4.2. COMMON PHOTODETECTORS 137
Figure 4.3: (a) Ap–nphotodiode under reverse bias; (b) variation of optical power inside the
photodiode; (c) energy-band diagram showing carrier movement through drift and diffusion.
4.2.1 p–nPhotodiodes
A reverse-biasedp–njunction consists of a region, known as thedepletion region, that
is essentially devoid of free charge carriers and where a large built-in electric field
opposes flow of electrons from then-side to thep-side (and of holes frompton).
When such ap–njunction is illuminated with light on one side, say thep-side (see Fig.
4.3), electron–hole pairs are created through absorption. Because of the large built-in
electric field, electrons and holes generated inside the depletion region accelerate in
opposite directions and drift to then- andp-sides, respectively. The resulting flow of
current is proportional to the incident optical power. Thus a reverse-biasedp–njunction
acts as a photodetector and is referred to as thep–nphotodiode.
Figure 4.3(a) shows the structure of ap–nphotodiode. As shown in Fig. 4.3(b),
optical power decreases exponentially as the incident light is absorbed inside the de-
pletion region. The electron–hole pairs generated inside the depletion region experi-
ence a large electric field and drift rapidly toward thep-orn-side, depending on the
electric charge [Fig. 4.3(c)]. The resulting current flow constitutes the photodiode re-
sponse to the incident optical power in accordance with Eq. (4.1.1). The responsivity
of a photodiode is quite high (R∼1 A/W) because of a high quantum efficiency.
The bandwidth of ap–nphotodiode is often limited by the transit timeτtrin Eq.
(4.1.9). IfWis the width of the depletion region andvdis the drift velocity, the transit
time is given by
τtr=W/vd. (4.2.1)Typically,W∼ 10 μm,vd∼ 105 m/s, andτtr∼100 ps. BothWandvdcan be opti-
mized to minimizeτtr. The depletion-layer width depends on the acceptor and donor
concentrations and can be controlled through them. The velocityvddepends on the
applied voltage but attains a maximum value (called thesaturation velocity)∼ 105 m/s
that depends on the material used for the photodiode. TheRCtime constantτRCcan be