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Name
Student ID_______________________________
Page 1 of 6
EE340 Lec Al
Midterm Exam Fall Term 2007
—
Wednesday, Nov7, 2007
Notes
I.
This is a 50 minute exam, starting at 10:00 am and ending at 10:50 am. The exam is worth a total
of 25 marks, broken down as follows:
Question 1
Question 2
Question 3
Question 4
-
-
-
-
6 marks
6 marks
6 marks
7 marks
2.
This is a closed-book exam. Only the exanf pages, a formula sheet (one side of an 8.5”x 11” sheet,
provided by you), a calculator, and writing instruments are allowed. Use the backs of the exam
pages for rough calculations, if necessary. No scrap paper allowed.
3.
Please write your solutions on the test paper in the spaces provided (write on the back of pages
where necessary).
Marking
Question 1
/6
Question 2
/6
Question 3
/6
Question 4
/7
Total
25
Name
Student 1D
Page 2 of 6
Ouestion 1
Consider the circuit below. Dl is a silicon diode with saturation current Is=lO’
5 [A], ideality factor n=l,
and you can assume room temperature operation.
vs
LA
a.
Calculate the DC supply voltage V that results in the small signal currents a and
Also calculate the ratio ‘A”B of the DC currents under these conditions, (4 marks)
b.
For the circuit in part a., assume that the small signal approximation requires the peak small signal
voltage across the diode is less than 5 [mV]. Calculate the corresponding maximum peak voltage of
the small signal source
(2 marks)
b
being equal.
Name
Student 1D
Page 3 of 6
Oucstion 2
For the circuit shown, assume the BJT has 3= 100 in forward active mode and exhibits k=l [mA] at
VBE=0,7 [VJ
+15 V
-15 V
a.
Calculate the scale current of the BJT. (2 marks)
b. Design the circuit (choose RE and Rc as accurately as possible based on the information given)
such that 25
c= [mA] and Vc=+5 [VI. (4 marks)
1
Name
Student ID
Page 4 of 6
Ouestion 3
Consider the circuit shown, and assume that the BJT can have j3 in the range 50 to 150 (ie. 5O<<l50).
÷ 10 V
+5 V
‘C
-Lf -1I;.E
-
‘B
a.
Calculate the maximum value for R that ensures the BJT is in the forward active mode. (3
marks)
b,
Assuming f3=l00 and using the R value calculated in part a,, what is the maximum (negative)
swing in the output voltage to avoid entering saturation? (3 marks)
____________________________________
Name
Student ID______________________________
Page 5 of 6
Question 4 (1 mark per multiple choice question. 7 marks total)
Select the appropriate answer (by circling one of a, b, c, or d) for each of the questions below:
I. Which statement is true regarding carrier diffusion?
a.
Diffusion occurs because any non-flat carrier concentration profile implies that there is an
electric field present.
b.
Diffusion occurs because particles (electrons or holes) are more likely to migrate in the
direction where they have more space to move around (making them less likely to collide with
each other).
c.
Diffusion occurs because of the random thermal kinetic energy of a collection of particles,
which (absent otherforces) drives them towards a uniform distribution in space.
d.
Diffusion occurs because otherwise space-charge fields would cause all electrons and holes
to congregate in one region, which is non-physical.
IL In semiconductor physics, a ‘hole’ refers to:
a.
A defect in the silicon crystal that arises at the site of a dopant atom, in the case that the
dopant atom is smaller than a silicon atom.
b.
A valence bond that is not occupied by an electron.
c.
An electron that has bridged the energy gap and can move freely around the crystal.
d.
A charge carrier that emanates from the positive terminal of a battery or source.
III. Which statement is false, regarding a BJT in the forward active mode?
a,
The current at each of the three terminals is exponentially related (approximately) to the
voltage across the emitter-base junction.
b.
The Early effect causes the base current to rise gradually as the voltage across the emitterbase junction is increased.
c.
The collector-base junction essentially robs current from the emitter-base junction.
d.
The ratio of the collector current to the base current (at a given base current) depends on the
temperature, the collector current, and the collector voltage.
IV. Which statement is
regarding the depletion region of a pn junction?
a,
It exists mainly on the mare lightly doped ride of the junction.
b.
Inside the depletion region, there is a very steep gradient in the volume densities of electrons
and holes.
c.
Electrons and holes are present with equal but opposite volume densities, which is why we
also call the depletion region the space charge layer’.
d.
When a small bias (forward or reverse) is applied to the pn junction, the voltage drops abnost
entirely across the depletion region.
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Name
Student ID________________________________
Page 6 of 6
V. Which statement is false, regarding minority carrier lifetime in semiconductors?
a.
The numerical value of the minority carrier lifetime depends strongly on the magnitude of the
excess minority carrier population.
b.
The minority carrier 4fetime is one of the main factors that determines the response time of a
forward biased pn junction.
c.
The numerical value of the minority carrier lifetime is dependent on doping density,
crystalline defects, and temperature.
d.
The minority carrier lifetime is the average time that a minority carrier exists prior to
recombination with a majority carrier.
VI. In semiconductor conduction problems, the concept of drift velocity arises because:
a.
Charge carriers are accelerated by an electric field. but decelerated by collisions with atoms.
The ensemble of charge carriers moves with an effective velocity along the direction of the
electnc field.
b.
Due to relativistic effects, charge carriers that are being accelerated in an electric field
quickly attain a terminal velocity.
c.
Assuming there is no carrier concentration gradient, then the volume density of charge
carriers (electrons or holes) must remain uniform everywhere. This is only possible if they
are moving at a single velocity.
d.
Free electrons are accelerated by an electric field, but occasionally recombine with a hole.
The combination of acceleration, recombination, and thermal generation produces a net drift
velocity.
VII. For a pn junction at sufficiently high forward bias:
a.
A very large currentflows due either to Zener or avalanche breakdown mechanisms.
b.
The built-in voltage is reduced to zero, and current is limited only by the series resistance of
the p and n regions.
c.
Electrons tunnel between the metal contacts, and the semiconductor regions are completely
bypassed.
d.
The current becomes completely dominated by majority carrier diffusion processes.