tesi R. Miscioscia.pdf - EleA@UniSA

24 Working Principles
In the [x0,L] region, the density of free charge is very low if compared
with the channel ([0,x0]), then we can approximate the character of IDS
as shown in (eq. 26):
eq. 26
E
x
( x)
>> E ( x ) ≈E
( 0)
⇒ I
⇒ I
DS
DS
∝
x
0
W
≈ ε chμn
t
L − x
( ( ) )
( ) 3
VDS
−V
x0
L − x
0
x
0
ch
2
∀x
> x ⇒
2 ( E ( L)
) W ⎛V
( L)
−V
( x )
x
This explains the power-law behavior shown in figure 3 for high D to
S fields.
1.4 Thermally activated conduction and
compensation rules.
The dependence of charge mobility in organic semiconductor
channels from temperature and gate fields for OTFT devices is one of
the subjects which have attracted the attention of the recent literature
studies [4][5][6][7] in the last years but still today, the charge
transport in such materials is still not fully understood.
In organic semiconductors, such dependence has been described in
terms of multiple charge trapping [14]), hopping [7], charge diffusion
processes [4] etc. by starting from common evidence which is the
starting point for all these models developments: the dependence of
the thermal activation energy Ea for the mobility from the gate
voltage. In particular, we are referring to a phenomenon which for the
matter of simplicity we will call X (i.e. conductivity activation in the
active region of the OTFT) and evidencing a thermally-activated
behavior and following an exponential Arrhenius-like law versus the
temperature (see eq. 27).
eq. 27
2
0
≈ ε chμn
t
L − x
0
ch
⎜
⎝
L − x
0
0
⎞
⎟
⎠
2
⇒