5.1 Device Structure/Operation

Overdrive voltage $|V_{OV}| = |V_{GS}|-|V_t|$ is the quantity that governs the operation of the MOSFET

• For the MOSFET to operate in the saturation region ($|V_{DS}| ≥ |V_{OV}|$), and the resulting:

  $$ i_D = \frac{1}{2} \mu n C{ox}(\frac{W}{L})V_{ov}^2 $$

• For PMOS, replace $\mu _n$ with $\mu _p$

• If $|V_{DS}| < |V_{OV}|$, the MOSFET operates in the triode region, which together with cutoff is used for operating the MOSFET as a switch

NMOS

PMOS

5.2 IV characteristics

• In saturation, $i_D$ shows some linear dependence on $V_{DS}$ as a result of the change in channel length
• The channel-length modulation becomes more pronounces as $L$ decreases
• Modeled by ascribing an output resistance $r_o = \frac{V_A}{I_D}$ to the MOSFET model
  • Each the Early voltage $|V_A| = |V_A’|L$ where $|V_A’|$ is a process-dependent parameter

5.3 Circuits at DC

• If a MOSFET is conducting but its region of operation (saturation or triode) is not known, we assume saturation-mode opeartion
• Then, we solve the problem and check to determine whether the assumption was justified
  • If not, then the transistor is operating in the triode region, and we must redo analysis accordingly