Transistors are semiconductor devices with three or more terminals. The operation of normal transistors has already been discussed, but there are several transistors with special properties that should be explained. As with diodes, a discussion of all the developments in the transistor field would be impossible. The unijunction transistor (UJT) and the field effect transistor (FET) will be discussed because of their widespread application in Navy equipment. Many other special transistors have been developed and will be discussed in later NEETS modules.
The Unijunction Transistor (UJT)
The UNIJUNCTION TRANSISTOR (UJT), originally called a double-based diode, is a three-terminal, solid-state device that has several advantages over conventional transistors. It is very stable over a wide range of temperatures and allows a reduction of components when used in place of conventional transistors. A comparison is shown in figure 3-38. View A is a circuit using conventional transistors, and view B is the same circuit using the UJT. As you can see, the UJT circuit has fewer components. Reducing the number of components reduces the cost, size, and probability of failure.
Figure 3-38A. - Comparison of conventional transistors and UJT circuits.
Figure 3-38B. - Comparison of conventional transistors and UJT circuits.
The physical appearance of the UJT is identical to that of the common transistor. As shown in figure 3-39, both have three leads and the same basic shape; the tab on the case indicates the emitter on both devices. The UJT, however, has a second base instead of a collector.
Figure 3-39. - Transistor and UJT.
As indicated in the block diagram shown in views A and B of figure 3-40, the lead differences are even more pronounced. Unlike the transistor, the UJT has only one PN junction. The area between base 1 and base 2 acts as a resistor when the UJT is properly biased. A conventional transistor needs a certain bias level between the emitter, base, and collector for proper conduction. The same principle is true for the UJT; it needs a certain bias level between the emitter and base 1 and also between base 1 and base 2 for proper conduction.
Figure 3-40. - Transistor and UJT structure.
The normal bias arrangement for the UJT is illustrated in figure 3-41, view A. A positive 10 volts is placed on base 2 and a ground on base 1. The area between base 1 and base 2 acts as a resistor. If a reading were taken between base 1 and base 2, the meter would indicate the full 10 volts as shown in view B. Theoretically, if one meter lead were connected to base 1 and the other lead to some point between base 1 and base 2, the meter would read some voltage less than 10 volts. This concept is illustrated in figure 3-42, view A. View B is an illustration of the voltage levels at different points between the two bases. The sequential rise in voltage is called a voltage gradient.
Figure 3-41A. - UJT biasing.
Figure 3-41B. - UJT biasing.
Figure 3-42A. - UJT voltage gradient.
Figure 3-42B. - UJT voltage gradient.
The emitter of the UJT can be viewed as the wiper arm of a variable resistor. If the voltage level on the emitter is more positive than the voltage gradient level at the emitter-base material contact point, the UJT is forward biased. The UJT will conduct heavily (almost a short circuit) from base 1 to the emitter. The emitter is fixed in position by the manufacturer. The level of the voltage gradient therefore depends upon the amount of bias voltage, as shown in figure 3-43.
Figure 3-43. - Forward bias point on UJT voltage gradient.
If the voltage level on the emitter is less positive than the voltage gradient opposite the emitter, the UJT is reverse biased. No current will flow from base 1 to the emitter. However, a small current, called reverse current, will flow from the emitter to base 2. The reverse current is caused by the impurities used in the construction of the UJT and is in the form of minority carriers.
More than 40 distinct types of UJTs are presently in use. One of the most common applications is in switching circuits. They are also used extensively in oscillators and wave-shaping circuits.
Q.26 The UJT has how many PN junctions?