Why Transistors Fail.
- Section 7.1 Why Transistors Fail
- • Manufacturing Faults.
- • Component Age.
- • External Causes.
- • Circuit Design.
- • Power vs. Reliability.
- • Semiconductor Faults.
- Section 7.2 Meters for Transistor Testing.
- • Typical Transistor Faults.
- • Digital and Analogue Multimeters.
- • Available test features.
- • Checking meter polarity.
- Section 7.3 Testing Transistors.
- •The Two Diode Model for BJTs .
- •Identifying Transistor Connections.
- • Testing BJTs.
- • Testing JFETs.
- • Testing MOSFETs
Why do Transistors Fail?
All semiconductor devices are extremely reliable. Provided they are operated correctly there is no reason for them to fail at all; but of course they do fail and this can be for a variety of reasons.
Manufacturing faults do (very occasionally) occur, usually in new equipment. If there is a fault in a new transistor, it will often show up in the first few hours of use. If it operates correctly for this period then the chances are that it will continue to do so. A large proportion of manufacturing faults can be detected by "soak testing" new equipment. That is running it on a test bench for a number of hours to make sure no early failures occur. Items that survive these tests can confidently be put into regular use.
There is no real reason that transistors should suffer from aging. A slice of silicon 10 years old should be the same as a 1-year-old slice. However older systems containing transistors do begin to give more problems. The reason for this is that other components such as resistors may change their values with age, especially if they are subject to heating effects caused by current flow. This may eventually result in a transistor operating outside its normal parameters, for example running at a higher than permitted temperature. It is then that transistors may fail. In such circumstances it is wise to investigate the reasons for the failed transistor rather than just replacing it. Always check the voltages at the transistor terminals after replacement to make sure there are no abnormal readings.
Sometimes external causes may damage or even destroy transistors. Bad handling of FETs can lead to damage by electrostatic discharge. Sometimes this will result in a transistor (or a circuit board) not working when fitted within a system. This can be because the very thin insulating layers within the device have broken down completely due to high voltage static electricity, carelessly applied to the terminals. What is more sinister is that sometimes such discharges do not cause immediate destruction of the device, but do damage the insulation to such a degree that the device fails sometime (hours or years) later.
In mains (line) powered equipment very short lived high voltage pulses can occur from time to time, caused by such events as lightning strikes (even some distance away from the damage location) can damage semiconductors. Also voltage spikes caused locally by such events as inductive equipment such as motors starting or stopping. Most mains powered (and even some low power) circuits liable to such damage have some sort of protection built in that prevents damage. In most cases this protection works well, but it is rarely 100% effective.
Many faults can be found, especially in equipment produced for the home user, by referring to databases of recurrent faults published in technical magazines on the Internet. The reason these recurrent faults occur is basically down to design. Home products are designed to be produced at a profitable price, and to give trouble free operation for a time. Manufacturers are able to produce products that perform to carefully worked out strategies. Some faults will occur due to the product exceeding its "designed life" whilst others will occur prematurely. Designing an electronic product for a particular life span, under conditions that will be very variable (e.g. our homes) and over which the designers have no control, is not a precise science. However such faults as do occur usually follow a distinct pattern, and careful recording of previous faults can be a good indication of future ones. These failures can affect transistors just as easily as any other component.
Power vs. Reliability
When considering an item of faulty equipment, always remember that the reliability of any component is proportional to the power it dissipates. In other words, "If it normally gets hot it normally fails". Such a rule suggests that a failed transistor is more likely to be in the output stages of a circuit than the low voltage, low power stages that precede it. Any circuit which uses either high voltages, high current or both, puts much more stress on semiconductors than low voltage, low current circuits. Although the devices used in these circuits are designed to withstand such use, they do so less well than those devices having a relatively easy life in low power situations. Main problem areas are power supplies and output stages. When faced with a faulty circuit and very little circuit information, a quick check on semiconductors in these stages can save much work.
When a diode or a transistor fails, one of two things usually happens:
• A junction (or junctions) go short circuit (its resistance becomes very low or zero).
• A junction (or junctions) go open circuit (its resistance becomes very high or infinity).
Of course this list could be extended to include that junctions may become leaky (slightly low resistance), though this is rare. In practice this condition is usually followed fairly soon by a complete short circuit.
The above suggests that diodes and transistors can be tested by simple resistance measurements, in most cases this is true. A set of resistance tests can show with a great degree of certainty whether a semiconductor is serviceable or faulty. True, some other faults can occur, and other tests made, but these will be discussed after the all important resistance tests.