Transistor Comparisons

Author: Leonard Krugman

Fig. 2-8. (A) Typical point-contact V_G-I_C characteristic.

To understand the factors which cause the relatively high collector resistance of the junction transistor as compared to the relatively low collector resistance of the point-contact transistor requires the aid of typical collector current-voltage characteristics. Figure 2-8 (A) illustrates the V_c-I_c characteristic of a typical point-contact transistor. As the collector voltage is raised above 5 volts, the current continues to increase, although at a diminishing rate due to the lack of available electrons in the transistor. As discussed previously, the holes in the point-contact transistor set up a positive space charge in the vicinity of their flow paths, attracting electrons from the more remote areas of the pellet. Thus, the electrons available for collector current decrease gradually.

Fig. 2-8. (B) Typical junction V_c-I_c characteristic.

Figure 2-8 (B) illustrates the V_c-I_c characteristic of a typical junction transistor. Here the V_c-I_c characteristic again follows an Ohm's law relationship at small values of collector voltage. The point of electron exhaustion is reached very abruptly, since there is no hole space-charge effect in the junction transistor to increase the available supply of electrons. After the critical voltage is attained, a large increase in collector voltage causes only a very small increase in collector current. The collector resistance is equal to the change in collector voltage divided by the resulting change in collector current: . For the typical junction transistor characteristic illustrated, the collector resistance from point A to B is 0.5 /(210^-3) = 250 ohms, and from point B to C the collector resistance is 50 / (0.0510^-3) = 1000000 ohms.

Because of the large collector resistance, and the resultant high resistance gain, the junction transistor is capable of far greater voltage and power gains than the point-contact types. Commercially available transistors with collector resistances in the neighborhood of 3 megohms are common; silicon-type junction transistors are inherently capable of far greater values.

The basic transistors have an upper frequency limit due to the small but finite time it takes the current carriers to move from one electrode to another. This limit, called the "alpha cutoff frequency," defines the point at which the gain is 3 db down from its low frequency value. The frequency response characteristics of transistors are considered more fully in Chapter 7.

In this chapter and those that follow, the germanium point-contact and junction transistors are considered at great length. This is not intended to create an impression that the entire field of semiconductors is limited to these two fundamental types. However, since their characteristics are basic to other semiconductor devices, a thorough understanding of the prototypes is essential. At this time, it appears that an unlimited number of variations of the original transistors is possible. Several of the more significant devices will now be considered.