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Homework
3
Due: Thursday, 9/14/00
NOTES: Please put your answers
in standard units like mA, mV, mm, cm, rather than scientific
notation. Also, make the
homework you turn in very neat, like something you would be happy to
give your boss. Sloppiness
will kill you on the job, and it makes the grader’s job tough. For the remainder of your
assignments, the grader has been instructed to penalize you for turning
in sloppy homework. It does
not have to be type-written or anything like that, just take an extra
second to make it easy to read and follow.
1. A silicon sample at room temperature is doped
with Boron acceptor atoms at a concentration of 5x1017 cm-3.
a)
Is the material p-type or n-type?
b)
What is the majority carrier concentration?
c)
What is the minority carrier concentration?
d)
Where is the Fermi level relative to the intrinsic Fermi level? Give answer in eV, and indicate
whether it is above or below the intrinsic level.
2. A pn junction diode is doped with NA=1015
and ND=5x1017.
a)
What is the built-in potential, Y0?
b)
If we assume this is a one-sided step junction, on which side (p
or n?) will most of the depletion layer be?
c)
What is the depletion layer width?
Assume one-sided step junction with Y0 = 0.7V.
d)
What is the reverse bias breakdown voltage if Ecrit =
1.25x105 V/cm? i.e.,
use the equations in the notes to calculate the VR needed to
make Emax = Ecrit.
e)
What is the forward bias voltage required to produce a diode
current of 100nA if the reverse saturation current is 10-15 A?
3. A BJT has a base current of IB = 7mA and a gain of bF = 120 V/V.
a)
If the BJT is operating in the forward active region, what is the
value of the collector current, IC? Assume we can ignore base width
modulation effects for now.
b)
What is the value of the emitter current, IE?
c)
If VA = 125, what is IC at VCE =
5V?
4. It is known that an npn BJT has IC0
= 5x10-13 A.
a)
What is the value of the collector current if the B-E voltage is
0.5V.
b)
What is the gain ,bF, of this device if IB = 1mA at VBE = 0.5V?
c)
What assumption have you made to calculate the gain in part (b). Hint, it has something to do
with the regions of operation.
d)
Assuming your answer to part (a) was 100mA, if IC = 120mA at VCE=5V, what is the
Early voltage, VA, of this device.
This homework is rather short because we spent a lot of
time discussing physics during the last week and there are not a lot of
calculations you can do. Please take the extra time you have from the short assignment
and read over the notes that were posted on the class web page. I can PROMISE you that your
first exam will have several questions over the physics and theory which
will not be covered significantly in homework assignments.
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