WPE effect on NMOS vth

[VLSI geek]

Can someone explain, how and why Vth of nmos increases due to WPE? and how S/D oriented device plays a role in defining Id?

 

[danadakk]

https://inst.eecs.berkeley.edu/~ee290d/fa13/LectureNotes/Lecture12.pdf

Well Proximity Effect - Siliconvlsi

Well proximity effect (WPE) is due to lateral non-uniformity in well-doping and causes the MOSFET threshold voltages and other electrical characteristics to vary with the distance of the transistor to the well-edge.

siliconvlsi.com

Regards, Dana.

 

[D.A.(Tony)Stewart]

Well Proximity Effect (WPE) on Vth: from ChatGPT​

and me in italics

1. Capacitance Effect:
   • The additional capacitance (Cwpe) due to WPE can be modeled using a simple formula:Cwpe=εox⋅A⋅dwpe
   • where:
     • εox is the permittivity of the oxide,
     • A is the area of the well-channel overlap,
     • dwpe is the separation between the well and the channel.
     • This ratio of area/gap is true for all capacitance effects in PCB's and parasitics
2. Electric Field Distortion:
   • The electric field distortion (Edist) can be related to the change in voltage (Vgs) and the channel length (L)
   • :Edist∝Vgs/L
     • This is always how E fields are defined in [V/m] or [V/μm]
   • An increase in Edist contributes to a higher threshold voltage.
3. Threshold Voltage Shift:
   • The overall shift in threshold voltage (ΔVth) can be expressed as a combination of various factors:
   • ΔVth=Cwpe/Cox⋅(Vgs−Vth0)+Edist/Eox
   • where:
     • Cox is the oxide capacitance,
     • Vth0 is the threshold voltage without WPE,
     • Eox is the electric field due to the oxide.

Source/Drain (S/D) Orientation and Drain Current (Id):​

1. Drain Current (Id) in Saturation:
   • The drain current in saturation (Idsat) can be expressed as:
   • Idsat=β/2⋅(Vgs−Vth−Vds/2)²
   • where:
     • β is the transconductance parameter. What about Ohmic region? Ids =β * (Vgs - Vth)²/2. when Vds~0 true/false?
2. Ohmic Region Behavior:
   • In the ohmic region, the drain current (Id) is approximately given by Ohm's Law:
   • Id=μ⋅Cox⋅W/L⋅(Vgs−Vth−Vds/2)
3. Transition from Ohmic to Saturation:
   • The transition from ohmic to saturation occurs when Vds≥Vgs−Vth. Beyond this point, the transistor enters the saturation region.

These formulas provide a quantitative understanding of how WPE affects threshold voltage (Vth) and how the drain current (Id) behaves in different regions. The actual values of parameters (β, μ, etc.) depend on the specific characteristics of the transistor and the technology used in its fabrication.

Comparing all diodes, FETs, and electrolytics, I have found a correlation in the product of R*C = Tau is constant for a given family or topology and geometry which also changes with Vmax rating. Using RdsOn, Coss or Ciss or ESR and C (0V) at some test temperature. The fundamental phenomena in physics is the ratio of C ∝Area/gap. In e-caps, capacitors with low ESR has to do with surface roughness (acid eteched) with a small gap.

 

[VLSI geek]

Sorry, but still I didn't understand how Vth for NMOS is increasing. From my understanding, if during N-well implant if some Phosphorus atoms enter in P-sub(which is doped with Boron) so it will reduce the number of holes in PSUB which will lead to application of less VGS in order to create a n-channel.

Can you please add some expalantion.