Page 47 - Organic Electronics in Sensors and Biotechnology

24 Chapter One

because the tunneling occurs through the pentacene channel which
is semiconducting with charges induced from gate control. In usual
cases, Fowler-Nordheim tunneling happens through a barrier which
is made of insulator, and therefore the charges contributing to
current are solely injected from electrodes, which is similar to space-
charge limited current (SCLC) and follows the same square law
vs. source-drain voltage. In the devices shown as Fig. 1.14d, the
charges constituting the drain current are induced in the channel
by applying gate bias, and thereby the prefactor of this tunneling
current is a linear term. The onset field (V /L) of tunneling slightly
ds
increases with V , probably due to the fact that the ability of the
g
drain bias to make the barrier thinner is screened by the gate-
induced charge, and the slope at high field decreases with V , cor-
g
responding to the gate-lowered tunnel barrier. On the other hand,
in Fig. 1.14c, at large V the slope increases with V , which indi-
ds g
cates that in this case the transport mechanism is not tunneling
through the contact barrier, but rather is determined by the intrin-
sic properties of pentacene, which suggests that in the devices of
the first group the metal electrode and the organic semiconductor
form good contacts.
For nanoscale pentacene transistors with non-ideal contacts, the
overall transport is limited by the gate-modulated metal/organic
semiconductor barrier, as described in Fig. 1.14b and d. If the contacts
are good, transport is not injection-limited (Fig. 1.14a and c). In long-
channel polycrystalline organic transistors, the overall transport is
limited by the transport through grain boundaries rather than that
through the bulk of the grains. 82, 83 On the other hand, when channel
lengths are less than the average grain size, transport through a single
grain may become important; however, the injection properties of
the contact (which is possibly affected by the local morphology of the
pentacene grain) are the most important factor that influences the
electrical response. The grain sizes of the pentacene layer were found
to be from 40 up to 200 nm, much larger than the channel lengths of
the measured devices (from 8 to 35 nm). Therefore for these devices
the number of grain boundaries inside a channel is significantly
reduced; i.e., a channel is composed of only one grain or has only a
single grain boundary inside. For these nanoscale transistors (chan-
nel length from 8 to 35 nm) with small W/L ratios, field-effect mobil-
ity calculated for each of the measured devices was found to have no
significant correlation to the channel length. The histogram in Fig. 1.15
gives a distribution of mobility by statistical counts of the number of
devices in every mobility interval. Nearly 50% of the measured
2
devices performed with a mobility higher than 0.01cm /(V⋅ s). The
wide-range distribution of mobility is attributed to the variability of
the semiconductor-electrode interface nature and the local morpho-
logy of pentacene coverage on the channel.