# Electronic Devices and Circuits Questions and Answers – The Body Effect

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The Body Effect”.

1. The _____________ of a MOSFET is affected by the voltage which is applied to the back contact.
a) Threshold Voltage
b) Output Voltage
c) Both threshold and output voltage
d) Neither of the threshold nor the output voltage

Explanation: The voltage difference between the source and the bulk, VBS changes the width of the depletion layer and therefore also the voltage across the oxide due to the change of the charge in the depletion region. This results in a difference in threshold voltage which equals the difference in charge in the depletion region divided by the oxide capacitance.

2. The variation of the threshold voltage with the applied bulk-to-source voltage is typically observed by plotting the _________________ as a function of the source-to-drain voltage.
a) drain current
b) square root of the drain current
c) square of the drain current
d) natural logarithm of the drain current

Explanation: The change in threshold current is directly proportional to the square root of the drain current. For further assistance check the mathematical expression for the same.

3. The SI units of the body effect parameter is
a) Volt
b) Volt X Volt
c) √Volt
d) It has no units

Explanation: Vt = Vt0 + k[√(Vsb + 2φf) – √2φf]. In this expression k is the body effect parameter hence its units can be determined.

4. An NMOS transistor has Vt0 = 0.8 V, 2 φf = 0.7 V, and γ = 0.4 V1/2. Find Vt when VSB = 3 V.
a) 0.12 V
b) 1.23 V
c) 2.34 V
d) 3.45 V

Explanation: Vt = Vt0 + k[√(Vsb + 2φf) – √2φf]. use this expression to obtain the desired result.

5. The threshold voltage is
a) Increases on increasing temperature
b) May increase or decrease on increasing temperature depending upon other factors
c) Temperature independent
d) Decreases on increasing temperature

Explanation: The threshold voltage depends only on the temperature and it decreases by roughly 2 mV for every degree Celsius increase in the temperature.
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6. As the voltage on the drain is increased, a value is reached at which the pn junction between the drain region and substrate suffers avalanche breakdown known as
a) Weak avalanche
b) Strong avalanche
c) Weak storm
d) Punch-through

Explanation: As the voltage on the drain is increased, a value is reached at which the pn junction between the drain region and substrate suffers avalanche breakdown. This breakdown usually occurs at voltages of 20 V to 150 V and results in a somewhat rapid increase in current (known as a weak avalanche).

7. A breakdown effect that occurs in modern devices at low voltages (of around 20 V) is
a) Weak avalanche
b) Strong avalanche
c) Weak storm
d) Punch-through

Explanation: Punch-through occurs in devices with relatively short channels when the drain voltage is increased to the point that the depletion region surrounding the drain region extends through the channel to the source. The drain current then increases rapidly. Normally, punch-through does not result in permanent damage to the device.

8. At ______________ the drain current is no longer related to the Vgs by square law relationship.
a) When the temperature is high (around 700 Celsius)
b) When temperature is very low (around -50 Celsius)
c) Velocity saturation
d) None of the mentioned

Explanation: At velocity saturation the current depends linearly on the Vgs.

9. In MOSFETs a breakdown may occur at around 30 V. This is due to
a) Velocity saturation
b) Breakdown of the gate diode
c) Sudden decrease in the depletion region
d) Fall of the threshold voltage due to impurities

Explanation: The breakdown of the oxide at the gate may occur when the voltage is around 30 V. This may also permanently damage the device.

10. Which of the below issues may not be experienced when using MOSFETs?
a) Weak avalanche
b) Velocity saturation
c) Punch-through
d) All of the mentioned

Explanation: All of the mentioned are some of the common issues that one may face while dealing with MOSFETs.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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