When the oxide thickness and W of a MOSFET are both doubled, which of the following statement(s) is(are) correct? (Select all that apply)

Question

When the oxide thickness and W of a MOSFET are both doubled, which of the following statement(s) is(are) correct? (Select all that apply)

BlackTom AI · Accepted Answer

To tackle this question, I’ll break down what happens when both oxide thickness and channel width/length are doubled, and how that affects VDsat and IDsat.
Option 1: VDsat increases. In many MOSFET models, VDsat is the drain-source voltage at which the device enters saturation. Doubling W and L leaves the W/L ratio unchanged, but doubling t_ox halves the oxide capacitance Cox, which tends to raise the threshold voltage Vth. A higher Vth would typically reduce the overdrive (VGS − Vth) and, in turn, reduce or not increase VDsat. Therefore, claiming that VDsat increases is not aligned with the expected effect of lowering Cox via thicker oxide; this option is not supported by the common understanding of how VDsat behaves with these changes.
Option 2: VDsat decreases. With Cox halved due to the thicker oxide, Vth tends to rise, which lowers the effective overdrive. Since VDsat is tied to the overdrive (roughly VDsat ≈ VGS − Vth in the simple saturation picture), a larger Vth would reduce the voltage at which saturation begins, i.e., VDsat decreases. This aligns with the intuition that a higher threshold makes it harder for the channel to saturate, pushing VDsat downward. Thus, this statement is consistent with the described device modifications.
Option 3: IDsat increases. In the square-law model, IDS at saturation is proportional to μn C_ox (W/L) (VGS − Vth)^2 / 2. Doubling W and L keeps W/L constant, but halving Cox reduces C_ox, and increasing Vth reduces (VGS − Vth). Both of these effects tend to reduce IDS at a given VGS, not increase it. Therefore, claiming IDSat increases contradicts the standard relation and is not supported by the change in Cox and Vth.
Option 4: IDsat decreases. As just noted, Cox decreases when oxide thickens, which lowers IDSat through the direct Cox factor, and the increased Vth further reduces the overdrive (VGS − Vth), leading to a smaller IDSat. This is consistent with the expected behavior when Cox is reduced and the overdrive shrinks. Consequently, this statement accurately describes the effect on IDSat under the given changes.

EESM5200 (L2) Part II: Topic 8 - I-V Characteristics Implied by the Carrier Velocity Saturation Model

When the oxide thickness and W of a MOSFET are both doubled, which of the following statement(s) is(are) correct? (Select all that apply)

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