Beta value of integrated NPN transistor

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Beta value of integrated NPN transistor [Copy link]

The current amplification capability of a transistor is equal to the ratio of the collector current to the base current. This ratio has many names, including current gain and beta. Different authors use different symbols for it, including β and h FE. A typical integrated NPN transistor has a beta value of about 150. Some special devices can have beta values exceeding 10,000. The beta of a transistor depends on the two compound processes shown in Figure 1.20. Base recombination occurs primarily in the base region between the two depletion regions, which is called the neutral base region. Three factors affect the recombination rate in the base region: the width of the neutral base, the base doping, and the concentration of recombination centers. A thin base shortens the distance that minority carriers need to travel, and also reduces the probability of recombination. Similarly, a lightly doped base reduces the probability of recombination due to the lower concentration of majority carriers. The Gummel number Q B reflects these effects simultaneously. It is calculated by integrating the impurity atom concentration along a line that traverses the neutral base region. In the case of uniform doping, the Gummel number is equal to the base impurity concentration multiplied by the width of the neutral base. Beta is inversely proportional to the Gummel number. The switching speed of a transistor is primarily related to how quickly the excess minority carriers can be removed from the base region. This removal occurs either through the base termination or through recombination. Gold is sometimes intentionally doped into bipolar transistors to increase the number of recombination centers. The increased recombination rate helps the transistor switch faster, but it also reduces the transistor's beta. Few analog integrated circuits are doped with gold because of the low beta. Bipolar transistors usually have a lightly doped base and a heavily doped emitter. This is done to ensure that most of the current consisting of carriers passing through the base-emitter junction is injected from the emitter into the base, rather than vice versa. Heavier doping increases the recombination rate in the emitter, but this effect is limited because only a few carriers are injected into the emitter. The ratio of the current injected into the emitter to the current injected into the base is called the emitter injection efficiency. Most NPN transistors use a wide, lightly doped collector, a heavily doped emitter, and a thin, moderately doped base. The lightly doped collector forms a wide depletion region in the neutral base. This allows a relatively high collector operating voltage to be achieved without avalanche breakdown of the collector-base junction. The asymmetrical doping of the emitter and collector also explains why bipolar transistors do not work properly when the terminals are reversed. (9 This is only partly the reason; the effective base width is also increased when it is in reverse active mode.) A typical integrated NPN transistor with a beta of 150 has a reverse beta of less than 5. This difference is mainly due to the drastic reduction in emitter injection efficiency caused by the lightly doped collector instead of the heavily doped emitter. Beta is also related to collector current. Beta is reduced by low leakage current and low recombination rate in the depletion region. With moderate current, these factors are unimportant and the transistor's beta rises to a peak value determined by the mechanisms discussed above. Large collector currents produce a high-level injection effect that causes beta to roll off. When the concentration of minority carriers in the base approaches the concentration of majority carriers, additional majority carriers accumulate to maintain charge balance. These additional base majority carriers reduce emitter injection efficiency, which in turn reduces beta. Most transistors operate at a moderate current to avoid beta roll-off, but power transistors must often operate at high-level injection due to size limitations. PNP transistors are very similar to NPN transistors. A PNP transistor of the same size and doping level will have a smaller beta than an NPN transistor because holes are less mobile than electrons. In most cases, PNP transistors perform poorly because NPN transistors are optimized and PNP transistors are not. For example, the material used in the base region of an NPN transistor is often used to make the emitter of a PNP transistor. As a result, the final emitter is relatively lightly doped, with low emitter injection efficiency and high-level injection at the appropriate current conditions. Despite these shortcomings, PNP transistors are very useful devices and can be produced on most bipolar processes. Source: Integrated Circuit Education Network (www.ICedu.net)