Light Detectors

LIGHT DETECTORS

For processing the light signal at the receiver end of the fibre link we require a device to convert the light signals to electrical wave forms. This task is done by the photo-detectors.

Definition

It is a device which converts light signal into electrical wave forms.

Types of photo-detectors

Photo-detectors are of three types:

(i) Photo emissive

(ii) Photo conductive

(iii) Photo voltaic

Photo-Emissive Photo-Detector

The emission of electrons from a photo cathode by the incident photon is called photo-emission.

Examples of such devices

(a) Photo-tubes

(b) Photo-multiplier tubes

The size of these is normally very large and hence not suitable for use as fibre optic detectors.

Photo-Conductive Devices

These types of devices have variation of resistance due to incident light on the photo-conductive materials.

Example of photo-conductive materials

Materials like CdS,

Intrinsic semiconductor materials like PIS, PhTe

Extrinsic semiconductor like doped Ge and Si

They are not suitable for use in fibre optic communication purposes since they have low frequency response.

Photo-Voltaic Devices

Semiconductor junction photo diodes are called as photo-voltaic devices

They are almost ideal for fibre systems.

We will study three forms of these devices.

1. PN junction photo detector

2. PIN photo diode

3. Avalanche photo diode (APD)

PN junction photo diode as in fig.4.12 explains the basic detection mechanism of a junction detector.

When reverse biased, the potential energy barrier between the p and n regions increases. Free electrons (which normally reside in the n region) and free holes (normally in the p region) cannot climb the barrier, so no current flows.

The junction refers to the region where the barrier exists. Because there are no free charges in the junction, it is called the depletion region.

Figure 4.12 shows an incident photon being absorbed in the junction after passing through the p layer. The absorbed energy raises a bound electron across the bandgap.

Quantum efficiency

The quantum efficiency η is defined as the number of electron-bole pairs generated by the number of incident photons, i.e.,

where, i = photo current developed

e = charge on electron

P = incident optical power

hv = energy of incident photon.

PIN Photo Diode

The frequency response can be improved if the pn junction is separated by an intrinsic region.

The introduction of the intrinsic region decreases the junction capacitance. This is called Positive Intrinsic Negative (PIN) photo diode. (fig 4.13).

A PIN diode has an intrinsic semiconductor at the centre and p-type and n-type regions at the end as shown in figure 4.14. It is reverse biased (5 - 20 V). The reverse biasing is used to attract the charge carriers from the intrinsic regions.

When light is incident on the PIN diode, the intrinsic region receives more amount of light because of its large size. The photons incident on the intrinsic region produces electron-hole pair.

The electron is raised from the valence band to the conduction band, leaving the hole. The electrons are attracted by the reverse biasing and hence move away from the junction. The movement of electrons in the conduction band creates the flow of charge and hence the light energy gets converted into electrical energy.

Avalanche Photo-Diodes (APD)

Fig 4.14 explains the working of Avalanche photo-diode. It is much more sensitive than PN or PIN diodes.

The avalanche photodiode is based on the principle of avalanche multiplication of the current. It consists of heavily doped p+ and n+ regions.

The depletion region is lightly doped, almost intrinsic. The diode is reverse biased using 50-300 V. The light is made to incident on the depletion region. The incident light produces electron and hole pair.

The electrons move towards the p region. Due to the strong reverse biasing, there is a depletion of charge carriers in the p region. The electrons in the p region undergo avalanche multiplication because of high reverse bias.

The holes move towards the p+ regions without producing further multiplication.

The avalanche photodiode has better noise performance, because the carrier multiplication is limited to electrons only.

Photo-Transistor

Photo-Transistor is another type of photo detector.

A transistor photo diode with its characteristic curves is shown in fig. 4.15.