Which temperature related effects impact transistor threshold voltage and mobility in analog design, and how do designers compensate?

• A. Vth increases with temperature; mobility increases current; compensation via negative feedback, bias stabilization, and degeneration.
• B. Vth decreases with temperature; mobility reduces current; compensation via negative feedback, bias stabilization, and degeneration.
• C. Vth decreases with temperature; mobility increases current; compensation via increased bias current.
• D. Vth constant; mobility reduces with temperature; compensation via cooling.

Answer: (B)

Temperature shifts push threshold voltage downward while hurting carrier mobility. In MOS devices, drain current in saturation roughly follows Id ∝ μ (Vgs − Vth)^2. As temperature rises, mobility μ decreases because of increased phonon scattering, which tends to lower the current. The threshold voltage Vth also falls with temperature, which would push the current higher for a fixed gate bias, but the mobility loss usually dominates, so the overall drive current tends to drop as temperature goes up.

To keep the operating point stable across temperature, designers use several strategies. Negative feedback automatically adjusts the bias to resist changes in current, helping hold the output near the desired level. Bias stabilization networks establish a robust bias point that tracks temperature variations, reducing drift. Degeneration, such as a resistor in the source, lowers transconductance and makes the current less sensitive to changes in Vgs or Vth, smoothing out temperature effects.

Other choices don’t fit because they imply the opposite temperature trends (threshold increasing with temperature or mobility increasing with temperature) or rely on impractical remedies (like cooling) rather than built‑in circuit techniques.