Nanotechnology-Enabled Sensors

3.5 Solid State Transducers 107
When p-type and n-type semiconductors are placed next to each other to
form a p-n junction, the majority carries in the p-type material (holes) diffuse
a certain distance into the n-type material, and similarly at the n-type
material, majority carries (electrons) diffuse into the p-type material. Once
this diffusion is balanced, a depletion region forms (Fig. 3.29). A potential
barrier then exists between the p- and n-doped materials, which must be
exceeded for the current to conduct. Devices having one p-n junction are
termed diodes, whilst those contained two, in the form of p-n-p or p-n-p
junctions, are referred to as BJTs. BJT’s have the added advantage of internal
current amplification as well.
Fig. 3.29 A Schematic reorientation of a p-n junction.
From the Shockley equation, the current flowing is given by: 20
qV / nkT
saturation e
( ) = ( −1)
I V I , (3.76)
where q is the electron charge, V is the voltage, n is the ideality factor, k is
Boltzmann’s constant, T is the temperature in Kelvin, and Isaturation is the
saturation current of the device..
Devices such as diodes and BJTs are commonly employed for monitoring
charge 21 and widely used in sensing electromagnetic irradiation. The
application of these devices in monitoring irradiation has already been described
in Chap. 2 in the section pertaining to photoconductive and photovoltaic
devices. They may also be employed, albeit less commonly, for
chemical and pressure sensing applications.
Diode and transistor based transducers are extensively used in irradiation
spectroscopy, 22 which will be described in further detail in Chap. 5. In
many such spectroscopic measurements, the amount of light to be monitored
may be very low. Using a photodiode in conjunction with an amplifier
may rectify such limitations. However, low irradiation intensity
produces small currents which are inherently difficult to amplify externally,
if electrical noise is to be avoided. In such situations, phototransistor
may be used as an alternative to external signal amplification. A transistor