handbook of modern sensors

14.2 Photodiodes 413
when i = 0), we will observe that with increasing optical power, the voltage changes
in a quite nonlinear fashion. In fact, variations are logarithmic. For the short-circuit
conditions (V = 0), [i.e., when the diode is connected to a current-to-voltage converter
(Fig. 5.10B of Chapter 5)], current varies linearly with the optical power. The currentto-voltage
response of the photodiode is given by [3]
i = i 0 (e eV/k bT − 1) − i s , (14.9)
where i 0 is a reverse “dark current” which is attributed to the thermal generation of
electron–hole pairs, i s is the current due to the detected optical signal, k b is Boltzmann
constant, and T is the absolute temperature. Combining Eqs. (14.8) and (14.9) yields
i = i 0 (e eV/k bT − 1) − ηeP
hv , (14.10)
which is the overall characteristic of a photodiode. An efficiency of the direct conversion
of optical power into electric power is quite low. Typically, it is in the range
5–10%; however, in 1992, it was reported that some experimental photocells were
able to reach an efficiency as high as 25%. In sensor technologies, however, photocells
are generally not used. Instead, an additional high-resistivity intrinsic layer is
present between p and n types of the material, which is called a PIN photodiode (Fig.
14.5). The depth to which a photon can penetrate a photodiode is a function of its
wavelength which is reflected in a spectral response of a sensor (Fig. 14.2).
In addition to very popular PIN diodes, several other types of photodiode are
used for sensing light. In general, depending on the function and construction, all
photodiodes may be classified as follows:
1. The PN photodiodes may include a SiO 2 layer on the outer surface (Fig. 14.6A).
This yields a low-level dark current. To fabricate a high-speed version of the
diode, the depletion layer is increased, thus reducing the junction capacitance
(Fig. 14.6B). To make the diode more sensitive to ultraviolet (UV) light, a p
layer can be made extra thin. A version of the planar diffusion type is a pnn +
diode (Fig. 14.6C), which has a lower sensitivity to infrared and higher sensitivity
at shorter wavelengths. This is due primarily to a thick layer of a low-resistance
n + silicon to bring the nn + boundary closer to the depletion layer.
Fig. 14.5. Structure of a PIN photodiode connected to a current-to-voltage converter.