Low Voltage Organic Field-Effect Transistors for Printed Electronics

June 2014 to August 2014

Supervisor-Dr. Leszek Majewski Microelectronics and Nanostructure Group

School of Electrical and Electronic Engineering The University of Manchester

Vishisht M. Tiwari (Year-1 Undergraduate)

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Contents 1. Background…………………………………………………….....1

1.1. Introduction……………………………………………....1 1.2. Operation of OFETs….………………………….….…..1 1.3. Device Parameters……..……………………………….2 1.4. Transfer Characteristics Curve………………………...3 1.5. Output Characteristics Curve…………………………..4

2. Experiment………………………………………………………...5 2.1. Si/SiO2 with PTAA ………………………………………5

2.2. Al/Al2O3 with PTAA and with Top Contacts………......7

2.3. Al/Al2O3 with PPDDTT and T.N.T recognition Seq…..8

3. Results…………………………………………………………….10

3.1. Analysis of OFET with SAM and without SAM………10

3.2. Analysis of Contact resistance……………………......11

3.3. Analysis of Bio Sensors with T.N.T recognition Seq..13

3.4. LabVIEW Program…………………………....……......14

4. Conclusion………………………………………………………..15

5. Future Work……………………………………………………....15

6. References………………………………………………………..15

Special Thanks to: Jesse Opoku

And Malachy Mcgowan

1

Background

Introduction

The field of nanoelectronics and microstructures has seen some great

inventions in the past few decades. One such invention is the development of

the Organic Field Effect Transistors (OFETs). These transistors have provided

the basic building block for flexible integrated circuits and displays. Intensive

research is being carried out almost everyday in this field and more and more

viable options are being prepared for industry purposes. Working in this field has

helped me to gain valuable knowledge about this upcoming subject and also

helped me broaden my horizon about other fields that are connected to

nanostructures and microelectronics.

Operation of OFETs

An Organic field-effect transistor (OFET) is a field effect transistor that uses

organic semiconductors in its channel. OFETs can be prepared by either

electro-polymerisation, by solution-based deposition of polymers, by vacuum

evaporation of small molecules or by Langmuir-Blodgett Technique. These

devices have many advantages over its non-organic counterparts such as easy

fabrication, mechanical flexibility, biodegradability and lower cost.

There are 3 different geometries in which FETs can be used:

The MISFET- Metal-Insulator-Semiconductor Junction

The MESFET- Metal-Semiconductor Field Effect Transistor

The TFT- Thin-Film Transistor

For this project, the OFETs were fabricated in thin-film transistor geometry.

Substrate

Gate

Insulator

Source Drain Semiconductor

Fig. 1. Schematic view of the Thin-Film Transistor

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In this geometry, the source electrode and drain electrode are deposited on the

thin film of insulator. This layer of insulator separates the gate from the

electrodes. The layer of semiconductor is then deposited on the insulator and

electrodes.

Unlike in other geometries, TFTs operate in accumulation regime rather than in

the inversion regime and there is no depletion region to isolate the device from

the substrate. On applying a zero bias, the electrons are expelled from the

surface due to difference in the Fermi-level of the semiconductor and the metal.

In this case, no carrier movement is observed between the source and drain. On

applying a positive charge, the conduction band bends downward forming a very

conductive channel on the interface.

Device Parameters

Parameters are empirical properties of OFETs that define how well a transistor

can work by showing its different electrical properties. Most of these parameters

can be derived by observing transfer and output characteristics of the transistor.

Few of the important parameters are as follows:

MOBILITY-Mobility is the measure of ease with which holes can move from the

source to the drain when electric field is applied. Mobility of an OFET, in both

linear region and saturation region, can be calculated using different properties

of the transistor such as drain current (ID), width of the channel (W), length of the

channel (L) and gate voltage (VG). The formula for calculating mobility is as

follows:

Linear Mobility

Saturation Mobility

ON-OFF CURRENT RATIO-It is the ratio of the value of the current when the

device saturates to that value of the current when the device is not switched on.

This ratio can be calculated using the transfer characteristics curve.

