Transistor Life Expectancy

Author: Leonard Krugman

One of the outstanding features of the transistor is its practically indefinite life expectancy. Long life was originally predicted on the basis of the transistor construction and its conduction mechanics, which indicate there is nothing to wear out. Although the transistor is still very young, enough experimental data is now available to back the initial long life predictions.

The usual transistor failure occurs gradually over a long period of time and after thousands of hours of operation. The performance degradation generally shows up as an increasing saturation current. (The effect of increasing saturation current was covered in the transistor amplifier chapter.) While the various self and fixed biasing methods may be used to minimize the effects of increasing I_co, the system's efficiency and gain suffer. In an amplifier circuit, this factor decreases the available volume. Gradually, as the limit of the automatic biasing arrangement is reached, there is also a noticeable increase in the distortion content.

Another variation in the transistor performance characteristics is a gradually decreasing output resistance. In systems designed for an image impedance match (R_g - r_i and R_L = r_o), this change introduces a mismatch loss. In the usual amplifier design, however, the output resistance is in the order of 20 to 50 times the load resistance. The decrease in r_o, therefore, is less serious than the increase in I_co. The best single maintenance check is a measurement of the current gain.

Sudden failures of transistors are not common in normal operation, although open emitter and collector junctions were not too rare in the early transistors. These defects were attributed to faulty assembly during manufacture. Present manufacturing and quality control techniques have practically eliminated open junction defects. Transistor shorts are more common since they are usually caused by overloading. When the transistor power rating is exceeded, the junction temperature rises quickly. The increased heating effect encourages diffusion of collector region impurities into the base layer, which, in time, will destroy the junction. In brief then, open circuited transistors generally result from poor production; short-circuited junctions generally mean improper circuit design.