Nanotechnology-Enabled Sensors

108 Chapter 3: Transduction Platforms
can amplify a current internally through its base (Fig. 3.30) owing to its internal
current gain, according to:
I = βI
. (3.77)
collector
where β is the current gain. Irradiation impinges directly at the p-n junctions.
One of the junctions is reversed biased which generates a large current
from the changes of the forward biased p-n junction current. Owing to
recent developments in fabrication technology, electrical noise in such
transducers has been dramatically reduced.
Impinging light
p
n
Fig. 3.30 A schematic of a phototransistor.
base
3.5.2 Schottky Diode based Transducers
p
Emitter
Base
Collector
A diode constructed from a metal-semiconductor junction is called
Schottky diode. The metal-semiconductor junction forms a rectifying barrier
that only allows current to flow in one direction. As shown in Fig.
3.31, for an Schottky diode of n-type semiconductor, the barrier height, φb,
depends on the difference between the work function of the metal, φm, and
electron affinity of the semiconductor, χs. 20 In most cases, the barrier
height is controlled by the density of surface states located in the metalsemiconductor
interface. 23