5.0.3 Summary to: Required Reading to Chapter 5

Essentials of the bipolar transistor:
band diagram bipolar transistor
High emitter doping (NDon for npn transistor here) in comparison to base doping NAc for large current amplification factor g = IC/IB.
NDon/NAc » k = injection ratio.  
g   »   NDon
  ·   æ
1  –  dbase
Small base width dbase (relative to diffusion length L) for large current amplification.  
Not as easy to make as the band-diagram suggests!  
Essentials of the MOS transistor:  
Basic MOS transistor

band diagram of inversion condition
Band diagram for inversion
Gate voltage enables Source-Drain current  
Essential process. Inversion of majority carrier type in channel below gate by:
  • Drive intrinsic majority carriers into bulk by gate voltage with same sign as majority carriers.
  • Reduced majority concentration nmaj below gate increases minority carrier concentration nmin via mass action law
nmaj · nmin  =  ni2
  • An inversion channel with nmin > nmaj develops below the gate as soon as threshold voltage UTh is reached.
  • Current now can flow because the reversely biased pn-junction between either source or drain and the region below the gate has disappeared.
The decisive material is the gate dielectric (usually SiO2). Basic requirement is:  
High capacity CG of the gate electrode - gate dielectric - Si capacitor = high charge QG on electrodes = strong band bending = low threshold voltages UG  
QG  =  CG · UG  
It follows:
  • Gate dielectric thickness dDi Þ High breakdown field strength UBd
  • Large dielectric constant er
  • No interface states.
  • Good adhesion, easy to make / deposit, easy to structure, small leakage currents, ...
U = 5 V, dDi = 5 nm  Þ  E = U/dDi = 107 V/cm !!

er(SiO2)  =  3.9

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© H. Föll (Electronic Materials - Script)