Basics of CMOS Inverter and CMOS as SWitch.

This PPT has basics of CMOS inverter.

1. VLSI Design Nonideal I-V Effects

2. Nonideal I-V Effects

3. Short Channel Effects – Velocity Saturation (1) Basic Concept In a MOSFET (or any semiconductor), electrons (or holes) move through the channel under the influence of an electric field. Normally, the drift velocity of carriers is proportional to the electric field: This is true for low electric fields , where carriers behave linearly.  

4. Short Channel Effects – Velocity Saturation (2) High Electric Field Condition When the electric field in the channel becomes very high (usually near the drain in a MOSFET under high VDS​), the carriers gain more energy. They start colliding more frequently with the lattice , and eventually their velocity cannot increase linearly with the field anymore. At this point, the carrier velocity saturates at a maximum value vsat ​, typically: Electrons in silicon: 6-10 x 10 6  cm Holes in silicon: 4-6 x 10 6  cm This phenomenon is called velocity saturation .

5. Short Channel Effects – Velocity Saturation (3) Effect on MOSFET Operation

6. Short Channel Effects – Velocity Saturation (4)

7. Short Channel Effects – Mobility Degradation What is Mobility? Mobility (μ) tells how easily carriers (electrons or holes) move in a semiconductor under an electric field. At low electric fields, mobility is nearly constant. At higher fields or in confined regions (like a MOSFET inversion layer), mobility can decrease → this is called mobility degradation .

8. Short Channel Effects – Mobility Degradation Surface Scattering (Vertical Field Effect) In MOS transistors, carriers move close to the silicon-oxide interface. As the gate voltage increases, carriers are pulled closer to the surface. Imperfections (roughness and interface traps ) at the Si-SiO₂ interface leads to scattering of carriers, reducing their mobility. Vertical Electric Field (Coulomb Scattering) As the gate voltage increases, the vertical electric field between the gate and the channel also increases. A higher vertical field forces carriers closer to the silicon-oxide interface, increasing the likelihood of scattering events and reducing mobility. This field-induced scattering is more prominent in highly scaled devices.

9. Short Channel Effects – Mobility Degradation Mathematical Model of Mobility Degradation Mobility degradation is typically modeled using a mobility reduction factor, often expressed as: Where: is the effective mobility ​ is the low-field mobility (intrinsic mobility without degradation) is a fitting parameter that represents the degree of mobility degradation is the effective gate voltage  

10. Short Channel Effects – Channel Length Modulation   The current in saturation region, according to channel length modulation is given by, Where, – Saturation Current in the absence of channel length modulation λ is the channel length modulation parameter (with units of )