Page 335 - Organic Electronics in Sensors and Biotechnology

312 Cha pte r Ei g h t

Before programming; After programming; charge
no charge stored stored in FG/electret
G

FG +++++++
S D
Closed
Open
Semiconductor substrate
(a)

S D

Closed
Open
Semiconductor layer
Electret ++++++++++
G
(b)

FIGURE 8.7 Methods against memory loss. The basic transistor is a device in which
a small voltage applied at the control gate (G) modulates a much larger current fl ow
from source (S) to drain (D) through a semiconductor substrate. (a) In fl ash memories,
an amount of charge is trapped on a fl oating gate (FG) that modifi es the control voltage
required for current to fl ow from S to D. Whether current fl ows or not defi nes a boolean
1 or 0. The memory of this state persists as long as the charge remains trapped on the
fl oating gate. (b) In Baeg and colleagues’ organic device shown, the same principle is
used, but the charge is trapped locally on a thin electret of chargeable polymer, rather
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than on an isolated fl oating gate. (Reproduced with permission from Ref. 77. Copyright
2004, Nature Publishing Group.) (See also color insert.)

EFETs (which can be seen as crude first steps toward organic flash
elements) and FeFETs.

EFETs
EFETs are field-effect transistors with a space charge electret as gate
dielectric. The external field of the charged electret alters the conduc-
tance of the semiconductor channel between the source and the drain
electrode, enabling applications in nonvolatile memories and sensors.
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Such an electret-based memory element developed by Baeg et al.
employs a thin layer of electret instead of a floating gate in between a
gate dielectric and semiconductor, as depicted in Fig. 8.7b, and comes
quite close to architectures used in silicon flash memories. However,
operation voltages are still much too high to be of practical use.
First attempts to use polarizable gate insulators in combination
with organic semiconductors were reported by Katz et al. in a most
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important seminal publication. The FETs showed floating gate effects,