Page 219 - Organic Electronics in Sensors and Biotechnology

196 Cha pte r S i x

Incident photons strike a photocathode, which then emits electrons into
the surrounding vacuum. The emitted electrons are directed by a focus-
ing electrode onto a series of electrodes known as dynodes that are held
at successively higher potentials. When the electrons strike the dyn-
odes, secondary emission occurs, causing the number of electrons (and
hence the current) to multiply rapidly. The cascade of dynodes gener-
ates a virtually noise-free gain that can exceed 1 million with a band-
width of more than 1 GHz. As a result, PMTs are an excellent choice for
measurements that must be made at high speed or in low light levels.
Photodiodes are solid-state devices that are normally based on
either p-n or p-i-n type architectures (Fig. 6.2). In a p-n junction, the
2
absorption of photons with energy greater than the semiconductor
band gap generates an electron in the conduction band and a hole in
the valence band. A high electric field exists in the depletion layer of
the p-n junction which drives the photogenerated electrons and holes
in opposite directions toward their respective electrodes, from where
they are then extracted into the external circuit in the form of a cur-
rent. The p-i-n devices behave in a similar way, except a lightly doped
layer of near-intrinsic semiconductor is “inserted” between the p- and
n-doped regions. This increases the width of the (high field) depletion
layer and so increases the photoactive thickness of the device, result-
ing in improved efficiency. In addition, since the electric potential is
dropped over a greater distance, the junction capacitance is reduced,
which, as we will see later, allows for faster performance.
In normal operation, p-n and p-i-n photodiodes generate at most
one electron-hole pair for every absorbed photon, but they can be made
to exhibit internal gain if they are operated at sufficiently high reverse
biases. In this mode of operation, the photogenerated electrons and
holes collide with atoms in the semiconductor crystal, generating

Depletion layer
p i n
p n
– – ––
– – –
E c
– –
+ + + + v E c
hv
+ + E v hv +
hv + + + E v
hv
(a) (b) (c)
FIGURE 6.2 (a) Typical Si photodiodes. (b) Band diagram of p-n type photodiode; the
main photoactive region is defi ned by the high fi eld depletion zone which drives
electrons and holes to the cathode and anode, respectively. (c) Band diagram of
p-i-n type photodiode; the central intrinsic layer increases the thickness of the high
fi eld region, resulting in an extended photoactive region and improved
photosensitivity. The increased thickness also reduces the capacitance of the
device, resulting in faster response. (Image (a) courtesy of Hamamatsu Photonics
KK., All Rights Reserved.)