ONSET VOLTAGE-The gate voltage at which the transistor starts to conduct is

called the onset voltage. This parameter can also be calculated using transfer

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characteristic curve. At this voltage the current starts to increase exponentially.

SUBTHRESHOLD SLOPE- Subthreshold slope shows the change in drain

current (ID) with unit change in gate voltage (VG) it can be calculated by

calculating the slope of the transfer characteristic curve of a transistor. The

voltage is plotted on the x-axis while the current is plotted on the y-axis in

logarithmic form. The formula can be represented as follows:

TRANSCONDUCTANCE-Transconductance relates the transistor output

current with the input gate voltage. It is defines as the rate of change of drain

current with respect to the gate voltage. This parameter can be calculated using

the transfer characteristics curve. However it can also be calculated

mathematically by using width (W) and length of the channel (L), capacitance of

the transistor (C) and the mobility of the transistor. The formula can be

represented as follows:

OPERATING VOLTAGE-Operating Voltage is the peak gate voltage till which

the device can be used as a transistor. It is sometimes also referred to as the

breakdown voltage because the transistor can breakdown if this voltage is

exceeded.

Transfer Characteristics Curve

To understand how well a transistor is working, the relation between the

controlling variable and the controlled variable has to be established. Transfer

characteristics help to derive this relation. The curve shows the dependence of

drain current over a range of values of gate voltage. Conclusions that can be

drawn out from the transfer characteristic curves are as follows:

Drain current is almost zero when gate voltage is less than the threshold

voltage.

The voltage from where the current starts to rise is the onset voltage.

Once the transistor starts to conduct, the current increases exponentially.

The ratio of the current at the highest point to the current at the lowest

point can give us the on-off ratio.

Subthreshold slope can be derived by finding the slope of the transfer

characteristics curve.

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Graph.1. Transfer Characteristics Curve for a Silicon Device

Output Characteristics Curve

The relationship between drain current and drain voltage is shown by the output

curves. Each curve is taken out at a particular value of gate voltage. The graph

shows the drain current for all the values of gate and drain voltages. Output

curves are important as they help to determine the different regimes in which the

transistor works. The regimes have been explained as follows:

At VG<VT the drain current is almost negligible and the transistor is said to

be in cut-off region. In this region no increase in drain current is observed

with increase in drain voltage.

At VG>VT and at high values of VDS, the transistor works in the saturation

region. In this case also drain current is virtually independent of drain

voltage

At VG>VT and at lower values of VDS, the transistors works in the linear

region. In this region there is a direct dependency between drain current

and drain voltage.

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Graph.2. Output Characteristics Curve for a Silicon Device

Experiment

During the project, different substrates, semiconductors and geometries were

used to fabricate devices. The Experimental Procedure for different devices

have been described below:

Si/SiO2 Transistors

PTAA Blend as Organic Semiconductor

Top Contact Electrodes

Silicon substrates were cut into the desired sizes of 2x2 cm.

The substrates were cleaned using acetone and methanol in an ultra-

sonic cleaner.

SAM (Self-Assembled Monolayer) was prepared using ODTS (1-

octadecane-thiol) and TCE (trichloroethylene).

o Solution: 4.5 ODTS in 4mL of TCE.

SAM was spin-coated on the substrate using a spinner.

o Spin-coated 2 times for 10sec with a solution of 250 of SAM.

o Speed was kept at 3000rpm with an acceleration of 2500 rounds

per minute2.

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The substrate was then annealed at 105 oC for 20min.

After annealing, the substrate was cleaned again in an ultrasonic cleaner

using methanol and IPA (isopropyl alcohol).

PTAA was used as the organic semiconductor.

o Solution: 10mg of PTAA in 1ml of 12-

dochlorobenzene.

A layer of this PTAA was spin-coated on the

substrate using a spinner.

o Spin-coated for 1min with a solution of

250 of PTAA

o Speed was kept at 2000rpm with an

acceleration of 255 rounds per

minute2.

The substrate was again annealed but this

time at 100 oC for an hour.

The environment was ensured to be oxygen

free.

Gold was cleaned in an ultrasonic cleaner

using acetone and methanol

Gold was then evaporated on the substrate for drain and source

electrodes.

o 60nm of gold was thermally evaporated in a vacuum chamber.

o 15 devices of channel length of 20um, 40um, 60um, 80um and

100um were formed on the substrate.

