Because the barrier is smaller, electrons enter the base, raising the concentration of electrons in the base above the normal equilibrium level and setting up a concentration gradient of electron density across the base. This forward bias V BE reduces the barrier φ n that opposes entry of electrons into the base. The base-emitter junction is forward biased, that is, the base is made positive with respect to the emitter, attracting electrons. Likewise, the reverse bias V CB splits the electron Fermi level in the bulk collector from the hole Fermi level in the field-free region of the base. The forward bias V BE splits the hole Fermi level in the base from the electron Fermi level in the emitter. The Fermi levels are separated by the application of bias voltages across the junctions. When bias is applied, the relative energies of the different regions are modified, upsetting equilibrium and causing the band edges to adjust in response. For more detail, see the articles semiconductor and semiconductor diode. If the biases are reduced to zero, these Fermi levels all coincide.
#Transistor base emitter collector widths free
The majority carrier Fermi levels in the various regions are shown as determined by the impurity dopant levels: E Fn for electrons in the field-free bulk of the emitter, E Fp for holes in the field free portion of the base, and E Fn for electrons in the field-free bulk of the collector. If no bias is applied, the band edges vary because impurity atoms set the number of carriers, and the bands must adjust position to insure the correct carrier densities. The CB and VB vary in position within the transistor for two reasons: variations in doping levels going from n- to p- to n-type layers, and also variations in electrical potential through the structure. These two energy levels are separated by the semiconductor energy gap, a region of forbidden energy for an electron. The valence band labeled VB shows the highest energy for electrons in the semiconductor valence band. The conduction band labeled CB shows the lowest energy of an electron (in electron volts, or energy divided by electron charge) in the conduction band of the semiconductor as a function of position in the npn transistor. It is a one-dimensional section vertically through the center of the emitter. The diagram shows an npn transistor biased in active mode. Using a band diagram as shown to the right, the operation can be understood. This mode is commonly used in analog circuits. Here focus is upon the active mode in which the emitter-base junction is injecting and the collector-base junction is collecting. The bipolar transistor can operate in a number of modes, distinguished by which junctions are injecting (forward bias of emitter-base or collector-base or both) and which are collecting (reverse bias of emitter-base or collector base, or both). Operationīand diagram for npn bipolar transistor biased in active mode. More of the history and development of this device can be found in an historical article by Shockley and a more recent history. Prior to its invention in 1947 by Bardeen, Brattain and Shockley at Bell Laboratories, semiconductor devices were only two-terminal devices, like diodes and rectifiers.
The bipolar transistor was the historically first transistor invented. Moreover, the current consumed by the base in normal operation is very small, so the device serves well to amplify either a current or a voltage signal applied to the base. Very small changes in the emitter-base junction voltage have an exponential influence over the number of carriers injected from the emitter, and so the base has enormous control over the current diffusing across the base to the collector. Moreover, the center region is thin enough to allow carriers injected from one of the end layers (the emitter E) to actually diffuse across the center region (the base B) and be collected by the other end region (the collector C). In concept it consists of two back-to-back pn-diodes, forming either a pnp or an npn sandwich, where p refers to a semiconductor doped to produce positively-charged carriers (holes) and n refers to a semiconductor doped to provide negatively-charged carriers (electrons). In electronics, the bipolar transistor, more completely the bipolar junction transistor, is a three-terminal semiconductor device used for switching and amplification. A planar npn bipolar junction transistor as might be constructed in a integrated circuit.
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