SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

148 CHAPTER 4. JUNCTIONS IN SEMICONDUCTORS:P-NDIODES

⇓

+

+

eχ

Evac

EFn

Junction

formation

(a)

Electrons

Fermi level is flat

Wp Wn

Holes

p-type n-type

(b)

Evac

Evac

Evac

Ev

Ev

Ev

Ec

Ec

Ec Ec

Ev

EFp

eφsn

eφsp

eχ

Depletion of

mobile charge

+

• ––––
  
  
  • 
    
    
    
    • ––
      
      
      • 
        

        –

        
        
      
      
      
      
    
    
    
    
  
  
  
  

+++++++

++

++++++++

• ––––––

Diffusion of electrons

Diffusion

Drift

• – – – – – –

+

++++

• – –

Drift

+++++++

Electron density, nn

Hole density, pn

Hole density, pp

Electron density, np

• – ––––––––

+ +

Figure 4.1: Forming ap-njunction (a) Thep-andn-type regions before junction formation.
The electron affinityeχand work functionseφspandeφsnare shown along with the Fermi
levels. (b) A schematic of the junction and the band profile showing the vacuum level and the
semiconductor bands.