A PhD proposal to fabricate AlGaN/GaN transistor-based chemical sensors in high temperature and pressure applications at the University of Western Australia.

1. AlGaN/GaN Chemical Sensors in
High Temperature and Pressure
Applications
Jeremy Gillbanks1
Supervised by: Giacinta Parish1, Brett Nener1,
and Matt Myers2
1Electrical, Electronic and Computer Engineering, University of Western Australia
2Commonwealth Scientific and Industrial Research Organisation

2. Chemical Sensor Technology
• Current applications
– Recycled water monitoring
– Industrial process monitoring
– Medical diagnostics
[1]

3. Previous Findings at UWA
No reference electrodes or solutions required
• pH sensitive (unfunctionalised)
• Calcium ions (functionalised)
• Nitrate ions (functionalised)
• Mercury ions (functionalised)
Sensor array design [2]

4. No Chemical Testing at
High Temperatures or Pressures
Potential Applications
• Oil and gas exploration
– Well characterisation (pH)
– In situ measurements
• Chemical process
monitoring
– Haber process
– Industrial chemical
processes above 30˚C
• Space
Offshore oil platform [3]

5. Drilling, Exploration, Characterisation
Current in situ sensing
• Glass electrodes
– Large, cumbersome
– Reference electrode issues
– Fragile
– Temperature and pressure
issues
• Optical dyes
– Requires light source
• Removal from well
– Irreversible change to
sample:
temperature and pressure
change
Oil well characterisation [4]

6. Why GaN?
• Chemically stable
• Low cost
– Utilises common
MOSFET fabrication
methods
• No reference electrode
• Temperature
dependent
• Piezoelectric
polarisation for
pressure
measurements2DEG
AlGaN/GaN device structure
AlGaN/GaN band structure

7. Aims (1/2)
1. Characterize the effect of temperature and
pressure on the response of AlGaN/GaN
chemical sensors.
Challenges:
• Get reliable and repeatable results with liquid in
situ testing.
• Low cost high temperature adhesives and
electrical connectors required for sensor
packaging

8. Aims (2/2)
2. Decouple these responses from the chemical
sensor and incorporate into a chemical
sensor array.
Challenge: Develop field testing capabilities.

9. Methodology (1/2)
1. Create new masks with temperature
decoupling capabilities
1. Resistance temperature detector (Pt-1000)
2. Thermocouple (Ni-Cr)
3. Schottky diode

10. Methodology (2/2)
2. Change passivation from SU-8 to SiNx.
3. Conduct temperature and pressure testing
and characterise the responses.
4. Create a sensor array to measure pH,
temperature, and pressure simultaneously.
5. Use commercially available low-cost (< $100)
data loggers for field-ready testing.

11. Preliminary results in air
• Temperature testing (1/2)
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120 140 160
Voltage(V)
Temperature (˚C)
Response

12. Preliminary results in air
• Temperature testing (2/2)

13. Preliminary results in solution
Challenge: Reduce sensor noise and drift.
0
50
100
150
200
250
0.00 2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00 16000.00 18000.00 20000.00
Response(mV)
Time (s)
GaN pH testing
6 45 3 2

14. Constant-voltage vs constant current
• Other workers (Jia, et al. 2016) used constant
voltage rather than constant current
• Challenges:
– Current mask designs assume constant current.
– What is the best way to create a constant voltage
sensor array?
– How can the array be kept in darkness with an
operator changing solution concentrations?

15. Data loggers
NIST Compact DAQ [5]
(ex. Laptop)
> $1000
Raspberry Pi [6]
(incl. peripherals)
< $100

16. New mask design
Thermocouple
Pt-1000 RTD
Sensor (3 mm x 1.5 mm)
Schottky diode

17. Summary
1. Create and characterise chip-based GaN-
independent temperature measurement
2. Characterise piezoelectric effect on GaN
structure
3. Demonstrate sensor array
4. Use low-cost data loggers

18. Picture Credits
1. Carl E. Thodal, N. (2018). USGS Fact Sheet 2009–3093: Monitoring for Pesticides in Groundwater and Surface Water in
Nevada, 2008. [online] Pubs.usgs.gov. Available at: https://pubs.usgs.gov/fs/2009/3093/ [Accessed 9 Apr. 2018].
2. Asadnia, M., et al., Ca2+ detection utilising AlGaN/GaN transistors with ion-selective polymer membranes. Analytica
Chimica Acta, 2017. 987: p. 105-110.
3. ‘South_Asian_Summit_on_Gas_Trading_and_Transportation’, by Arpna Sharma,
https://upload.wikimedia.org/wikipedia/commons/3/36/South_Asian_Summit_on_Gas_Trading_and_Transportation.jpg
. Licence at http://creativecommons.org/licenses/by/2.0
4. ‘Salt dome trap’, by MagentaGreen,
https://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Salt_dome_trap.svg/1280px-Salt_dome_trap.svg.png.
Licence at http://creativecommons.org/licenses/by/2.0
5. ‘RTD Data logger’, by National Instruments, http://sine.ni.com/images/products/us/usb-9219_withcable_l.jpg
6. ‘Raspberry Pi’, by Onepiece84, https://commons.wikimedia.org/wiki/File:Raspberry_PI.jpeg. Licence at
http://creativecommons.org/licenses/by/2.0

19. Phosphate testing (Jia et al. 2016)