Semiconductor Biological and Gas Sensors

There is significant interest in the development of new generations of semiconductor-based sensors that take advantage of microelectronic fabrication techniques to create robust sensor arrays that are integrated with wireless data transmission systems. Our work began with the development of GaN and ZnO-based hydrogen sensors for a NASA program and this led to the installation of hydrogen gas sensors based on Pt or Pd coated AlGaN/GaN transistor structures at an automobile dealership in Orlando, FL¹.

The sensors have been operational for almost two years, monitoring emissions from a fleet of hydrogen-fueled demonstration vehicles. The output from the sensors can be checked from remote locations and any alarm conditions sent to cell phones or personal digital assistants. The practical aspects of selectively detecting hydrogen in an atmosphere that contains other exhaust fumes and in which temperature is not controlled, required use of a differential pair of sensors, one which is activated for hydrogen detection while the other is sealed against gas adsorption but is subject to the same temperature variations in the facility. The differential output from the sensors is then able to cancel out changes in sensor current due to temperature changes².

The same AlGaN/GaN High Electron Mobility Transistor (HEMT) structures can be functionalized with selective chemical functional layers in the gate region to produce biomarker molecules. An example is shown in Figure 1 for the detection of botulinum toxin. In this case, the Au-coated gate area is functionalized with botulinum antibody on thioglycolic acid. Binding of botulinum molecules to the antibody produces changes in surface charge on the HEMT, leading to changes in channel current at fixed source-drain voltage.

Figure 1.Schematic of AlGaN/GaN HEMT sensor functionalized for detection of botulinum toxin. Only those toxin molecules will bind to the antibody, producing a sensor response.

This approach of using different functional layers to detect different biomarkers of interest has led to the detection of DNA, kidney injury molecules, prostate cancer-specific antigens, breast cancer markers, botulinum toxins, hydrogen gas, carbon dioxide, oxygen gas, mercury ions, and to measure the pH of exhaled breath condensates^3-7. A summary of detected species to date is shown in Table 1.The detection sensitivity depends on the species, but is usually in the micro-to nanogram per milliliter range.

Table 1. Summary of detected species to date

he HEMT-based sensors produce a fast (< 1 sec), unambiguous electrical signal that is free from some of the disadvantages of other sensor approaches, such as optical bleaching in fluorescence methods, the need for electrodes with electrochemical approaches or the slow turn-around and equipment-heavy requirements in most lab-based approaches. The HEMTs sensors are small (less than 100 square microns) and different sensors are readily integrated onto a single chip.

The HEMT sensors are promising candidates for handheld, programmable, single-chip sensors that are capable of wireless communication. Such sensors could revolutionize current clinical practice through real-time monitoring of patient health. For example, the sensor could be programmed in the doctor's office for a specific medical condition and then given to patients for use at home. Encrypted sensing results would then be transmitted directly back to the doctor to monitor the effectiveness of prescribed medicines, providing patients with better and more immediate health care.

Such devices may also reduce the number of unnecessary visits to the emergency room and the resulting cost to the national health system. Similar sensors would allow fast detection of toxins in the environment and greatly improve our ability to respond. Environmental safety monitoring would benefit from in-field deployable sensors.

While the sensors show excellent potential, there is still significant work to be done on testing of realistic samples, the robustness of sample collection, stability of some surface functional layers and the need for field trials.

References

1. X. Yu, C. Li, et al., "Wireless hydrogen sensor network using AlGaN/GaN high electron mobility transistor differential diode sensors", Sensors and Actuators B: Chemical, 135, 188 (2008).
2. H.T. Wang, T.J. Anderson, et al., "Robust Detection of Hydrogen Using Differential AlGaN/GaN HEMT Sensing Diodes", Appl. Phys.Lett.89, 242111(2006).
3. B. S. Kang, H. T. Wang, et al., "Prostate specific antigen detection using AlGaN/GaN high electron mobility transistors", Appl. Phys Lett. 91, pp. 112106, 2007.
4. B. S. Kang, H. T. Wang, et al., "Exhaled-breath detection using AlGaN/GaN high electron mobility transistors integrated with a Peltier element", Electrochem. Solid State Lett. 11, pp. J19, 2007.
5. H. T. Wang, B. S. Kang, et al., "Electrical detection of kidney injury molecule-1 with AlGaN/GaN high electron mobility transistors", Appl. Phys. Lett. 91, pp. 222101, 2007.
6. B. S. Kang, H. T. Wang, et al., "Enzymatic glucose detection using ZnO nanorods on the gate region of AlGaN/GaN high electron mobility transistors", Appl. Phys. Lett. 91, pp. 252103, 2007.
7. K. H. Chen, B. S. Kang, et al., "c-erB-2 sensing using AlGaN/GaN high electron mobility transistors for breast cancer detection", Appl. Phys. Lett. 92, pp. 192103, 2008.