Download Investigation of the potential of organic circuit

Investigation of the potential of organic circuit for RFID tags
Qintao Zhang and Sha Li
University of California Berkeley
Abstract-In recent years, RFID technology is one
of the attractive technologies in service industries.
Till now, the core of RFID technology is based on
the single crystal silicon IC technology. In order to
reduce the cost of RFID tags furthermore to less
than 1 cent, organic transistor technology is
introduced as one of the most promising alternative
techniques. In this project, RFID and organic
transistor technologies are reviewed and different
organic circuit schemes used in RFID are
simulated based on an amorphous silicon spice
model. The speed of organic circuit is going to be
investigated to tell the capability of the organic
circuit on high frequency RFID application and
the power consumption of the device will also be
evaluated.
I. Introduction
In recent years, RFID (Radio Frequency
Identification), which is a means of identifying a
person or object using a radio frequency
transmission, has gained a lot of attentions from
both the academia and industries due to its great
potential on detecting and tracking capability. RFID
can store more information about the object, like its
unique serial number, manufacture date and
composition for the commercial product, besides
being identification like the barcode and the
personal and medical information for the human
user.
In RFID, communication takes place between a
reader and a transponder (often called a tag), shown
in figure 1[1]. Inside the tag, which is the core of the
RFID technology, the information of the products is
stored and sent back to the reader or modified
according to the command from the reader.
Currently, there are two kinds of Tags, 1) active tags,
powered by a battery and 2) passive ones, powered
by the coupling magnetic field from the reader to the
antenna of the tag.
Figure 1, a typical RFID system
Figure 2 is the schematic diagram of a passive RFID
tag, which is more favorable than an active tag for
the future market due to its simple architecture. The
commands from the antenna, where not only the
signal but also the power is coupled from the reader
through a metal circle, are intercepted by the
decoder and stored into the sequencer, which is
basically a register. According to the command, the
sequencer reads the necessary information from
EPROM and sends it to the encoder and the
modulator to modulate the signal into the
corresponding radio frequency and broadcast it back
to the reader. In this process, the rectification of the
AC is done by the AC/DC converter and a power
control module is embedded to switch the DC
voltage from the converter to the working voltage of
the chip. The working frequency of the device is
determined by an on-chip frequency generator,
which is not shown in the figure.
Figure 2 the basic tag IC architecture [1]
Currently the application of the RFID is only
confined to the business of pallets and shipping
containers, and the reason is mainly because of the
high cost of tags. All the commercial tags are made
from the silicon process, leading to the cost per chip
25 cents typically and no less than 5 cents.
Compared to the goal, under 1 cent per chip, for
item level tracking, this conventional fabrication
technique can’t satisfy the requirement. Therefore, a
new technique which uses inherent organic
semiconductor and soluble metal nanoparticles to
print circuits and antennas on flexible substrates or
to fabricate chips and print circuitry on them is
under investigation. The organic tags would not only
cut the cost to 1 cent per piece or less eventually
after commercialized but also provide a flexible
substrate, which makes tags easy to use on all kinds
of surface or versatile products.
Organic circuits have been under research for more
than a decade now. Instead of using silicon as the
semiconductor material, organic transistors use
semiconductor polymer as the active material, which
gives these transistors the great properties like the
flexibility and low processing temperature.
Therefore, it can be fabricated on versatile substrates,
such as plastic and paper at very low temperature,
generally at room temperature or around 100 oC. And
because most of Organic materials are soluble in
particular
solvents,
such
as
P3HT
(Poly3hexylthiophone) in Chloroform, a cheap
fabrication method, inkjet printing, could be used for
the potential organics circuits. This maskless process
can reduce the cost dramatically.
According to the current research, organic cicuits
show great potential on the RFID application,
however, the inherent properties of the organic
semiconductors set some restrictions on the final
circuit. Therefore, a study on the potential
performance and design of the organic circuits for
RFID is necessary.
Figure 4 is the first demonstrated passive organic
RFID tag [4]. In the paper, Pentacene, a p-type
organic semiconductor is used.
Figure 4, a real organic RFID circuit based on
pentacene devices.
