SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

250 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

+++++

• –––––
  
  
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n = N~ dc

n = N~ de

p = N~ ab

Wb

Electron current

Hole current = Base current

n

n

p

(a)

(b)

EMITTER BASE COLLECTOR

• ––––


• –––––


• ––––


• –––– –––––

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+

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        • ––
        
        
        
        
      
      
      
      
    
    
    
    
  
  
  
  

Figure 6.4: (a) Band profile of an unbiasedn+p-nBJT.(b)BandprofileofaBJTbiasedinthe
forward active mode, where the EBJ is forward biased and the BJT is reverse biased.

WeremindourselvesthatiftheEBdiodeisasymmetricallydoped,theforwardbiascurrent
isessentiallymadeupofinjectionofelectronsintothep-side. Thisforward-biasedcurrent
canalsobealteredbyaverysmallchangeintheforwardbiasvoltagesincethecurrentdepends
exponentiallyontheforwardbiasvalue. The forward-biasedn+emitter injects electrons into the
p-base. Some of the electrons recombine in the base with the holes, but if the base region is less
than the diffusion length of the minority carriers, most of them reach the depletion region of the
p-nbase-collector diode and are swept out to form the collector current. The collector current is
proportional to the minority carriers (electrons) that reach the edge of thep−ndepletion region,
as shown in figure 6.4b. Since the injected minority carriers are due to the emitter current, we