

• 3. Parasitic structures

• 4. PCB faults

• 5. Contact resistance

• 6. Voltage spikes

• 7. Age of the transistor

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Manufacturing faults: Most electronic components, including transistors, face early-stage failures during manufacturing processes. Such faults could be due to improper packaging, inadequate mechanical stress, false circuitry, wire-related problems, thermal shocks, contamination, or damaged parts. Manufacturing faults reduce the performance capabilities of a transistor and lead to failures during the operation.

Hot electrons: Due to temperature variations, the majority charge carriers may gain sufficient thermal energy to cross the barrier potential. It may hinder the operation of a transistor even before biasing.

Parasitic structures: A parasitic structure forms inside the semiconductor that resembles another device. It hinders the operation of a transistor in its operating modes and makes it behave partially like another device. An example could be the base-emitter junction in BJT behaving like a PN diode, the body diode in MOSFET offering high current, failure of SCR to turn off, etc.

PCB faults: PCB is vulnerable to contamination due to environmental influences. Such faults may occur during the mechanical loads and soldering process.

Contact Resistance: The contact resistance of a transistor should ideally be low. Mechanical pressure and corrosion may alter the contact resistance.

Voltage spikes: When a transistor is turned off, a sudden high-frequency voltage spike may damage the device.

Age of the transistor: When a transistor is in use for multiple projects over a long time, chances are that it is overused. Most electronic components fail over time due to corrosion and physical damage.

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Overcoming the challenges of transistors

Soak-test: Manufacturers are directly responsible for manufacturing faults. To avoid such faults, industries deploy “soak testing”. Also known as the capacity test, the soak test evaluates the transistor operation for an elevated temperature. The test results help manufacturers to eliminate faulty transistors and choose the best ones for commercial use. Soak tests are best suited for power transistors during the early stages.

Self-test: Some faulty transistors may get delivered from manufacturers or damaged over time. You should regularly test your transistors and other related electronic components to save time and money.

Replacement: When a transistor fails, it does not operate in the desired range. The best solution is to replace it with another transistor. When your power device starts to heat frequently, there are high chances of damage.

How to test a power transistor?

Watch this video to learn how to test a transistor with a multimeter:

It is crucial to test a transistor before using it for a project. Whether it is a power transistor or a logic transistor, testing helps to eliminate possible failures. Most transistors, along with other components, are tested on the circuit board before their use. The most common way to test a power transistor is the diode test method. The result of the diode test function indicates either a fully-operational transistor, a dead transistor, or damaged junctions.

Step 1: Remove the transistor from the circuit board. If it is a power transistor, remove it from the protective covering.

Step 2: Set up a Multimeter using red and black probes.

Step 3: Set the selector knob to the diode/buzz symbol.

Step 4: Use the relevant clampers with the probes.

Step 5:

• Two of the probes must touch either of the terminals at the same time.

• When there is no reading, swap the probes and test again.

• If you don’t know the configuration of power BJT (NPN or PNP) or Power MOSFET, you can try using all the probe combinations. However, it is an unlikely situation.

• Reading between two terminals indicates that the transistor is in good condition.

• For Power BJT, there should be a reading between the collector and the emitter.

• For Power MOSFET, there should be a reading between the drain and the source.

Result 1: When the multimeter gives a small reading, the transistor is working. Depending upon the position of the probes, the transistor can be NPN or PNP.

Result 2: When there is no reading even after swapping the probes, the transistor does not work.

Result 3: The multimeter produces a buzzing noise in either or all of the tests. The buzzing sound indicates damage to the junction and failure of the transistor.

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