### 5.0.3 Summary to: Required Reading to Chapter 5

| Essentials of the 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 NAc | · | æ ç è | 1 – | dbase L | ö ÷ ø
|  | 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: |  |
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 C G 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, ... |  |
Example: U = 5 V, dDi = 5 nm Þ E = U/dDi = 107 V/cm !! er(SiO2) = 3.9

© H. Föll (Electronic Materials - Script)