A bipolar junction transistor consists of three doped semiconductor regions: the emitter region, the region of the base and collector region. These regions are, respectively, type P, type N and P in a PNP type, and type N, type P and type N in the NPN transistor. Each region of the semiconductor is connected to a terminal, called emitter (E), base (B) and collector (C), as appropriate.
simplified cross section of an NPN bipolar junction transistor. Where you can see how the base-collector junction is much wider than the base-emitter. The base is physically located between the emitter and collector and is made of lightly doped semiconductor material and high resistivity. The collector surrounds the emitter region, making it nearly impossible for the electrons injected into the core region escape being collected, which makes the resulting value of α is much closer to unity, and therefore give the transistor a large β.
The bipolar junction transistor, unlike other transistors, a device is usually not symmetrical. This means that interchanging the collector and the emitter makes the transistor to stop working in active mode and starts working in reverse. Because the internal structure of the transistor is usually optimized to run in active mode, swap the collector and emitter are the values \u200b\u200bof α and β in reverse mode are much smaller than could be obtained in active mode, many times the value of α in reverse mode is less than 0.5. The lack of symmetry is mainly rates due to doping between the emitter and collector. The emitter is highly doped
, while the collector is lightly doped, allowing it to be applied to a large reverse voltage on the collector-base before the collapse. The collector-base junction is biased in reverse during normal operation. The reason is that the highly doped emitter is to increase the efficiency of carrier injection from the emitter: the rate of carriers injected by the issuer in connection with those injected at the base. For a large current gain, most of the carriers injected into the base-emitter junction must come from the issuer. 
Low performance lateral bipolar transistors are often used in CMOS processes because they are designed symmetrically, which means that there is no difference between the operation in active mode and reverse mode.
Low performance lateral bipolar transistors are often used in CMOS processes because they are designed symmetrically, which means that there is no difference between the operation in active mode and reverse mode. Small changes in the voltage applied between the base-emitter terminals generates the current between the emitter and collector to change significantly. This effect can be used to amplify the input voltage or current. The BJT can be thought of as current sources controlled by voltage, but are characterized more simply as sources of current-controlled current, or power amplifiers, due to the low impedance base.
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