# Electronic Devices and Circuits Questions and Answers – The Continuity Equation

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

1. What does p/τ represent?
a) holes
b) time
c) holes per second lost
d) p per unit time

Explanation: Option (c) represents the holes per second lost by recombination per unit volume.

2. Which of the following is the Taylor’s expression?
a)
b)
c)
d)

Explanation: Option a represents the correct formula for the Taylor’s expression.

3. Calculate the number of coulombs per second if the area is 4cm2, recombination rate of hole is 1000 cm-3/s and the differential length is 2mm.
a) 1.28*10-23
b) 1.28*10-22
c) 1.28*10-21
d) 1.28*10-20

Explanation: number of coulombs per second= eAdxp/τ
=1.6*10-19*4*10-4*2*10-3*1000
=1.28*10-22.

4. The current entering the volume at x is I and leaving is I+Δi , the number of coulombs per second will be equal to δI. Is it true or false?
a) True
b) False

Explanation: Coulombs per second is known as the current. The differential current will be the current through the semiconductor.

5. The change in the carrier density is due to
a) Flow of incoming flux
b) Flow of outgoing flux
c) Difference of flow between incoming and outgoing flux
d) Difference of flow between incoming and outgoing flux plus generation and minus recombination

Explanation: The change in the carrier density describes the continuity equation which is equal to the difference between the incoming and outgoing flux plus generation and minus recombination.
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6. What of the following conditions satisfies when the number of holes which are thermally generated is equal to the holes lost by recombination?
a) I≠0
b) dp/dt≠0
c) g=p/τ
d) g≠p/τ

Explanation: Under the equilibrium conditions, I will be zero and then the dp/dt will aso be equal to zero in the continuity equation. Then, g= p/τ is left which is option c.

7. What is the diffusion length for holes when Dp=25cm2/s and τp=25s?
a) 25cm
b) 1cm
c) 0.04cm
d) 50cm

Explanation: Lp=√(Dpp)
=√(25*25)
=25cm.

8. Which of the following represents the continuity equation?
a) dp/dt=-(p-p0)/τp+Dp(d2p/dx2)-µpd(ρϵ)/dx
b) dp/dt=-(p-p0)/τp-Dp(d2p/dx2)-µpd(ρϵ)/dx
c) dp/dt=-(p-p0)/τp+Dp(d2p/dx2)+µpd(ρϵ)/dx
d) dp/dt=(p-p0)/τp-Dp(d2p/dx2)-µpd(ρϵ)/dx

Explanation: Option a represents the correct equation of the continuity equation for holes.

9. What is the diffusion length for electrons when Dn=10cm2/s and τn=40s?
a) 50cm
b) 25cm
c) 20cm
d) 15cm

Explanation: Ln=√(Dnn)
=√(10*40)
=20cm.

10. Which of the following represents the best definition for the diffusion length for holes?
a) Average distance which an electron is injected travels before recombining with an electron
b) Average distance which a hole is injected travels before recombining with an electron
c) Average distance which a hole is injected travels before recombining with a hole
d) Average distance which an electron is injected before recombining with a hole

Explanation: Diffusion length for holes is represented as the average distance which a hole is injected travels before recombining with an electron. It is the distance into the semiconductor at which the injected concentration falls to 1/ϵ of its value at x=0.

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