All the devices were patterned do increase the mobility of the devices.

Output and transfer characteristics were taken in ambient conditions

using a LabView program and different parameters of the devices were

obtained.

Thermally evaporating gold in vacuum chamber

7

Al/Al2O3 Transistors

PTAA Blend as Organic Semiconductor

Top Contact Electrodes

Glass substrates were cut into the desired sizes of 2x2 cm.

The substrates were cleaned using acetone and methanol in an

ultra- sonic cleaner.

Aluminium (Al) was

thermally evaporated to a

thickness of 150nm on the

substrate in a vacuum

chamber. This aluminium is

used as gates in the

devices.

The substrate was then kept

in the ozone chamber for

5min.

Substrates were then anodized to a desired voltage and a thin layer of

Al2O3 was deposited on the

aluminium gate.

For the purpose of this project,

the substrates were anodized to

3V, 5V, 7V and 13V to get an

operating voltage of 1V, 3V, 5V

and 10V respectively.

The substrates were then

cleaned again using IPA

(isopropyl alcohol).

SAM (Self-Assembled

Monolayer) was prepared

using ODTS (1-octadecane-thiol) and TCE (trichloroethylene).

o Solution: 4.5 ODTS in 4mL of TCE.

SAM was spin-coated on the substrate using a spinner.

o Spin-coated 2 times for 10sec with a solution of 250 of SAM.

o Speed was kept at 3000rpm with an acceleration of 2500 rounds

per minute2.

The substrate was then annealed at 105 oC for 20min.

After annealing, the substrate was cleaned again in an ultrasonic cleaner

using methanol and IPA (isopropyl alcohol).

PTAA was used as the organic semiconductor.

o Solution: 10mg of PTAA in 1ml of 12-dochlorobenene.

Aluminium Gates after being thermally evaporated on glass substrates

Glass substrates cleaned in an ultrasonic cleaner

8

A layer of this PTAA was spin-coated on the substrate using a spinner.

o Spin-coated for 1min with a solution of 250 of PTAA

o Speed was kept at 2000rpm with an acceleration of 255 rounds

per minute2.

The substrate was again annealed but this time at 100 oC for an hour.

The environment was ensured to be oxygen free.

Gold was cleaned in an ultrasonic cleaner using acetone and methanol

Gold was then thermally evaporated on the substrate for drain and

source electrodes.

o 60nm of gold was thermally

evaporated in a vacuum

chamber

o 15 devices of channel length of

20um, 40um, 60um, 80um and

100um were formed on the

substrate.

Output and transfer characteristics

were taken in ambient conditions using

a LabView program and different

parameters of the devices were obtained.

Al/Al2O3 Transistors

PPDDTT Blend with T.N.T Recognition Sequence

Bottom Contact Electrodes

Glass substrates were cut into the desired sizes of 2x2 cm.

The substrates were cleaned using acetone and methanol in an ultra-

sonic cleaner.

Aluminium (Al) was thermally evaporated to a thickness of 150nm on the

substrate in a vacuum chamber. This aluminium is used as gates in the

devices.

The substrate was then kept in the

ozone chamber for 5min.

Substrates were then anodized to a

desired voltage and a thin layer of

Al2O3 was deposited on the

aluminium gate.

For the purpose of this project, the

substrates were anodized to 3V, 5V,

7V and 13V to get an operating

voltage of 1V, 3V, 5V and 10V

respectively.

Characterisation of devices

Anodisation

9

The substrates were then cleaned again using IPA (isopropyl alcohol).

Gold was cleaned in an ultrasonic cleaner using acetone and methanol

Gold and chromium were then evaporated on the substrate for drain and

source electrodes.

o 5nm of chromium and 55nm of gold were thermally evaporated in

a vacuum chamber.

o 15 devices of channel length of 20um, 40um, 60um, 80um and

100um were formed on the substrate.

2 layers of SAM are coated on the devices

The first layer of SAM (Self-Assembled Monolayer) was prepared using

ODTS (1-octadecane-thiol) and TCE (trichloroethylene).

o Solution: 4.5 ODTS in 4mL of TCE.

