Superior electronic, chemical and thermal properties of wide bandgap gallium nitride (GaN), compared to silicon, have resulted in much interest for developing high power and high frequency switching transistors as well as high performance chemical micro-sensors.
The initial material is grown by metal organic chemical vapor deposition (MOCVD) on silicon, sapphire or silicon carbide substrates. When a heterojunction of AlGaN/GaN is grown a high conductivity electron channel is formed at the interface known as two-dimensional electron gas (2DEG). The sheet carrier density of the channel is typically >1013 cm-2 and the electron mobility >1500 cm2/Vs. An epitaxial structure for device fabrication is shown in figure 1.
Figure 1. Epitaxial structure of AlGaN/GaN device
Utilizing this structure a high electron mobility transistor (HEMT) can be fabricated. A schematic of such device is shown in figure 2.
Figure 2. Schematic of AlGaN/GaN HEMT
The wide bandgap of GaN (3.4 eV) enables the devices to operate at much higher temperatures that are possible using Si (1.12 eV) based field effect devices. Chemical stability of GaN further allows for devices to operate in harsh environments that contain caustic, corrosive or acidic vapour. These properties make GaN a highly favourable material for field effect transistor (FET) type chemical, gas sensor development.
The AlGaN/GaN HEMT can be transformed into a gas sensors by selecting an appropriate gas sensitive layer to be used as the gate electrode of the transistor. Catalytically active metals such as Pt, Pd or Ir can be used for a wide range of gas sensors to detect e.g. H2, NH3, NO2. Figure 3 shows a schematic of HEMT based gas sensors with a catalytic metal gate and example fabricated device.
Figure 3. Schematic of Pt-HEMT gas sensors (a). Fabricated sensors (b).
This research is focused on the design fabrication and testing of gas sensors based on GaN HEMT transducer. We have investigated the impact of sensor design on the transducer performance. Example designs are shown in figure 4.
Figure 4. Examples of HEMT sensor designs
We have observed that device geometry has a large impact on gas sensing performance. Hydrogen was used as test gas in this work. Gas measurement results of devices with different geometries of gate electrode are shown in figure 5.
Figure 5. Dynamic response of Pt-HEMT sensor to H2 with different geometries
The response mechanism involves the interaction of the gas molecules with the catalytic gate electrode. Pt causes molecule dissociation into atoms which then diffuse into metal-semiconductor interface where they form a dipolar layer, causing a shift in the metal work function, which then causes variation of the drain current of the HEMT. The schematic of gas sensing mechanism is shown in figure 6.
Figure 6. The detection principal of Pt HEMT sensor. Left figure shows catalytic dissociation of H2 atoms followed by diffusion to the interface. Right figure shows the gate band diagram under air and H2 ambient.
Important application of chemical micro sensors is in the field of industrial safety. Continuous industrial growth results in rising levels of toxic gas e.g. CO, NOx, SO2, NH3 and H2S release into the atmosphere, hence emissions control regulations are imposed. Portable, low-cost gas sensors with high accuracy are necessary for on-site monitoring. Therefore we have investigated the use of Pt-HEMT sensors for detection of H2S, a colorless, flammable, toxic gas.
The devices showed excellent response to low ppm concentrations of H2S. The drain current variation with exposure to increasing gas concentration is shown in figure 7.
Figure 7. HEMT sensor response to H2S gas
With increasing operating temperature the sensor response characteristics are improved due to more rapid catalytic processes. Hence it is beneficial to operate the sensors at high temperature above 200°C as seen from sensor responses shown in figure 8.
Figure 8. Dynamic sensor response to H2S at various temperatures.
In summary, GaN material system is highly promising for developing new generation on miniaturized gas and chemical sensors capable of operation at high temperature and hash environments. These devices have advantages of higher sensitivity and lower detection limits beyond offered by Si based sensors. Applications of environmental monitoring, air quality, various industrial processes would benefit from continuous monitoring that would be possible by integrating GaN based sensor chips with electronic readout and wireless data transfer and storage in one integrated solution.