2nd order effects vlsi

1. Threshold Voltage
• The threshold voltage is the minimum gate-to-source voltage VGS
differential that is needed to create a conducting path between the
source and drain terminals. It is abbreviated as Vth.
• Dependence on oxide thickness
• Dependence on temperature
• Dependence on random dopant fluctuation

2. Latchup
• Latchup refers to short circuit formed between power and ground
rails in an IC leading to high current and damage to the IC.
• Latch up is the phenomenon of low impedance path between power
rail and ground rail due to interaction between parasitic pnp and npn
transistors.

3. Electromigration (EM)
• Electromigration is the forced movement of metal ions due to an electric
field.
• Ftotal = Fdirect +Fwind
• Metal atoms(ions) travel toward the positive end of the
conductor while vacancies move toward the negative end.

4. Mobility Degradation
• Mobility is important because the current in MOSFET depends upon
mobility of charge carriers(holes and electrons).
• Lateral Field Effect: In case of short channels, as the lateral field is
increased, the channel mobility becomes field-dependent and
eventually velocity saturation occurs .
• Vertical Field Effect: As the vertical electric field also increases on
shrinking the channel lengths, it results in scattering of carriers near
the surface. Hence the surface mobility reduces .