Sensing of Biomolecules - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 63

drop of DNA
solution

polymer passivation
of the electrodes

D

S

SiO 2

I

Fig. 2.5 FET label-free sensor of DNA

dependence can be described by

G Š ˛ C ˇm; (2.15)

where ˛ is 307 nS and ˇ is 30 pS. The detection limit of the CA15.3 biomarker
is 1.7 nS, corresponding to 60 pM. So, the label-free FET nanowire biosensor is
an extremely sensitive technique, able to sense cancer biomarkers in very low
concentrations.
Many biomolecules, such as DNA or proteins, and biological key parameters
such as pH, gas concentration, etc., were sensed using high-electron-mobility
(HEMT) transistors ( Kang et al. 2008 ; Pearton et al. 2010 ).
The HEMT is a FET transistor with a channel consisting of a 2DEG generated
at the interface between a doped wider-bandgap semiconductor and an undoped
narrower-bandgap semiconductor that form a heterostructure. Technologies devel-
oped for AIII-BV semiconductors or nitrides can create a HEMT channel at
the interfaces. n-AlGaAs/GaAs and AlGaN/GaN are typical examples of such
heterostructures. The HEMT working principle is based on the fact that the Fermi
level in the narrower-bandgap semiconductor is lower than the Fermi energy in the
wider-bandgap semiconductor, so that the electrons are transported from the doped
toward the undoped semiconductor, leaving positively charged donors in the doped
semiconductor. As a result, the energy bands of the heterostructure bend and trap
the electrons in the quantum well that develops at the interface (see Fig. 2.6 a). The
spatial separation of negative and positive charges at the interface, as well as the fact
that the Fermi energy in the well is higher than the first resonant level, generates an
electron gas in the quantum well. More precisely, the electrons are free to move in
the well, with a low rate of scattering processes due to the separation of the well
from donor impurities. In this 2DEG, the electrons behave ballistically, reaching
very high mobilities.