Figure 3, basic structure of organic transistors; top
contact (a) and bottom contact (b). [5]
Two typical structures of organic transistors is
shown in figure 3, a) top contact and b) bottom
contact. The organic transistor works in the
accumulation mode. Instead of forming inversion
layer in the channel, the accumulation layer is forced
by the gate voltage, and the drain current reaches to
the saturation region when the source drain voltage
is high enough. Organic transistors show enhanced
characteristics in the past ten years [5], mobility has
been improved from 10-5cm2/VS to 1.5cm2/VS,
which is already same as the mobility of amorphous
silicon transistors. The Ion/Ioff ratio of 106 can be
very easily obtained too. The improvement on the
organic semiconductor material and fabrication
make the whole-organic circuit possible. The first
all-inkjet-print organic transistor and organic
transistors on fiber have been reported in organic
research group in Berkeley.
II. Motivation
III. Proposed work
The main digital circuit inside the RFID would be
the memory, decoder, buffer and the oscillator. In
this project, we will first examine the properties of a
single MOSFET of the organic transistor and then
expand the discussion to the inverter and the
oscillator. Eventually, we are going to design a
memory cell and corresponding decoder.
For the organic transistors, both of the most popular
materials being used and studied, Pentance and
Polythiophene, which has high mobility, are p-type
semiconductors. So all-pmos circuit is inevitable for
organic RFID if no comparable n-type organic
material emerges. So in the project, an all-pmos
circuit is designed first and the trade off of this
design will be discussed. However, people are still
researching for n-type organic transistors and
soluble inorganic n-type transistors, such as ZnO
nanoparticles done in Berkeley. The nmos transistor
will then be added into the circuit in order to extract
the maximum speed of the organic circuit for the
investigation of the performance.
The communication frequencies used by RFID
ranges from 125KHz to 2.45 GHz, depending on
applications. In US, 13.56MHz is the most attractive
frequency because of the FCC rules and the trade-off
between power and detection range. The speed of
the organic RFID will be compared with this
frequency range.
n-type organic and soluble inorganic are on their
beginning, n-type parameters are going be not real
number. Actually, the mismatch of the n and p mos
is another problem for organic transistor design.
Power will be another issue for the RFID especially
the passive one because the power collected from
the antenna is decreased quadratic to the distance so
the working region of RFID is determined by the
power dissipation directly. We will discuss the basic
power consumption for the circuit. On the other
hand, the leakage is a big concern for the current
organic RFID because of the ohmic contact between
the source-drain contact and the channel and the
high working voltage, higher than 10V, due to the
high threshold voltage generates quite high gate
leakage. But for these phenomena, there are no good
model for them now so we will only qualitatively
discuss the influence of the leakage.
Summary
The spice model of organic transistor is not ready to
be used, so in this project, we are going to use the
AIMSPICE amorphous silicon transistor model to
simulate the organic circuits. Amorphous silicon
transistors show same mobility as the organic
transistors, and because they are all amorphous
crystal, they show similar interface trapping
mechanism between crystalline. Thus it’s reasonable
to use amorphous silicon crystal model to simulate
the circuit, although it can’t simulate correctly the
power consuming.
Spice parameters of pentacene are going to be
extracted and optimized based on data from
Professor Vivek Subramanian’s group. Because the
In this project, we are going to investigate the speed
of the organic transistors and tell the feasibility of
organic RFID. Because organic RFID has not been
simulated before, some circuit diagram is going to
be proposed and suggested.
Reference:
[1] “A Basic Introduction to RFID Technology and
Its Use in the Supply Chain,” Laran RFID white
paper
[2] Patrick A. Toensmeier, “As RFID applications
increase, suppliers look to lower it cost,” 12 Plastics
Engineering, February 2005.
[3]
K.
Finkenzeller,
“RFID
handbook:
Fundamentals and applications in contactless smart
cards and identification,”
[4] P. F. Baude, D. A. Ender, M. A. Haase, and et al,
“Pentacene-based radio frequency identification
circuitry”, Applied Physics Letters, Vol 82, number
22, 2003.
[5] C. D. Dimitrakopoulos, and D. J. Mascaro,
“Organic Thin-Film Transistors: A Review of Recent
Advances,” IBM J. Res. & Dev., Vol. 45, No. 1, Jan,
2001