![]() ![]() If the base voltage falls below approximately 0.6 V for a silicon transistor, the large emitter-collector current ceases to flow. Moreover, modulating the small base current produces a larger change in collector current. Most of the emitter current of electrons diffuses through the thin base into the collector. Also, few electrons entering the base flow directly through the base to the positive battery terminal. Few electrons injected by the emitter into the base of an NPN transistor fall into holes. A few majority carriers in the emitter, injected as minority carriers into the base, actually recombine. In our NPN transistor example, electrons leaving the emitter for the base would combine with holes in the base, making room for more holes to be created at the ( ) battery terminal on the base as electrons exit. If the base region were thick, as in a pair of back-to-back diodes, all the current entering the base would flow out the base lead. This voltage source needs to exceed 0.6 V for majority carriers (electrons for NPN) to flow from the emitter into the base becoming minority carriers in the P-type semiconductor. This is similar to forward biasing a junction diode. Normally we forward bias the emitter-base junction, overcoming the 0.6 V potential barrier. In Figure below(a), a voltage source has been added to the emitter base circuit. ![]() ![]() There is no current flow, except leakage current, in the collector circuit. The reverse bias voltage could be a few volts to tens of volts for most transistors. Note that this increases the width of the depletion region. It is customary to reverse bias the base-collector junction of a bipolar junction transistor as shown in (Figure above(b). (b) Apply reverse bias to collector base junction. The device in Figure below(a) has a pair of junctions, emitter to base and base to collector, and two depletion regions. We cannot over emphasize the importance of the thin base region. The key to the fabrication of a bipolar junction transistor is to make the middle layer, the base, as thin as possible without shorting the outside layers, the emitter, and collector. In fact, it is far easier to build a pair of back-to-back diodes. If this were the only requirement, we would have no more than a pair of back-to-back diodes. It is as if a third layer were added to a two layer diode. The bipolar junction transistor shown in Figure below(a) is an NPN three layer semiconductor sandwich with an emitter and collector at the ends, and a base in between. commercially the type PR1 is a development sample produced by National Semiconductor.\).The IT124 is made by Intersil Incorporated and is available They are both discrete npn devices type IT124 super-beta dual monolithic silicon planar transistor and type PR1 super-beta transistor. Two types of super-beta transistors are used in this research. The goal of this research is to study the parameters of the super-beta transistor de characteristics, small signal behavior, large signal behavior, and noise - in order to better understand the device. The device requires low bias currents so the undesired output offset voltage can be minimized, and the high current gain can be important to minimize noise. It has application in the first stage of an operational amplifier. It has been shown that circuit techniques are available, namely bootstrapping and cascode connections, that take advantage of the high-current gain of one transistor type and the high breakdown voltage of the other, producing the equivalent of a high gain, high voltage device. However, it can be fabricated simultaneously with high-voltage transistors. This unusually low breakdown voltage precludes its use in standard circuit designs. The super-beta transistor has the disadvantage of collector-to-emitter voltage breakdown of less than 5.0 volts. However, by stopping the emitter diffusion in time, a super-beta transistor can be practically made. When the emitter is deeply diffused, the depletion region of the collector-base junction may penetrate the base and reach through to the emitter, resulting in a collector-emitter short. But there is a trade-off between breakdown voltage and current gain. As the emitter is diffused more deeply into the base, reducing the base width, the current gain will increase. It is well-known that a narrow base width results in a high-beta transistor. These devices are generally called super-beta or super-gain transistors. Transistors with current gains of 1,000 to 10,000 at collector current levels as low as 1 ♚ can now be made in discrete and in monolithic form. ![]()
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