Direct Energy, 2018a

6 PHOTOVOLTAICS 133
made from gas lled vacuum tubes, and these detectors are called phototubes
[10, p. 646]. A voltage is placed across electrodes in the tubes.
When light shines on the phototube, energy from a photon of light can rip
o an electron from a gas atom. The electron and ion ow towards the
electrodes, thereby producing electricity. The most common type of phototube
is the photomultiplier tube. This device has multiple electrodes, and
when an electron hits one of these electrodes, additional electrons are emitted.
These electrons can hit additional electrodes to produce even more
electrons. Because eachincoming photon produces a cascade of electrons,
photomultiplier tubes have high internal amplication.
Another way to classify photodetectors depends on whether incoming
photons have enough energy to rip o electrons or just excite them. The
rst type of detectors are called photoelectric detectors, and they operate
based on a process called photoelectric emission [10, p. 645] [27, p. 171].
In these detectors, incoming light has energy greater than or equal to the
energy from the valence band to the ground level at the top of an energy
level diagram. These detectors convert light to electricity because incoming
photons of light rip electrons o their atoms, and the ow of the resulting
electrons is a current. The second type of detectors are called photoconductive
detectors or sometimes photovoltaic detectors, and they operate based
on a process called photoconductivity [10, p. 647]. In these detectors, incoming
light has energy equal to the dierence between the valence and
conduction bands, not enoughto rip o electrons. These detectors convert
light to electricity because incoming photons excite electrons, and the
conductivity of the detector is higher when light shines on it. Solid semiconductor
photodetectors can operate based on either photoelectric emission
or photoconductivity, but most operate based on photoconductivity.
Phototubes typically operate based on photoelectric emission.
Some photodetectors have a single element while others are made from
an array of elements. A digital camera may contain millions of individual
photodetectors. These elements are integrated with a charge-coupled device
(CCD), which is circuitry to sequentially transfer the electrical output of
each photodetector of the array [9, p. 359]. The CCD was invented in 1969
by Willard S. Boyle and George E. Smith. For this invention, they shared
the 2009 Physics Nobel Prize with Charles K. Kao, who was awarded the
prize for his work on optical bers [80].
Eyes in animals are photodetectors. The retina of the human eye is an
array composed of around 120 million rod cells and 6 to 7 million cone cells
[81]. These cells convert light to electrical impulses which are sent to the
brain.