Page 38 - Organic Electronics in Sensors and Biotechnology

Scaling Effects in Organic Transistors and Transistor-Based Chemical Sensors 15

0
270 nm
–2
500 nm
–4
1 μm
–6
2 μm
–8
5 μm
–10
V th (V) –12
–14
–16
–18
–20
–22
50 100 150 200 250 300
T (K)
FIGURE 1.9 The temperature dependence of threshold voltage for scaled
channel lengths. Threshold voltages V were extracted from the
th
transconductance plots (I vs. V for V < V − V ; (I ) vs. V for V ≥ V − V )
1/2
d g ds g th d g ds g th
on high end of V for each channel. V does not signifi cantly change with
ds th
longitudinal fi eld. Channels of different lengths follow the same trend;
namely, V shifts to high gate bias quasi-linearly with decreasing
th
temperature. (Reprinted with permission from Ref. 60. Copyright 2007,
American Institute of Physics.) (See also color insert.)
geometry, morphology, and interfaces. Most experimental studies per-
formed so far have emphasized measurement and interpretation of
temperature-dependent field-effect mobility. 52–55 In partially ordered
organic semiconductors, however, it is the electric field dependence of
the mobility at various temperatures that sheds the most light on the
understanding of transport phenomena. 56–57 Temperature dependence of
the mobility alone is insufficient to draw some of the important conclu-
sions. Previous reports 58–59 contained evidence of a field-dependent
mobility, but the measurements were not accurate enough for quan-
60
titative work. The work reported recently by Liang Wang et al. rep-
resented the first quantitative measurement of the electric field
dependence of the mobility in organic thin-film transistors when
device geometries are scaled along the direction of charge transport
and when accumulated carrier densities are at levels of practical
importance. With systematic measurements, they reported tempera-
ture and electric field dependence of field-effect mobility in polycrys-
talline pentacene thin-film transistors of scaled channel geometries,
and their results show a Frenkel-Poole type dependence of mobility
on electric field.
In purified single crystals of polyacenes such as anthracene,
band-like transport has been experimentally observed by Karl and
coworkers and subsequently interpreted in terms of polaronic trans-
port models. 61–62 In this picture, band transport can exist at low