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AlGaN/GaN heterostructure cell-based biosensor

June 2011

The Microelectronic Research Group (MRG) at the ANFF WA Node has investigated the ion sensitivity of ungated, uncapped AlGaN/GaN heterostructure-based devices.

Sensors based on AlGaN/GaN High Electron Mobility Transistor (HEMT) structures have high gate charge sensitivity, stability in harsh environments and are believed to have low toxicity. The conducting channel of an AlGaN/GaN HEMT consists of a two dimensional electron gas (see Fig. 1), the density of which is modulated by changes in the surface potential of the HEMT. Thus, transistor amplification can be achieved by directly sensing charged particles adsorbed onto the exposed gate area. A four contact bar structure device (see Fig. 2b) was fabricated to measure voltage between two points while applying constant current along the bar as function of pH. These measurements revealed a bath-tub like dependence of sheet resistance with pH, indicating the sheet resistance increased as a function of ionic concentration, regardless of whether the pH was acidic or basic, and showing that the uncapped AlGaN/GaN heterostructures are demonstrating selectivity towards negative ions over positive ions.

Figure 1 Figure 2
Figure 1. AlGaN/GaN band structure diagram Figure 2. Photograph of ungated AlGaN/GaN heterostructure-based
devices (a) and measurement configuration for Kelvin probe measurements (b)

Such selectivity towards negative ions can therefore be employed in cell-based bio-sensor applications via the detection of negative ion transport through the cell membrane/ion channels. Living cells are extremely organised microstructures containing a high concentration of chemicals, including enzymes, nucleic acids, ions, many types of proteins and small organic molecules. They process multiple incoming signals by means of parallel activation of different pathways and respond with a reaction pattern according to the type of stimulus, physical or chemical. In spite of the many difficulties and complications involved in using whole cells, including their limited lifespan, the most important reason for developing cell-based biosensors is that only by using living components capable of a direct response to incoming information can the effect of an external physical or chemical stimulus on living system be investigated.

Compatibility of living cells and AlGaN/GaN heterostructures for this application has been investigated through a number of qualitative optical observations, scanning electron microscopy (SEM) and focused ion beam (FIB) technique. The image of the semi-conductor/cell interface (see Fig. 3) achieved by FIB/SEM demonstrates the cell attachment to AlGaN/GaN surface.

Figure 3
Figure 3. FIB/SEM cross-section of chemically fixed HEK-293 cell on AlGaN/GaN 

Investigation of this interface is crucial for understanding the cell-based biosensor performance and future optimisation. MRG researchers have also performed quanti-tative flow cytometry experiments to assess bio-compatibility of cells and AlGaN/GaN wafers with different Al mole fractions. The viability of cells on the AlGaN surface has been seen to be comparable for different Al mole fractions. These provide flexibility in design and optimisation of the AlGaN/GaN hetero-structure for specific ion detection.

The four contacts bar structure device used in the pH sensitivity measurements was packaged in a special cell-compatible way and live cells were seeded on its active area. This device monitored the cells growth and proliferation inside the cell-culture incubator for more than 15 hours. Response to the added drug of 1.25mV was detected. Other signals obtained from this measurement indicate different stages of cell-life activity, however further data processing is needed to extract more information.

To conclude our story we would like to highlight the following:

Article by Anna Podolska, PhD student - School of Electrical, Electronic & Computer Engineering, The University of Western Australia