Question

If fixed positive charges are present in the gate oxide of an n-channel enhancement type MOSFET, it will lead to

• A. a decrease in the threshold voltage
• B. channel length modulation
• C. an increase in substrate leakage current
• D. an increase in accumulation capacitance

Hint:

Any positive charge (either on the gate or fixed in the oxide) helps form the electron channel in an NMOS device, thus lowering its threshold voltage. Conversely, any negative charge would increase the threshold voltage.

Answer 1

The Correct Option is A

Step 1: Understand the function of gate voltage and threshold voltage. The threshold voltage (\(V_{th}\)) is the minimum gate-to-source voltage required to create a conducting channel (inversion layer) of electrons in an n-channel MOSFET. Applying a positive gate voltage attracts electrons to the silicon surface under the oxide.
Step 2: Analyze the effect of fixed positive charges in the oxide. Fixed positive charges (\(Q_f\)) within the gate oxide will create their own electric field. This field will also attract negative charges (electrons) to the silicon surface, just as a positive gate voltage would.
Step 3: Conclude the impact on threshold voltage. Since the fixed positive charges are already helping to attract the electrons needed to form the channel, less external positive voltage needs to be applied to the gate to reach the point of inversion. Therefore, the required threshold voltage is reduced.