Test: Ids Versus Vds Relationships - Electrical Engineering (EE) MCQ

Ids depends on both Vgs and Vds. The charge induced is dependent on gate to source voltage Vgs also charge can be moved from source to drain under influence of electric field created by Vds.

Ids can be given as charge induced in the channel(Qc) divided by transit time (Ʈ). Ids is equivalent to (-Isd).

Transit time (Ʈ) can be given by lenght of channel(L) by velocity(v). Transit time is the time required for an electron to travel between two electrodes.

Velocity can be given as the product of electron or hole mobility(µ) and electric field(Eds). It gives the flow velocity which an electron attains due to electric field.

 Electric field(Eds) can be given as the ratio of Vds and L. Eds is the electric field created from drain to source due to volta Vds.

The value of mobility of proton or hole at room temperature is 240 cm2/V sec. This gives the measure of how fast an electron can move.

 In non saturated or resistive region, Vds lesser than Vgs – Vt where Vds is the voltage between drain and source, Vgs is the gate-source voltage and Vt is the threshold voltage.

The condition for saturation is Vds = Vgs – Vt, since at this point IR drop in the channel equals the effective gate to channel voltage at the drain.

The threshold voltage for nMOS depletion denoted as Vtd is negative.

The current Ids remains fairly constant as Vds increases in the saturation region.

 In linear region of MOSFET, the channel is uniform and narrow. This is the concentration distribution.

In MOSFET, in saturation region, when Vds > (Vgs – Vth), the channel pinches off that is the channel current at the drain spreads out.

Increasing the threshold voltage, leads to small leakage current when turned off and reduces current flow when turned on.

MOSFET is used as current source. Bipolar junction transistor also acts as good current source.