Q41GATE 2010MCQ

In a uniformly doped BJT, assume that $N_{E},N_{B}$ and $N_{C}$ are the emitter, base and collector doping in atoms/ $cm^{3}$ , respectively. If the emitter injection efficiency of the BJT is close unity, which one of the following condition is TRUE

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📖 Explanation

$\gamma=\frac{1}{1+\frac{N_{B}}{N_{E}}}$ To achieve $\gamma=1$ $N_{E} \gt \gt N_{B}$

Q42GATE 2009MCQ

Consider the following two statements about the internal conditions in a n-channel MOSFET operating in the active region. S1 : The inversion charge decreases from source to drain. S2 : The channel potential increases from source to drain. Which of the following is correct?

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Q43GATE 2008MCQ

The cross section of a JFET is shown in the following figure. Let $V_{G}$ be -2V and let $V_{P}$ be the initial pinch -off voltage. If the width W is doubled (with other geometrical parameters and doping levels remaining the same), then the ratio between the mutual trans conductances of the initial and the modified JFET is

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📖 Explanation

\begin{aligned} g_{m} &=g_{m 0}\left[1-\left(\frac{-V_{G}}{V_{P}}\right)^{\frac{1}{2}}\right]\$ \\ \text { where, } g_{m o} &=\frac{2 a z}{\rho L} \end{aligned}

a $\rightarrow$ Half channel height L $\rightarrow$ Length of channel z $\rightarrow$ Depth of channel $\rho \rightarrow$ Resistivity of channel since all the parameters are same except width 'W'. So.

\begin{aligned} W &\propto a\\ \therefore \quad g_{m 0}&=\frac{2 W z}{\rho L} \\ g_{m}&=g_{m o}\left[1-\left(-\frac{V_{G}}{V_{p}}\right)^{1 / 2}\right]\$ \\ g_{\text {initial }}&=W\left[1-\left(-\frac{V_{G_{1}}}{V_{P_{1}}}\right)^{1 / 2}\right]\$ &\ldots(i) \end{aligned}

Now, the width is doubled. So,

\begin{aligned} g_{\text {mmodifed}}&=2 W\left[1-\left(-\frac{V_{G_{1}}}{4 V_{P_{1}}}\right)^{1 / 2}\right]\$ &\ldots(ii)\\ &\qquad \left[\because V_{p}=\frac{q N_{0} a^{2}}{2 \epsilon}\right]\$ \\ \frac{g_{\text {mmodifes }}}{g_{\text {minitial }}}&=\frac{1}{2}\left[\frac{1-\sqrt{\frac{2}{V_{p}}}}{1-\sqrt{\frac{1}{2 V_{p}}}}\right] \end{aligned}

Q44GATE 2008MCQ

The drain current of MOSFET in saturation is given by $I_{D}=K(V_{GS}-V_{T})^{2}$ where K is a constant. The magnitude of the transconductance $g_{m}$ is

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📖 Explanation

\begin{aligned} g_{m} &=\left.\frac{\partial i_{d}}{\partial V_{G S}}\right|_{V_{D S}} \\ &=\frac{\partial K\left(V_{G S}-V_{T}\right)^{2}}{\partial V_{G S}} \\ &=2 K\left(V_{G S}-V_{T}\right) \end{aligned}

Q45GATE 2008MCQ

The measured trans conductance $g_{m}$ of an NMOS transistor operating in the linear region is plotted against the gate voltage $V_{G}$ at a constant drain voltage $V_{D}$ . Which of the following figures represents the expected dependence of $g_{m}$ on $V_{G}$ ?

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📖 Explanation

$I_{D}$ is given by, $I_{D}=K_{n}\left[\left(V_{g s}-V_{t}\right) V_{D S}-\frac{V_{D S}^{2}}{2}\right]\$ -$ inlinear region

\begin{aligned} \text{and }g_{m}&=\frac{\partial I_{D}}{\partial V_{g S}} \text{ at constant }V_{D S} \\ g_{m}&=\frac{\partial I_{D}}{\partial V_{g s}}=K_{n}\left(V_{G S}-V_{t}\right) V_{D S} \\ \therefore g_{m} &\propto V_{G S} \end{aligned}

So, linear relation.

Q46GATE 2007MCQ

The figure shows the high-frequency capacitance-voltage (C-V) characteristics of a Metal/ $SiO_2$ /silicon (MOS) capacitor having an area of $1\times 10^{-4}cm^{2}$ Assume that the permittivities ( $\varepsilon_{0}\varepsilon _{r}$ ) of silicon and SiO2 are $1\times 10^{-12}$ F/cm and $3.5\times 10^{-13}$ F/cm respectively. Consider the following statements about the C-V characteristics plot : S1 : The MOS capacitor has as n-type substrate S2 : If positive charges are introduced in the oxide, the C-V polt will shift to the left. Then which of the following is true?

