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Electronic active circuits based on organic thin film transistors
M. Berliocchi, M.Manenti, A. Bolognesi, A. Di Carlo, P. Lugli
INFM and Dept. of Electronic Engineering University of Rome “Tor Vergata”
Via del Politecnico, 1 - 00133 Rome, Italy
Simulation Model
Abstract
We simulate organic devices using an industry
standard device simulation tool, namely ISETCADTM, a
package able to resolve the standard drift-diffusion
equations coupled with Poisson’s equation in two and
three dimensions.
ISETCADTM is not capable to handle charge transport
in organic material, so we have implemented a new
model based on the following properties:
Organic field effect transistors have been realized and characterized since they are
excellent candidates to study the transport properties of the organic material used
as active layer. Basic circuits as inverters, in addition, were realized and have been
investigated by experimental and numerical analysis. Devices are fabricated using
the same manufacture processes in a bottom contact configuration. First of all,
source and drain multifinger contacts were prepared by Cr/Au vacuum evaporation
using a standard photolitographic process over thermally grown silicon dioxide
substrates. After the contact deposition, we sublimated (in vacuum atmosphere) a
thin layer (about 30 nm at a deposition rate of 0.2 Å/sec) of organic molecules.
Moreover a hybrid logic family was demonstrated.
•Organic/Inorganic Band Alignment
J n   q n  qDnn
J p   q p   qD p p
  ( 0 r  )   q( p  n  N D  N A )
n 1
   J n  (G  R )
t
q
p
1
    J p  (G  R )
t
q
•Density of States (equal to molecule density)
To characterize the devices we performed two dimensional drift diffusion
simulations taking into account field dependent mobility, trap states (both interface
and bulk trap states) and surface charge density at the interface beetween organic
material and silicon dioxide. Numerical calculations for the quasi static output
characteristics of single organic transistor show a satisfactory agreement with
experimental results and allow us to estabilish a procedure of extraction of field
dependent mobility. With this informations we can simulate more complex
configurations such as inverters and ring oscillators. Here the most interesting point
is given by the time dependence of the gate current that is influenced strongly by
(de)trapping of charge carriers.
Drift Diffusion Equation
•Monte Carlo extracted Field dependent Mobility (see
panel above)
Pentacene Band Structure
•Trap states at the interface between organic
material and silicon oxide (see panel below).
Vacuum
Level
We apply this model to the simulation of pentacene
organic thin film transistor. In particular we simulate
a device with a channel length of 12 m and oxide
thickness of 250 nm. The same device was realized
and measured from our group.
2.6 eV
Source/Drain
Metal
LUMO
0.1 eV
2.4 eV
HOMO
Pentacene TFT:Simulation and Experimental Device
Geometric Characteristics
Multifinger structure for source-drain
contacts (5nm Cr; 45nm Au)
Active layer material
Measured and simulated
output characteristic
Pentacene
ALDRICH
All measurements were carried out in
vacuum
atmosphere
at
room
temperature with Agilent Semiconductor
Analyzer 4155C.
Measured transfer
characteristic @ Vds=-30 V
Square root of
saturation corrent
From square root of IDSS is possible
to calculate pentacene mobility
using the expression:
Inverters
Conclusions
Vcc
Vout
Vin
-18,5
Rc
-4
-19,0
-19,5
-6
-20,0
-8
Vout (V)
Vout
Voltage (V)
Vin
-2
-20,5
-21,0
Vin = 10V
-21,5
-10
-12
-14
-22,0
-16
-22,5
-23,0
Vin: 250 mHz; Vpp=20 V
Vin = -10V
-18
-60
0
5
10
15
20
-40
-20
0
Vin (V)
25
Time (sec)
Vcc: -20V
Rc: 30 MΩ
Channel lenght: 12 μm
Vcc: -20V
Rc: 50 MΩ
Channel lenght: 12 μm
Voltage (V)
Vin: 250 mHz; Vpp=20 V
Vout
Vin
10
5
Vin: DC
0
Vcc: -20V
-5
Rc: 100MW
-10
Channel lenght: 20mm
-15
-20
0
10
20
Time (sec)
30
40
20
We simulate organic devices using an
industry standard device simulation tool,
namely ISETCADTM. This package is able
to resolve the standard drift-diffusion
equations
coupled
with
Poisson’s
equation in two and three dimensions.
We have realized and characterized
single organic transistors and basic
circuits like inverters using commercial
Pentacene.
We
settled
the
basis
for
the
development of a complete family of
circuits like three stages ring oscillators.