ECE 3150 Lab 2 3/26/2010
MOS T
RANSISTOR
M
ATCHING
AND
S
ERIES
/P
ARALLEL
MOS N
ETWORKS
1
Objectives
In this lab, you will examine how closely the characteristics of nominally identical MOS
transistors on your chips match one another. You will also examine the characteristics of closely matched
MOS transistors connected in series and in parallel.
2
Prelab
1.
DiodeConnected MOSFET
. Recall that a semiideal diode has an exponentially increasing current
when the voltage across it is positive (i.e., when it is
forward biased
) with a slope of
η
×
60mV/decade of current (
η≥
1
is the ideality factor), but it conducts only a tiny saturated
current when the voltage across it is negative (i.e., when it is
reverse biased
). It is common to use
an MOSFET as a “diode” by connecting its drain and gate together. Explain why the MOSFET is
always in saturation and why it behaves like a diode when connected in this manner. Draw a diode
symbol and show how you would replace it with a diodeconnected
n
MOSFET and with a diode
connected
p
MOSFET.
2.
MOS Transistors in Series and Parallel
. Consider the pair of
matched
n
MOSFET, shown on the
left in each part of Fig. 1. By calling these transistors matched, we mean that their characteristics
are identical (e.g., they have the same
W/L
, threshold voltage, normalized threshold current,
V
Dsat
,
normalized
I
Dsat
, etc. and they are operating at the same temperature), so that, if the same terminal
voltages were applied to both devices, their channel currents would be identical. For this question,
we shall assume that the Early effect is negligible for now. Recall that we can express the channel
current of an MOSFET in a
source/drain symmetric
form as in the EKV model:
(
29
(
29
(
29
,
,V
V
f
,V
V
f
I
I
DB
GB
SB
GB
s

=
(1)
where
I
S
is approximately the channel current at
V
th
and
is a function that assumes an exponential
form in weak inversion and a quadratic one in strong inversion. For an
n
MOSFET, the bulk is
connected to ground, so the three potentials in the model would simply be
V
G
, V
D
and
V
S
.
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 Spring '07
 SPENCER
 Microelectronics, Transistor, MOS transistors

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