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Q47GATE 2006MCQ

The phenomenon known as "Early Effect" in a bipolar transistor refers to a reduction of the effective base-width caused by

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Q48GATE 2005MCQ

A MOS capacitor made using p type substrate is in the accumulation mode. The dominant charge in the channel is due to the presence of

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Q49GATE 2004MCQ

Consider the following statements S1 and S2. S1 : The threshold voltage ( $V_{T}$ ) of MOS capacitor decreases with increase in gate oxide thickness. S2 : The threshold voltage ( $V_{T}$ ) of a MOS capacitor decreases with increase in substrate doping concentration. Which Marks of the following is correct ?

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📖 Explanation

$S_{2}: V_{T}$ of $\mathrm{N}-\mathrm{MOS}$ is increased by adding extra p-type impurities i.e. $V_{T}$ of a MOS increases by increase in substrate doping concentration. (True) $S_{1}: C_{o x}=\frac{\epsilon_{o x}}{t_{o x}} \quad t_{o x}=$ gate oxide thickners $V_{T}=\phi_{G C}-2 \phi_{F}-\frac{Q_{R}}{C_{O x}}-\frac{Q_{o x}}{C_{O x}}$ If $C_{\text {ox }}$ increases $\frac{Q_{B}}{C_{\text {ox }}}$ and $\frac{Q_{\text {ox }}}{C_{\text {ox }}}$ decreases and $V_{T}$ increases. $C_{o x}$ decreases when $t_{o x}$ increases.

Q50GATE 2004MCQ

Consider the following statements S1 and S2. S1 : The $\beta$ of a bipolar transistor reduces if the base width is increased. S2 : The $\beta$ of a bipolar transistor increases if the dopoing concentration in the base is increased. Which remarks of the following is correct ?

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📖 Explanation

$\beta=\frac{I_{C}}{I_{B}}=\frac{\alpha}{1-\alpha}$ When base width increases, recombination in base region increases and $\alpha$ decreases and hence $\beta$ decreases. If doping in base region increases , then recombination in base increases and $\alpha$ decreases, thereby decreasing $\beta.$

Q51GATE 2004MCQ

If for a silicon npn transistor, the base-to-emitter voltage ( $V_{BE}$ ) is 0.7 V and the collector-to-base voltage ( $V_{CB}$ ) is 0.2 V, then the transistor is operating in the

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Q52GATE 2004MCQ

The drain of an n-channel MOSFET is shorted to the gate so that $V_{GS}=V_{DS}$ . The threshold voltage ( $V_{T}$ ) of the MOSFET is 1 V. If the drain current ( $I_{D}$ ) is 1 mA for $V_{GS}$ =2V, then for $V_{GS}$ =3V, $I_{D}$ is

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📖 Explanation

\begin{aligned} I_{D} &=K\left(V_{G S}-V_{T}\right)^{2} \\ 1 m A &=K(2-1)^{2} \\ K &=1 \mathrm{mAN}^{2} \\ \text { Again, } \quad I_{D} &=K\left(V_{G S}-V_{T}\right)^{2}=4 \mathrm{mA} \end{aligned}

Q53GATE 2003MCQ

For an n - channel enhancement type MOSFET, if the source is connected at a higher potential than that of the bulk (i.e. $V_{SB} \gt$ 0), the threshold voltage $V_{T}$ of the MOSFET will

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📖 Explanation

$V_{T}=V_{T_{0}}+\gamma \sqrt{\left|-2 \phi_{F}+V_{S B}\right|}-\sqrt{\left|2 \phi_{F}\right|}$ $\gamma=$ substrate bias coefficient $V_{S B}=$ substrate bias voltage

Q54GATE 2003MCQ

When the gate-to-source voltage ( $V_{Gs}$ ) of a MOSFET with threshold voltage of 400 mV, working in saturation is 900 mV, the drain current is observed to be 1 mA. Neglecting the channel width modulation effect and assuming that the MOSFET is operating at saturation, the drain current for an applied $V_{Gs}$ of 1400 mV is

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📖 Explanation

\begin{aligned} I_{D} &=K\left(V_{G S}-V_{T}\right)^{2} \\ 1 \mathrm{mA} &=K(900-400)^{2} \\ K &=\frac{1 \mathrm{mA} / \mathrm{V}^{2}}{10^{4} \times 25} \\ \text { When } V_{G S} &=1400 \mathrm{V} \\ I_{D} &=K\left(V_{G S}-V_{T}\right)^{2}\\ &=\frac{1 \mathrm{mA}}{25 \times 10^{4} \mathrm{V}^{2}}(1400-400)^{2} \mathrm{V}^{2} \\ &=\frac{1 \mathrm{mA} \times 100}{25 \times 10^{4}} \times 10^{4} \\ I_{D} &=4 \mathrm{mA} \end{aligned}

Q55GATE 2001MCQ

The effective channel length of a MOSFET in saturation decreases with increase in

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📖 Explanation

At the edge of saturation i.e. when drain to source voltage reaches $V_{D_{sat}}$ the inversion layer charge at the drain end becomes zero (ideally). The channel is said to be pinched off at the drain end. If the drain to source voltage $V_{DS}$ is increased even further beyond the saturation edge so that $V_{DS} \gt V_{D_{sat}}$ , an even larger portion of the channel becomes pinched off and effective channel length is reduced.

Q56GATE 2001MCQ

MOSFET can be used as a

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