Biophotonics_Concepts_to_Applications

Example 5.2A high-speed InGaAs pin photodetector is made with a

0.15-μm thick depletion region. What percent of incident photons are

absorbed in this photodetector at 1310 nm if the absorption coefficient is

1.5μm−^1 at this wavelength?

Solution: From Eq. (4.1), the optical power level at x = 0.15μm relative to

the incident power level is

Pð 0 : 15 Þ

Pin

¼expðÞ¼asx exp½ð 1 : 5 Þ 0 : 15 Š¼ 0 : 80

Therefore only 20 % of the incident photons are absorbed.

When the energy of an incident photon is greater than or equal to the bandgap
energy Egof the semiconductor material, the photon can give up its energy and
excite an electron from the valence band to the conduction band. This absorption
process generates mobile electron–hole pairs, as Fig.5.2shows. These electrons
and holes are known asphotocarriers, since they are photon-generated charge
carriers that are available to produce a currentflow when a bias voltage is applied
across the device. The concentration level of impurity elements added to the
material controls the number of charge carriers. Normally these carriers are gen-
erated mainly in the depletion region where most of the incident light is absorbed.

Intrinsic region

Photo-

current

Penetration

depth

Exponential power

decay in the

intrinsic region

Very thin p region

n region

Fig. 5.1 TopRepresentation of apinphotodiode circuit with an applied reverse bias;BottomAn
incident optical power level decays exponentially inside the device

5.1 ThepinPhotodetector 121