This solution was spin-coated on the substrate using a spinner.

o Spin-coated 2 times for 10sec with a solution of 250 of SAM.

o Speed was kept at 3000rpm with an acceleration of 2500 rounds

per minute2.

The second layer of SAM is prepared PFBT (Poly(fluorine-alt-

benzothiadiazole)) and IPA (isopropyl alcohol).

o Solution: 6.67 (10mM) in 5mL of IPA.

o 200 solution of PFBT is sprinkled on the devices and is rested

for 5min before cleaning it with IPA again.

The substrate was then annealed at 105 oC for 20min.

After annealing, the substrate was cleaned again in an ultrasonic cleaner

using methanol and IPA (isopropyl alcohol).

Solution of PPDDTT and T.N.T recognition sequence was used as the

organic semiconductor.

o Solution: (7:3 ratio of PPDDTT and T.N.T recognition sequence)

3.5mg of PPDDTT and 1.5mg of T.N.T in 1ml of 12-

dochlorobenene.

A layer of this solution was spin-coated on the substrate using a spinner.

o Spin-coated for 2min with a solution of 250 .

o Speed was kept at 2000rpm with an acceleration of 255 rounds

per minute2.

The substrate was again annealed but this time at 120 oC for 30min.

The environment was ensured to be oxygen free.

Output and transfer characteristics were taken in ambient conditions

using a LabView program and different parameters of the devices were

obtained.

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Results

The Aim of this project was to understand the basic principle and working of an

Organic Field-Effect Transistor (OFETs). To gain a better understanding, many

different experiments and analysis were carried out during this project and many

results regarding OFETs were obtained. A compendium of these results have

been given below:

Analysis of OFETs with SAM and without SAM

Self-Assembled Monolayer (SAM) was used in our project to achieve a greater

mobility in our devices. SAM provides near crystalline ordering and has

hydrophobic terminal groups that make it easier for holes to transfer from drain

to source. To gain understanding of SAM, we prepared two set of Si/SiO2

devices, one with SAM and one without it, keeping all other factors constant.

Transfer curves were derived for these two devices and were then compared.

Graph.3. Transfer Characteristics Curve for Silicon Devices with SAM and without

SAM

The graphs clearly show the improvements in the performance of the devices

with SAM in it. The noticeable improvements are as follows:

The drain current in the samples with SAM is 10 times higher than the

samples without SAM.

On-Off ratio also has significant improvement when SAM is used. The

increase in on-off ratio is of the order of 10 when SAM is used.

Hysteresis is observed to be negligible in devices with SAM. On the other

11

hand, the devices without SAM have significant hysteresis.

Leakage current has also improved when SAM is used in devices.

On calculating the mobility, it was observed that devices with SAM had

mobility around 10-4cm2/V while the devices without SAM had mobility

around 10-5cm2/V. Hence mobility also increased by an order of 10 when

SAM was used in transistors.

Analysis of Contact Resistances for transistors

operating at 1.5V and 3V

With increase in charge-carrier mobility, limitations by contact resistances are

becoming a crucial parameter of any organic device and finding ways to reduce

this limitation is becoming an important issue. Contact resistances are often of

the order of 109 ohm.

1.5V Device

Graph.4. Contact Resistance graph for 1.5V devices

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Gate Voltage (V) Linear Contact

Resistance (Mohm)

Specific Linear Contact

Resistance (Mohm-cm)

-1.5 2.467 0.246

-1.4 2.895 0.289

-1.3 3.761 0.376

-1.2 6.572 0.657

Table.1. Contact Resistance table for 1.5V devices

In this experiment, two different sets of devices with operating voltage of 1.5V

and 3V were taken. There were 15 devices in each set with channel length of

20um, 40um, 60um, 80um and 100um. Current was measured at different

voltages and the following graph was obtained.

