Transistor Operation

npn- Transistor Operation

A forward-biased pn- junction is comparable to a low-resistance circuit element because it passes a high current for a given voltage. In turn, a reverse-biased pn- junction is comparable to a high-resistance circuit element. By using the Ohm's law formula for power (P = I²·R) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a crystal were to contain two pn- junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. In this manner, a power gain would be obtained across the crystal. This concept, which is merely an extension of the material covered in the previous topics, is the basic theory behind how the transistor amplifies. With this information fresh in your mind, let's proceed directly to the npn transistor.

collector

base

emitter

forward bias

Figure 1: The forward-biased junction in an npn transistor

Just as in the case of the pn- junction diode, the n- material comprising the two end sections of the npn transistor contains a number of free electrons, while the center p section contains an excess number of holes. The action at each junction between these sections is the same as that previously described for the diode; that is, depletion regions develop and the junction barrier appears. To use the transistor as an amplifier, each of these junctions must be modified by some external bias voltage. For the transistor to function in this capacity, the first pn- junction (emitter-base junction) is biased in the forward, or low-resistance, direction. At the same time the second pn- junction (base-collector junction) is biased in the reverse, or high-resistance, direction. The letters of these elements indicate what polarity voltage to use for correct bias. For instance, notice the npn transistor shown in figure 2:
The emitter, which is the first letter in the npn sequence, is connected to the negative side of the battery while the base, which is the second letter (npn), is connected to the positive side. However, since the second pn- junction is required to be reverse biased for proper transistor operation, the collector must be connected to an opposite polarity voltage (positive) than that indicated by its letter designation (npn). The voltage on the collector must also be more positive than the base, as shown beside:
We now have a properly biased npn transistor.
In summary, the base of the npn transistor must be positive with respect to the emitter, and the collector must be more positive than the base.

collector

base

emitter

reverse bias

forward bias

Figure 2: The junctions in an npn transistor.

npn forward-biased junction

An important point to bring out at this time, which was not necessarily mentioned during the explanation of the diode, is the fact that the n- material on one side of the forward-biased junction is more heavily doped than the p material. This results in more current being carried across the junction by the majority carrier electrons from the n- material than the majority carrier holes from the p material. Therefore, conduction through the forward-biased junction, as shown in figure 3, is mainly by majority carrier electrons from the n- material (emitter).

hole flow

electron flow

Bild 3: Strom durch den pn- Übergang in Durchlasspolung

With the emitter-to-base junction in the figure biased in the forward direction, electrons leave the negative terminal of the battery and enter the n- material (emitter). Since electrons are majority current carriers in the n- material, they pass easily through the emitter, cross over the junction, and combine with holes in the p material (base). For each electron that fills a hole in the p material, another electron will leave the p material (creating a new hole) and enter the positive terminal of the battery.

npn reverse-biased junction

collector

base

emitter

reverse
biased
junction

hole flow

electron
flow

Figure 4: The reverse-biased junction in an npn transistor.

The second pn- junction (base-to-collector), or reverse-biased junction as it is called (figure 4), blocks the majority current carriers from crossing the junction. However, there is a very small current, that does pass through this junction. This current is called minority current, or reverse current. As you recall, this current was produced by the electron-hole pairs. The minority carriers for the reverse-biased pn- junction are the electrons in the p material and the holes in the n- material. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the p material enter the n material, and the holes from the n- material enter the p- material. However, the minority current electrons play the most important part in the operation of the npn transistor.

npn- junction interaction

The bias batteries in the figure 5 have been labeled V_CC for the collector voltage supply, and V_BB for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts. This difference in supply voltages is necessary to have current flow from the emitter to the collector.

collector

base

emitter

reverse
bias

forward
bias

hole flow

electron
flow

V_BB

V_CC

Figure 5: npn transistor operation is basically the action of a relatively small emitter-base bias voltage controlling a relatively large emitter-to-collector current.

The current flow in the external circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of the supply batteries to the n-type emitter. This combined movement of electrons is known as emitter current (I_E). Since electrons are the majority carriers in the n- material, they will move through the n- material emitter to the emitter-base junction. With this junction forward biased, electrons continue on into the base region. Once the electrons are in the base, which is a p-type material, they become minority carriers. Some of the electrons that move into the base recombine with available holes. For each electron that recombines, another electron moves out through the base lead as base current I_B (creating a new hole for eventual combination) and returns to the base supply battery V.
The electrons that recombine are lost as far as the collector is concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped. This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons that move into the base region come under the influence of the large collector reverse bias. This bias acts as forward bias for the minority carriers (electrons) in the base and, as such, accelerates them through the base-collector junction and on into the collector region. Since the collector is made of an n-type material, the electrons that reach the collector again become majority current carriers. Once in the collector, the electrons move easily through the n material and return to the positive terminal of the collector supply battery V_CC as collector current (I_C).
To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage.

Figure 6: total current flow in the npn transistor

In summary, total current flow in the npn transistor is through the emitter lead. Therefore, in terms of percentage, I_E is 100 percent. On the other hand, since the base is very thin and lightly doped, a smaller percentage of the total current (emitter current) will flow in the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total current is base current (I_B) while the remaining 95 to 98 percent is collector current (I_C). A very basic relationship exists between these two currents:

I_E = I_B + I_C

In simple terms this means that the emitter current is separated into base and collector current. Since the amount of current leaving the emitter is solely a function of the emitter-base bias, and because the collector receives most of this current, a small change in emitter-base bias will have a far greater effect on the magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter-base bias controls the relatively large emitter-to-collector current.