Digital Signal Processing Reference
In-Depth Information
(b)
(a)
7
7
V
G
sweep @ V
D
=−10V
V
G
sweep @ V
D
=−2V
V
BG
sweep @ V
D
=−10V
V
BG
sweep @ V
D
=−2V
Original 3−pin
4−pin floating BG
4−pin BG=S
6
6
5
5
4
4
3
3
2
2
1
1
0
0
−20
−15
−10
−5
0
5
−20
−15
−10
−5
0
5
V
G
[V]
V
G
[V]
Fig. 2.16 a
Transistor current of an original 3-contact transistor, a 4-contact transistor with floating
backgate, and a 4-contact transistor with backgate connected to the source.
b
V
G
−
I
D
curves for
the original transistor (while
V
BG
=
V
S
) and for the additional transistor created by the backgate
(while
V
G
=
V
S
). The
straight lines
represent a
V
D
=−
10V the
dashed lines
a
V
D
=−
2V
contact and with the source and the drain. So the intuitively expected behavior of
the backgate is a parallel transistor behavior. The second transistor has the same W
and L dimensions, yet the insulator capacitance
C
ox
is about 5 times smaller due to
the thickness of the second insulator layer, as can be seen in Fig.
2.15
. The measured
V
G
−
I
D
curves of both transistors are visualized in Fig.
2.16
. The measurements of
the gate transistor (gate, drain, source) are performed while the backgate is connected
to the source. Those of the backgate transistor (backgate, drain, source) are measured
while the gate is connected to the source. It can be seen that for a
V
D
of
2Vthe
ratio between the currents more or less equals 5 as expected. However, for a
V
D
of
−
−
7, likely caused by a shift of the
V
T
. This supports the idea that other effects are present, which is an acceptable idea
since the electric fields of both transistors have a mutual influence on one another.
A better insight into the functionality of the backgate is acquired when the
V
G
−
10V the ratio becomes larger and amounts to
∼
I
D
curves are plotted for various values of the backgate voltage
V
BG
. The measured
transistor curves are plotted in Fig.
2.17
. The black line corresponds with the char-
acteristics of a transistor with a W/L of 140
m with the backgate connected
to the source, which is the ground level. The gray curves correspond to the same
transistor with a gradually varying backgate voltage
V
BG
. The leftmost curve corre-
sponds with a
V
BG
of 10V and the rightmost curve with a
V
BG
of
µ
m/5
µ
10V. The curves
in between correspond to intermediate backgate voltages with a 2V step in between
consecutive curves. It is remarkable that the curves shift in a very linear way. The
conclusion that can be drawn from this graph is that a linear
V
T
shift occurs which
is caused by the applied backgate voltage, i.e., the
V
T
becomes a linear function of
the source-backgate voltage
V
SBG
. This function is written in Eq. (
2.7
).
−
