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An ideal MOS capacitor (p-type semiconductor) is shown in the figure. The MOS capacitor is under...


Question: An ideal MOS capacitor (p-type semiconductor) is shown in the figure. The MOS capacitor is under strong inversion with VG1 = 2 V. The corresponding inversion charge density (Qinv1) is 2.2 µC/cm2. Assume oxide capacitance per unit area (Cox) as 1.7 µF/cm2. For VG2 = 4 V, the value of Qinv2 is ______ µC/cm2 (rounded off to one decimal place).
The Catch: Once a MOS capacitor enters Strong Inversion, the surface potential (φs) becomes pinned. Any further increase in gate voltage (VG) results in a linear increase in inversion charge (Qinv), while the depletion charge remains constant.
Step 1: Identify Given Parameters
  • Initial Gate Voltage (VG1): 2 V
  • Initial Inversion Charge (Qinv1): 2.2 µC/cm2
  • Oxide Capacitance (Cox): 1.7 µF/cm2
  • Target Gate Voltage (VG2): 4 V
Step 2: Use the Strong Inversion Relationship In strong inversion, the relationship between the change in voltage and the change in charge is defined by:
ΔQinv = Cox × Î”VG
Which can be rewritten as:
Qinv2 - Qinv1 = Cox(VG2 - VG1)
Step 3: Substitute the Values Substitute the known values into the equation to find Qinv2:
Qinv2 = 2.2 + [1.7 × (4 - 2)]
Step 4: Final Calculation
Qinv2 = 2.2 + [1.7 × 2]
Qinv2 = 2.2 + 3.4
Qinv2 = 5.6
Final Answer: 5.6 µC/cm2
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