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YOUR NAME________________________________
Department of Electrical Engineering and Computer Science
Massachusetts Institute of Technology
6.012 Electronic Devices and Circuits
Exam No. 2
Notes:
1. Unless otherwise indicated, assume room temperature and that kT/q is 0.025 V.
You may also approximate [(kT/q) ln 10] as 0.06 V.
2. Open book: 6.012 text and up to one shopping cart full of notes permitted.
3. All of your answers and any relevant work must appear on these pages.
Any
additional paper you hand in will not be graded.
4. Make reasonable approximations and assumptions.
State and justify any such
assumptions and approximations you do make.
5. Be careful to include the correct units with your answers when appropriate.
6. Be certain that you have all nine (9) pages of this exam booklet and make certain
that you write your name at the top of this page in the space provided.
6.012 Staff Use Only
PROBLEM 1
(out of a possible 40)
PROBLEM 2
(out of a possible 30)
PROBLEM 3
(out of a possible 30)
TOTAL
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Problem 1 
(40 points)
For Parts a, b, and c,
you are given an MOS capacitor made on uniformly doped
silicon and you are told that its flatband voltage, V
FB
, is + 1 V, and that its threshold
voltage, V
T
, is + 3 V.
You are also told that the thickness, t
ox
, of the gate insulator is 80
nm (8 x 10
6
cm) with a dielectric constant,
e
ox
, of 3.2 x 10
13
coul/Vcm.
a)
What is the doping type of the silicon, ntype or ptype? Explain your answer.
�
ntype
�
ptype
�
indeterminate, because
b)
What is the condition of the oxidesilicon interface when v
GB
, the voltage on gate
realtive to the silicon, is zero volts? Explain your answer.
�
inverted
�
depleted
�
accumulated, because
c)
For what range of v
GB
is the silicon surface depleted?
< v
GB
<
For parts d, e, f, and g,
you have an nchannel silicon MOSFET and a pchannel
silicon MOSFET which have the same value of Kfactor in their large signal models
[where the Kfactor is defined as (W/L)µ(
e
ox
/t
ox
)]. The transistors are identical in all
dimensions and doping level magnitudes except that the gate length, L, of one of the
devices is twice that of the other. The
a
factor for each device is one.
d)
Which transistor, if either, would you expect to be the one with the longer gate
length, and why?
�
nchannel
�
pchannel
�
neither, because
e)
What is the ratio of the electron to hole mobility in these transistors (i.e., what is
the ratio of the mobility of the electrons in the channel of the nchannel MOSFET to that
of the holes in the channel of the pchannel MOSFET)? Explain your answer.
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 Fall '03
 Prof.CliftonFonstadJr.
 mos capacitor

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