Digital Signal Processing Reference
In-Depth Information
(a)
(b)
Gate
Gate
Insulator
Insulator
Source
Drain
Semiconductor
Source
Drain
n
n
++
++
Inert
Bulk
Substrate
p
+
Substrate
Fig. 2.1 a
The cross-section of an NMOSFET which is embedded in a
Si
substrate.
b
The cross-
section of a TFT which is deposited on top of an inert substrate. Both the MOSFET and the TFT
are field-effect transistors
electronics technology. The presence of transistor models which are physically rather
than behaviorally inspired is discussed in Sect.
2.6
. Consequently the implemen-
tation of the passive components, i.e., resistors, capacitors, and inductors, in the
organic electronics technology is treated in Sect.
2.7
. Finally, Sect.
2.8
summarizes
this chapter.
2.1 Thin-Film Transistors
As indicated by the title, the transistors in the applied technology are thin-film tran-
sistors. The latter differs from the more common MOSFET transistors in
Si
standard
CMOS both in a physical and a behavioral way. The cross-section of an n-type MOS-
FET transistor (NMOS) and a thin-film transistor (TFT) is presented in Fig.
2.1
.The
NMOS transistor is embedded on a
p
+
-doped
Si
substrate. The two
n
++
-doped
areas represent the source and the drain of the NMOS and the transistor channel
is situated between them. A gate contact, typically manufactured in polysilicon, is
deposited on top of the channel. When the gate is biased positively a space charge
region is formed and the channel conducts electrons (
e
−
) from the source to the
drain while the transistor channel is in the inversion mode, i.e., the channel con-
ducts electrons
e
−
. MOSFET transistors also have a bulk contact which is the area
close to the transistor channel. The bulk of this NMOS is connected to the substrate,
so the bulk contacts of all NMOS transistors are connected. This can, if necessary,
be overcome through a triple well but that is beyond the scope of this dissertation.
p-type MOSFET transistors (PMOS) are fully complementary with NMOS transis-
tors. Their working principle is based on the conduction of holes.
In a thin-film transistor (TFT) the semiconductor is sufficiently thin so that it
can be fully depleted by the gate. It has ohmic drain and source contact regions.
Furthermore, it has no substrate contact. The basic architecture of a TFT is similar
to a MOSFET transistor, since the gate contact is electrically separated from the
channel by an insulator. Both are field-effect transistors (FET).
