Don't Underrate Transistors, September 1960 Electronics World

Transistors always have been and always will be prone to damage or destruction if operated at temperatures higher than their designed ranges. Modern foundry processes have made it possible for greater heat tolerances for a given transistor size, but care must be taken during circuit design to assure that the devices will under normal ambient conditions not exceed their intended temperature range. Often a heatsink is required in order to use a transistor at its full rated temperature, and sometimes extensive measures are needed to keep the heatsink within an acceptable maximum temperature. Take a look inside your computer for an example of how far heatsink technology has come. The liquid-cooled (desktop) and heat pipe-cooled (laptop) schemes are amazingly efficient and capable of dissipating heat from the CPU package, which would otherwise fry in milliseconds without it.

Explanation of maximum ratings of junction transistors and how to determine maximum operating capabilities.

Table I - Absolute maximum ratings of typical junction transistor used as example.

Normally transistors are not operated at their maximum ratings; however, there are many times when it may be desirable to operate a transistor very close to these maximum ratings. An understanding of the inter-relationship among the various maxima is necessary if the demise of many a good transistor is to be prevented.

Manufacturers of transistors supply specification sheets which provide information comparable to that found in a tube manual. Usually after a brief description of the transistor and its mechanical specifications, the absolute maximum ratings are listed. A typical list of such ratings for a germanium p-n-p junction transistor is shown in Table 1. The transistor is a medium-power unit intended for audio-output applications.

The storage temperatures are the two temperature extremes, low and high, to which the entire transistor-case, leads, whiskers, and semiconductor blank - can be subjected without permanent damage. The low-temperature extreme is usually determined by cold-induced mechanical stresses which may cause the germanium blank to crack. The effects which determine the high-temperature extreme are not as simple. For example, I_co, the a.c. parameters, and the life expectancy of the transistor may all deteriorate with extended storage at elevated temperatures. The results of transistor life tests, however, indicate that life expectancy is an exponential function of the ambient storage temperature and, therefore, the maximum storage temperature is usually chosen to yield a reasonably high life expectancy.

The storage temperatures also have the following influence on transistor operation. The transistor case temperature cannot be below the low-temperature extreme during operation and the transistor junction cannot exceed the high extreme.

Fig. 1 - Collector curves showing limits due to voltage rating.

Fig. 2 - Limits imposed by collector voltage and current maxima.

The absolute maximum collector-to-emitter voltage is the maximum reverse voltage which may be impressed between the emitter and collector without permanently damaging the transistor. The value of this maximum voltage is usually determined by what is known as punch-through. Punch-through occurs when the electric field generated by the collector voltage extends completely across the base region. When this point is reached, transistor action ceases and an extremely large current flows between the emitter and collector.

The common-emitter output characteristic curves for the transistor given in Table 1 are shown in Fig. 1. The diagonally hatched area indicates the region on the curves where operation is not possible without exceeding the absolute maximum voltage rating of the transistor.

The absolute maximum collector current is the maximum current which can flow through the base-collector junction without irreversibly damaging the transistor. This value is determined by the maximum current density the base-collector junction can tolerate. It is a function of base-collector junction area, semiconductor material resistivity, and is different for each type of semiconductor.

Fig. 2 shows the limitations placed on the output curves by both the absolute maximum collector voltage and the absolute maximum collector current.

The product of the two maxima in Fig. 2 gives a power rating of 3 watts. The manufacturer, however, has specified in Table 1 an absolute maximum collector dissipation of 200 milliwatts at an ambient temperature of 25°C (77°F. See Table 2 for converting centigrade to Fahrenheit.) The power dissipated within the transistor and termed collector dissipation is the familiar I²R loss of electrical power which is converted into heat. The maximum allowable collector dissipation is a function of the thermal resistance of the junction and is limited by the maximum allowable junction temperature. The maximum junction temperature is the same as the high temperature limit of the storage temperature ratings discussed earlier. The temperature of the junction is elevated above the ambient, or case, temperature when the transistor is operating, due to collector dissipation. The junction temperature may be calculated for any transistor using the equation: t_j = t_a + kP_C ... (1)

where t_j is the junction temperature, t_a is the ambient temperature, k is the thermal drop of the transistor, and P_C is the collector dissipation. Here k is a function of the mechanical de-sign and heat dissipating mechanism of the transistor., Two values for k are given in Table 1; one for when the transistor is used without a heat sink and the other for when an infinite heat sink is used. The junction temperature of this transistor, when operated at the absolute maximum collector dissipation specified by the manufacturer, can be obtained by substituting values in equation (1) :

Table 2 - Temperature conversion table.

It can be seen from the above that the manufacturer has stipulated a maximum collector dissipation rating that will provide safe junction temperature when the transistor is operated at normal room temperatures. To illustrate the limitations imposed by the maximum collector dissipation rating, a maximum power dissipation area is shown on the collector characteristic curves in Fig. 3. The cross-hatched area indicates the region on the curves where operation at an ambient temperature of 25°C is not possible without exceeding the absolute maximum collector dissipation rating.

If the transistor is operated at an ambient temperature that is higher than 25°C, the collector dissipation must be derated. For example, if the transistor is operated in a sealed equipment case that also contains operating vacuum tubes, the ambient temperature may be as high as 122°F. The maximum collector dissipation allowable for operation without a heat sink at this ambient temperature can be determined by a restatement of equation

Thus it is possible to operate the transistor in an ambient temperature of 122°F provided precautions are taken to limit the maximum collector dissipation to 75 milliwatts. The limitation imposed on transistor operation at 122°F ambient is illustrated in Fig. 4. The relationship between collector dissipation and ambient temperature shown in equation (2) indicates the possibility of safely operating the transistor beyond the manufacturer's absolute maximum collector dissipation rating if the ambient temperature is kept below 25°C.

One method of lowering the ambient temperature is to mount the transistor on a suitable heat sink and then immerse the entire assembly in ice water to a point just short of shorting out the transistor leads. Many mechanical arrangements can be devised to obtain this type of ice bath, and with it, it is possible to maintain an ambient temperature of 0°C (32°F).

The maximum allowable collector dissipation with a depressed ambient temperature can be determined by substituting the appropriate values in equation (2).

Fig. 3 - Limitation imposed by maximum collector dissipation when transistor is operated at 25° C without using heat sink.

Fig. 5 - Collector dissipation limitations for operation at an ambient temperature of 0° centigrade with a heat sink employed. Compare the region of excessive collector dissipation shown here with that obtained under operating conditions shown in the previous figure.

These calculations indicate that it is possible to operate the transistor with a collector dissipation of 1 watt if the ambient temperature is 0°C. The new value of k for operation with a heat sink was obtained from Table 1. Of course, for continuous operation precautions must be taken to insure that the 0°C ambient temperature is maintained Fig. 5 shows the output characteristic curves with the 1-watt power dissipation curve. Since the power output of a transistor power amplifier is approximately equal to collector dissipation times collector efficiency, any increase in collector dissipation without exceeding the manufacturer's absolute maximum ratings results in increased power output. Many manufacturers, rather than providing the values for k, give a collector dissipation-ambient temperature derating curve. Fig. 6 is such a curve for the transistor described in Table 1. From this curve it is possible to read the allowable maximum collector dissipation in milliwatts for any ambient temperature.

Fig. 4 - Limitation imposed by maximum collector dissipation when transistor is operated at 50° C without using heat sink.

Don't Underrate TransistorsSeptember 1960 Electronics World

Don't Underrate Transistors
September 1960 Electronics World