Page 222 - Organic Electronics in Sensors and Biotechnology

An Intr oduction to Or ganic Photodetectors 199

although the costs of these devices tend to be much higher than
those based on CdS. †
Just as all diodes are photoactive to some extent, so too are all
2
transistors. Phototransistors operate in a similar way to photodiodes
except, due to their transistor operation, they exhibit current gains of
up to a few thousand. (Note that this is a conventional amplification
effect as opposed to the low-noise internal gain exhibited by PMTs
and APDs.) In operation, a voltage is applied between the collector
and the emitter, and when the transistor is illuminated, a photocur-
rent flows between the base and the collector, which in turn causes an
amplified current to flow between the collector and the emitter. Pho-
todarlingtons are closely related devices, comprising a photoactive
input transistor followed by a secondary transistor, which can pro-
vide increased levels of current gain upward of several hundred
thousand. When phototransistors and photodarlingtons are used in
conjunction with suitable load resistors, their built-in gain allows
them to drive TTL and CMOS logic gates directly, which simplifies
circuitry considerably. However, the need for an applied bias between
the base and the collector gives rise to a constant (dark) current even
in the absence of light. This is usually several nanoamperes and sets
an approximate lower limit for the detectable photocurrent. As well
as having higher dark currents, phototransistors tend to exhibit
slower response speeds and worse linearity than photodiodes, and
suffer from significant device-to-device variability. In virtually all
cases, superior performance is achieved by separating the detection
and gain stages, using a photodiode for the former and a discrete
amplifier for the latter (although this clearly entails some additional
cost and circuit complexity).
The last kind of photodetector in common use is the charge-
coupled device (CCD) (Fig. 6.5) which forms the basis of virtually all
2
digital cameras and image sensors. The term CCD formally refers to
the specific architecture that is used to shuttle charges around the
sensor array rather than the individual photosensors themselves.
CCDs were originally conceived in the 1960s as a new kind of
memory circuit, but were soon applied to other tasks such as signal
processing and imaging. Today they are no longer of interest as mem-
ory elements (having been surpassed by a variety of superior tech-
nologies), but they have evolved into the current solution of choice
for imaging applications. In essence an imaging CCD is an array of
metal-insulator-semiconductor photocapacitors, which collect and
maneuver photogenerated charges. The CCD is usually ‘‘built’’ on a
grounded p-type silicon substrate (the semiconductor) with a surface

† Note, many countries are phasing out the heavy metals commonly used in
photoresistors for environmental reasons.