# VLSI Questions and Answers – Device Modelling and Performance Estimation -1

This set of VLSI Multiple Choice Questions & Answers (MCQs) focuses on “Device Modelling and Performance Estimation -1”.

1. MESFETs are _______ modulation devices.
a) channel area
b) channel voltage
c) channel current
d) channel variation

Explanation: MESFETs are channel area modulation devices and they depend upon the capacitance of the schottky barrier.

2. Gallium arsenide have _____ regions of operation.
a) two
b) three
c) four
d) five

Explanation: Gallium arsenide devices have three regions of operation – cutoff, linear and saturation.

3. Drain to source current is due to
a) flow of majority carriers from drain to source
b) flow of minority carriers from drain to source
c) flow of majority carriers from source to drain
d) flow of majority carriers from drain to source

Explanation: The current Ids results due to the flow of electrons, the majority carrier from source to drain. Ids can be given as ratio of charge induced in channel to electron transit time.

4. Transit time can be given as the ratio of
a) channel length to velocity
b) electron distance to velocity
c) source length to velocity
d) drain length to velocity

Explanation: The transit time is given as the ratio of channel length to velocity and the carrier velocity can be further given as the product of electric field and electron mobility.

5. The average potential is given as
a) Vgs – Vt
b) 0.5(Vgs – Vt)
c) 0.25(Vgs – Vt)
d) 2(Vgs – Vt)

Explanation: The average potential difference between the gate and the channel (Vgb) owing to the shape of the depletion layer can be given as 0.5(Vgs-Vt).
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6. Average electric field is _______ to implant depth.
a) directly proportional
b) indirectly proportional
c) does not depend
d) exponentially dependent

Explanation: The average electric field is indirectly proportional to implant depth and this electric field can be given as (Vgs-Vt)/a.

7. The range of kp in MESFET is
a) 0.1 to 1 mA/V2
b) 1 to 5 mA/V2
c) 0.1 to 0.5 mA/V2
d) 0 to 1 mA/V2

Explanation: β is a common parameter used in MESFET and it is denoted by kp. Kp is in the order of 0.1 to 0.5 mA/V2.

8. The hyperbolic tangent function is used to describe the
a) channel conductance
b) channel length
c) channel strength
d) channel depth

Explanation: The hyperbolic tangent function tanh(aVds) is used to describe the channel conductance at low drain to source voltage Vds.

9. The magnitude of the depletion region decreases when
a) Vgs decreases
b) Vgs increases
c) Vds increases
d) Vds decreases

Explanation: When the gate to source voltage Vgs increases, the magnitude of the depletion region beneath the gate decreases.

10. Current saturation occurs when
a) Vgs < Vt
b) Vgs > Vt
c) Vgs > Vds
d) Vgs = Vt

Explanation: When Vgs < Vt the increase in drain to source voltage above the saturation voltage leads to current saturation.

11. Velocity saturation occurs in
a) low electric field
b) high electric field
c) low magnetic field
d) high magnetic field

Explanation: The saturation of drain current with an increasing drain to source voltage is caused by velocity saturation which occurs in high electric field in the channel.

12. Knee voltage is the boundary between
a) active region and saturation region
b) linear and non linear region
c) linear and saturation region
d) linear and cutoff region

Explanation: The boundary between the linear and saturation regions defined by Vds=Vgs-Vt is referred to as the ‘knee voltage’.

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