3V Device

Graph.5. Contact Resistance graph for 3V devices

Gate Voltage (V) Linear Contact

Resistance (Mohm)

Specific Linear Contact

Resistance (Mohm-cm)

-3.0 5.677 0.567

-2.6 6.319 0.632

-2.2 7.750 0.775

-1.8 12.990 1.300

-1.4 32.980 3.298

Table.2. Contact Resistance table for 3V devices

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The graph and the table show the following properties:

Overall resistance decreases when channel length is decreased. This is

the case because channel resistance decreases as channel length

decreases

The graph shows that the relation between gate voltage and contact

resistance is inverse as when gate voltage increases, contact resistance

decreases. (There are few anomalies in the graphs regarding this

deduction)

Same relation can be found in contact resistance and drain voltage. The

graph shows that for the same gate voltage, the contact resistance

decreases with increasing drain voltage.

Analysis of Biosensors with T.N.T recognition

sequence

Among the plethora of uses of OFETs, one of the most important use is in the

field of biosensors. Biosensors are analytical devices whose electrical property

changes when they detect analyte. In this project, OFETs were used for the

detection of T.N.T. This could be done by mixing T.N.T with the organic

polymer. Following current vs time graph was observed when T.N.T was put

onto the 20um transistor.

Graph.6. Current vs Time graph when T.N.T recognition sequence is used

Current vs Time graph when T.N.T was used

Time (seconds)

Cu

rren

t (A

mp

ere)

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LabVIEW Program

Another important demand of the project was to develop a LabVIEW program

that can measure the drain current and source current with respect to time.

This was a necessary demand for experiment of the biosensors. The program

could measure the change in current at different drain, source and gate

parameter.

Fig.2. Snapshot of the LabVIEW program used to for Current vs Time Graphs

To carry out the measurements the program needed the following parameters:

Gate Voltage

Drain Voltage

Source Voltage

Time for which the measurements is suppose to be taken out

The program then supplies the voltage through the devices and obtain current

values. The following measurements were obtained from the program:

Graph for Drain Current

Graph for Source Current

Numerical value of Source Current at different voltages

Numerical value of Drain Current at different voltages

An excel file of all the values are saved in the computer hardware

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Conclusion

Organic transistors have been in talks for the last few decades but only recently

have their been considered to be used industrially. Low cost, easy production

and mechanical flexibility are few of the many advantages that have lead to

extensive research in this field in the past few years. In this project, I gained not

only a basic understanding of the working of an organic transistor but also

carried out experiment to achieve the best working transistor. First part of the

project was to gain knowledge about the field by reading research papers and

articles. This helped to gain a firm grip on the basics of how the idea was

thought of and how has it started to become viable in the present world. After

doing thorough research I started conducting experiments to achieve the best

working transistor by experimenting with SAM and organic semiconductors. The

next part of the project was to carry out research on contact resistances and

what role do they play in working of an OFET. I also got the opportunity to work

on how OFETs are used as biosensors and how they can be used as an

engineering solution. Working with Dr. Majewski has helped me to gain valuable

knowledge about this upcoming field and also helped me broaden my horizon

about many different fields that are connected to nanostructures and

microelectronics.

Future Work

Working on this project helped conduct many experiments and analyze many

different ways in which OFETs can be used or improved. However, delving

deeper into this field made me realize how broad this subject is and how much

research is being carried out everyday. I would like to state few of the worked

that I would like to do in the future if I get another opportunity to work in this

field.

Working with some new organic polymers and getting the highest mobility

Doing some more research in the field of contact resistance and able to

get some conclusive results about how contact resistance can be

decreased.

Work with plastic substrates and achieve conclusive results with this

substrate.

References 1. Horowitz G. (1998). Organic Field-Effect Transistors. Advanced Materials. 10

(5), p365-377.

2. Jaiswal M and Menon R. (2006). Polymer electronic materials: a review of

charge transport. Society of Chemical Industry. 55, p1371-1384.

3. Burghard M, Klauk H and Kern K. (2009). Carbon-Based Field-Effect

Transistors for Nanoelectronics. Advanced Materials. 21, p2586-2600.

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4. Moliton A and Hiorns R. (2013). The origin and development of (plastic) organic

electronics. Society of Chemical Industry. 61, p337-341.

5. Sirringhaus H. (2014). Organic Field-Effect Transistors: The Path Beyond

Amorphous Silicon. Advanced Materials. 26, p1319-1334.