Few at (b) flow on through the base to the (+) battery terminal as if the base were a resistor. Not shown, holes in the base may diffuse into the emitter and combine with electrons, contributing to base terminal current. A few at Figure above(a) fall into holes in the base that contribute to base current flow to the (+) battery terminal. These electrons face four possible fates entering the thin P-type base. Majority carriers within the N-type emitter are electrons, becoming minority carriers when entering the P-type base. (c) Most diffuse from emitter through thin base into base-collector depletion region, and (d) are rapidly swept by the strong depletion region electric field into the collector. This is but a small current compared to the emitter current.ĭisposition of electrons entering base: (a) Lost due to recombination with base holes. The base current flow corresponds to electrons leaving the base terminal for the (+) battery terminal. Electrons, majority carriers, enter the emitter from the (-) battery terminal. Though not shown, we assume that external voltage sources 1) forward bias the emitter-base junction, 2) reverse bias the base-collector junction. We have an enlarged view of an NPN junction transistor with emphasis on the thin base region. In Figure below we take a closer look at the current amplification mechanism. NPN junction bipolar transistor with reverse biased collector-base: (a) Adding forward bias to base-emitter junction, results in (b) a small base current and large emitter and collector currents. 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.
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