Biomedical Engineering Reference
In-Depth Information
I DSat D Wq i .0/ v thd D Wq i .0/Πv thd F 1=2 ./=F 0 ./;
(2.19)
where v thd D .2E th =m/ 1=2 is the unidirectional thermal velocity, F 1=2 .&/ is the
Fermi-Dirac integral, and D .E F E S 0 /=E th , with E S 0 the energy of the first
electron subband at the source.
The AlGaN/GaN HEMT was extensively used for various sensing applications
( Kang et al. 2008 ). The 2DEG is induced and can be tuned by piezoelectric
polarization of the strained AlGaN layer and the spontaneous polarization between
AlGaN and GaN. Sensing applications are favored in this HEMT transistor since
the 2DEG is located in the vicinity of the surface, and its density is tunable due
to polarization, being thus very sensitive to ambient conditions around the surface.
A gateless AlGaN/GaN HEMT is used as sensor by exposing its gate area to various
analytes. The HEMT biosensor is represented in Fig. 2.8 .
The HEMT configuration in Fig. 2.8 is used as a pH sensor, various cover layers
deposited above the gate area improving the sensitivity of the biosensor. DNA
detection and its hybridization were achieved also in such HEMT transistors ( Kang
et al. 2006 ). In this case, the HEMT structure consisted of a 3-m-thick GaN
buffer, a 3-nm-thick Al 0:3 Ga 0:7 N spacer layer, and a 22-nm-thick cap layer from
Si-doped Al 0:3 Ga 0:7 N. The Ti/Al/Pt/Au drain and source contacts have a gap of
4m between them, as shown in Fig. 2.9 . In the gap, 5 nm of gold, which allows the
immobilization of thiol-modified ssDNA, was deposited as gate metal. The drain
and source electrodes were then covered with a polymer, and only the gate region
was intentionally opened to allow the liquid, which contains the DNA solution,
to flow through the gap. DNA hybridization is detected by measurements of the
variation of the drain-source current when the transistor is exposed to matched
and mismatched DNA. In the former case, the drain current decreases in time until
the hybridization process is completed and remains constant afterward, the current
difference compared to the mismatched DNA case reaching 115A. This value is
D
S
gate exposed area
AlGaN
GaN
Si substrate
Fig. 2.8
The HEMT biosensor
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