EEE21: Novel Gallium Nitride based sensor for air pollution - - PowerPoint PPT Presentation

Student: Suresh Rao Subhash Nanyang Junior College Professor: Dr. K. Radhakrishnan School of Electrical Engineering

EEE21: Novel Gallium Nitride based sensor for air pollution monitoring

Background

As the usage of the fossil fuels continues to rise, the world’s cities are becoming more polluted by air.

The World Health Organisation estimates that 4.2 million people die as a result of ambient air pollution each year.

The WHO has a limitation for Nitrogen Dioxide of 20 ppb

Dangers of NO2 Gas

Catalyses formation of acid rain Forms photochemical smog

Damages the Respiratory system of a Biological body and the Environment!

Applications of Gas Sensors

Gas Leak in Pipelines and in Laboratories Vehicle exhaust systems Industrial exhaust

Qualities of a preferable gas sensor

❏ High Sensitivity ❏ Selectivity ❏ Fast Response time ❏ Fast recovery time ❏ Lifetime ❏ Energy consumption

Existing Metal Oxide sensors vs AlGaN/GaN HEMT sensors

Compared to the existing metal oxide sensors, AlGaN/GaN HEMT based sensors possess

• 1. Higher electron mobility
• 2. Wide operating temperature range
• 3. Good chemical inertness and mechanical stability
• 4. Longer life expectancy

Aim of the project

• 1. Characterise the AlGaN/GaN HEMT based sensors for NO2 gas sensing, by

studying the effect of humidity, temperature and gas concentrations on sensitivity, response time and recovery time.

• 1. To understand the underlying sensing mechanism.

Ohm’s Law - The principle in the project

Ohm’s law states that the potential difference across an ideal conductor is proportional to the current through it. It is given by: V = IR , where V is the potential difference between two points which include a resistance R.

Mechanism

• AlGaN/GaN heterostructure is formed when a wide band gap AlGaN barrier is grown coherently
• n the top of narrow band gap GaN channel.
• AlGaN grown layer on GaN buffer induces a polarization effect resulting in a positive charge on

the AlGaN/GaN interface and a negative charge on the top of the AlGaN layer. T

• his differently charged areas result in forming an electric field inside the AlGaN layer. This makes

the energy band, including the Fermi level, tilt towards the AlGaN/GaN interface.

• As the fermi level of GaN is lower than AlGaN, accumulated electrons in the lower AlGaN layer

will flow to the top of GaN layer forming two dimensional electron gas (2DEG) .This process will cease when the Fermi levels of both materials will align with each other.

Mechanism

• The close proximity of 2DEG to the surface and high carrier density makes the device extremely

sensitive to the surface charge.

• Platinum is used as a catalyst in AlGaN/GaN based sensor.
• When a gas compound diffuses to the interface, it results in a drain current response. The NO2

gas is adsorbed on the platinum layer.

• Dissociation reaction occurs, causing the formation of the O2
• ions, which diffuses into grain

boundaries and are adsorbed at the adsorption sites at the AlGaN/GaN surface.

• These adsorption sites are electrically charged due to the Galium or Nitrogen vacancies in the

AlGaN/GaN structure[3] and in turn affects the 2DEG density.

NO2 gas sensing at room temperature

• The device was able to

detect and react to the NO2 gas exposure.

• The resistance of the

device was not able to recover to baseline value completely, when the NO2 gas supply was at 0 ppm concentration.

• The sensor was able to

recover after heating it up to 300℃ and holding for a few minutes before cooling it down.

NO2 gas sensing at 300oC

• The sensor is able to recover

completely for each concentration of the gas.

• The response time and the

recovery time decreased with increasing temperatures, when exposed to differing concentrations of NO2 .

• Measurable resistance

change was observed even at a lower concentrations of NO2 (0.5 ppm).

• Sensitivity of 5.35% was

achieved for 10 ppm NO2 concentration at 300℃.

Relative Humidity

NO2 gas at 40% humidity NO2 gas at 60% humidity

• The sensitivity of the sensor to the gas is higher at 60% relative humidity than the sensitivity at 40%

relative humidity.

• The higher the humidity, the higher the sensitivity of the sensor to the gas.
• This, along with periodic heating, encourages the recovery of the sensor from NO2 exposure.
• Furthermore, it prevents the baseline changes due to the change in the humidity level.

Results analysis

Conclusion

• In this project, it is experimentally verified that the presence of high temperature and

humidity plays a major role in the proper and efficient functioning of the device.

• Moreover, The sensitivity of the fabricated sensor is 5.35% for 10 ppm concentration at

300℃.

• At 300oC, the response time of the sensor is less than 2.5 minutes for the lowest

concentration of 0.5 ppm.

• Compared to existing metal oxide sensors, which has a recovery time of around one

hour, the device uses much lesser time to recover, at just around 4 minutes.

• Hence, AlGaN/GaN HEMT has a good prospect for various applications as NO2 gas

sensor.

Thank you!