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: Define gate voltage and threshold voltage.The threshold voltage (\(V_{th}\)) represents the minimum gate-source voltage needed to establish a conductive electron channel (inversion layer) in an n-channel MOSFET. A positive gate voltage draws electrons towards the silicon surface beneath the oxide layer.
Step 2: Examine the influence of fixed positive oxide charges.Fixed positive charges (\(Q_f\)) present in the gate oxide generate an internal electric field. This field attracts electrons to the silicon surface, mimicking the effect of a positive gate voltage.
Step 3: Determine the resulting threshold voltage change.Because these fixed positive charges aid in attracting electrons for channel formation, a smaller external positive gate voltage is required to achieve inversion. Consequently, the threshold voltage is lowered.