Bipolar Junction Transistor (BJT)

 Bipolar Junction Transistor (BJT) has three terminals connected to three doped semiconductor regions. In an npn transistor, a thin and lightly doped p-type material is sandwiched between two thicker n-type materials; while in a pnp transistor, a thin and lightly doped n-type material is sandwiched between two thicker p-type materials. In the following we will only consider npn BJTs.


In many schematics of transistor circuits (especially when there exist a large number of transistors in the circuit), the circle in the symbol of a transistor is omitted.


The three terminals of a transistor are typically used as the input, output and the common terminal of both input and output. Depending on which of the three terminals is used as common terminal, there are three different configurations: common emitter (CE), common base (CB) and common collector (CC). The common emitter (CE) is the most typical configuration:

• Common-Base (CB)Two voltages  and  are applied to the emitter  and collector  of the transistor with respect to the common base . Te BE junction is forward biased while the CB junction is reverse biased.
  
  
  The behavior of the npn-transistor is determined by its two pn-junctions:
  
  
  
  • The forward biased base-emitter (BE) junction allows the free electrons to flow from the emitter through the PN junction to form the emiiter current .
  • As the p-type base is thin and lightly doped, most electrons from the emitter  (e.g. ) go through the base to reach the collector-base junction, only a small number of the electrons are combined with the holes in base to form the base current .
  • The reverse biased collector-base junction blocks the majority carriers (holes in the p-type base, electrons in n-type collector), but lets through the minority carriers, electrons in base and holes in collector, including most of the electrons from the emitter , and the reverse saturate current of the CB junction ,
  The relationship between the output  and the input  can be found as:
  
  
  
  
  
  
  The base current  is the small difference between two nearly equal currents  and :
  
  
  
  
  
  
  
• Common-Emitter (CE)Two voltages  and  are applied to the base  and collector  of the transistor with respect to the common emitter . The BE junction is forward biased while the CB junction is reverse biased. The voltages of CB and CE configurations are related by:
  
  
  
  
  
  
  
  
  The input current is , , and the output current is
  
  
  
  
  
  
  Solving for , we get the relationship between the output  and the input :
  
  
  
  
  
  
  Here  is the current-transfer ratio for CE (e.g.,  and ), and  is the reverse saturation current between collector and emitter. In summary:
  
  
  
  
  
  
  
  
  Source: http://fourier.eng.hmc.edu/e84/lectures/ch4/node3.html