Test methods

To simplify measurement use the common (grounded) emitter connection and divide transistors into two groups according to the kind of semiconductor used: germanium and silicon junction tran­sistors. Tests for point-contact transistors are omitted because these units have negligible commercial value. Engineers in industry feel that junction transistors eventually will replace point-contact units in most applications.

Measuring l_co and l_cbo

Fig. 401. Measuring dc leakage current between collector and base. Emitter is open circuited.

Fig. 401 shows Ico, the dc leakage between collector and base when the emitter is open-circuited. lco increases with temperature, and should be smaller than lcbo—the smaller it is, the "cleaner" the transistor. Clean transistors have longer life because they are relatively free of contaminating materials.

Germanium Junction Transistors

Early plastic-encapsuled transistors were unreliable. Moisture and impurities, trapped during the encapsulation process, "poisoned" the germanium heart of the transistor. This slow killing process caused unstable operation and failure. Several im­portant facts were learned during the evaluation of plastic-coated transistors. First, transistors must be assembled under surgically clean conditions. Second, rigorous factory tests were necessary if quality and performance were to be maintained. Third, the units had to be enclosed in hermetically sealed containers. For example, transistors encapsuled in plastic had relatively large Ico and Icbo readings. When the same units were assembled under surgical conditions and mounted in hermetically sealed cases, these current readings dropped appreciably.

Fig. 402. Measuring dc leakage current between collector and emitter. Base is open circuited.

Fig. 403. Circuit tests p-n-p junctions. Scope is in dc position with slow sweep.

Transistor noise.

Let's clarify the confusion that exists about transistor noise. The confusion began only because the point-contact unit appeared on the market before the junction type. Point-contact transistors have poor noise properties—a great disadvantage. Junction transistors have excellent noise properties. Noise in many junction transistors measures only 3 or 4 db above theoretical (Johnson) noise. Compare this with about 8 db of noise from the 1620 vacuum tube— the best tube as far as noise is concerned. But tube noise increases when the electrodes are subjected to vibration or shock. In junction transistors, noise is related to the ratio of the sizes of collector and emitter junctions. Hence, noise for a given transistor is fixed and does not change appreciably with vibration or shock. Nominally, noise in good junction transistors is from about 10 to 20 db.

Fig. 404. Drawing at the top shows transistor static characteristics. The cir­cuit diagram below it illustrates a method of measuring incremental cur­rent (dc) gain b.

Duality

The duality concept is useful when comparing transistors and vacuum tubes. Base current bias is the dual of grid voltage bias; collector current the dual of plate voltage and collector voltage replaces plate current. In other words, current and voltage func­tions are interchanged when comparing static characteristics of transistors and tubes.

Fig. 404 shows part of a set of transistor static characteristics. The diagram shows how to measure incremental dc current gain. Rl adjusts base bias current from about 1 to 130 amp. With Ec at —6 volts, adjust Rl so that Ic is about 0.3 ma. Note the values of Ic and Ib. Then change Rl slightly and note the new readings in Ic and Ib. Differences in readings are the incremental changes Alc and Ib Values of b vary widely for various transistors of a given type. For this test, practical working limits for b are from about 18 to 140. If b is less than 18, insufficient gain will be obtained. If it is greater than 140, the transistor probably is unstable.

The small-signal current gain beta in low-powered transistors is the dual of  in a triode vacuum tube. A circuit for the measure­ment of  is shown in Fig. 405. The choke should be a UTC HQB-6 or equivalent, and the capacitors pyranol or equivalent non­polarized. The beta factor is defined as collector-to-base current gain with the output short-circuited. Adjust Rl so that Ic is 1 ma. Set El to 1 volt at 1 kc. Resistor R2 essentially shorts out the output.

Fig. 406.   Measuring output impedance. Impedance values for transistors cover a wide  range  with  an  average  of about 40,000 ohms.

Fig. 408. A 2N57 attached to a chassis. The paper clip furnishes an indication of comparative sizes. The useful ac output power of this transistor is approximately six watts.   The mounting technique is an important factor in heat dissipation.

Silicon Junction Transistors

Available silicon junction transistors are of the n-p-n grown junction type. Their principal value is that they can be used at higher temperatures—around 150° C for example. This is because silicon has a higher energy gap (1.1 electron volts) than germanium (0.72 electron volt) . So since the energy gap between filled and conduction bands is large, the intrinsic contribution to conductivity is reduced greatly.

As far as test methods are concerned, we are interested in the same parameters mentioned earlier. The same test circuits can be used (except that battery and meter connections must be reversed for n-p-n transistors).

At room temperatures, Ico and Icbo for low- and medium-powered units should be less than 2 amp with 22.5 volts on the collector.

When a silicon unit is operated at 100° C, Ic„ should be less than 12 Liamp. Noise in silicon units is slightly higher than in germanium units, but is objectionable only in units with high beta factors. Since p adequately describes the performance of available silicon units, and since leakage currents are so small, the dc and incremental dc current gain tests can be postponed until high-power (10 watts or greater) units are available.

Fig. 409. Measuring current gain (dc). The  current  gain  (beta) should   be between 10 and 20.

The spread in  for both low- and medium-powered silicon units is from about 4 to more than 75. There is no official recommendation regarding practical working limits. Some designers and service technicians use an unofficial rule of thumb from about 18 to 36 (as measured with 6 volts on the collector).