Organic Thin-Film Transistor Technology: Properties and Functionality - Analog Organic Electronics - page 41

5

BG=S
BG=G

4

V G =−2V:−2V:−10V

3

2

1

0

−20 −15 −10 −5 0

V D [V]

Fig. 2.20 Measured V D − I D characteristics of a 140 µ m/5 µ m gate-backgate connected transistor

and a 140 µ m/5 µ m transistor with a backgate connected to the ground. The V G varies between − 2

and − 10V

C BG is one-fourth of the gate capacitance C G according to the thicknesses of the
respective insulator layers. This means that the parasitic capacitances on the contacts
in the gate-backgate connected transistor are increased with 25% which typically
slows down the speed of analog circuits. However, the current through the transistor
is increased with 80%, hence both effects result in a favorable balance for the speed
in the transistor.
The connection of the backgate to the gate influences the behavioral transistor
parameters and accordingly it will influence the specifications of analog circuits.
An increase of the transistor current and of the g m has been demonstrated. The ratio
g m / I D , however, remains unchanged. The same reasoning is valid for the impedance

r sd of the transistor. It decreases proportionally with the transistor current. Another
effect also influences the value of r sd . The presence of the backgate, independent of
the biasing scheme, increases the V E with a factor 2

4. The r sd is determined by this
value and by the current I ; hence it increases with a factor 1

.

33. The only possible
drawback of this topology is found in the increase of the gate capacitance with 25%.
The effect of applying this technique in a single-stage analog amplifier on the gain,
the gain-bandwidth product, and the bandwidth of that amplifier can be predicted
through Eqs. ( 2.19 )-( 2.21 ):

.

A = g m × r sd (2.19)

1

BW = (2.20)

2 π × r sd × C L

g m

GBW = (2.21)

2 π × C L