Venugopala Rao

Dept of CSE

SSE, Mukka

Electronic Circuits10CS32

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Photodiode types:There are various types of photodiodes depending on the

construction.PN PhotodiodesPIN PhotodiodesSchottky type Photodiodes Avalanche Photodiodes.

PN PhotodiodesComprises of PN junction as shown

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When light with sufficient energy is made to fall on diode, photoinduced carriers are generated.

These include electrons in the conduction band of P - type material and holes in the valence band of the N-type material.

When photodiode is reverse biased, photoinduced electrons move downwards from P-type material to N-type material.

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Similar action will take place w.r.t. holes and they move from N-side to P-side

Shorter wavelength signals are absorbed at the surface itself an longer wavelength signals penetrate into the diode.

Applications:Used in medical instrumentation, analytical instrumentation etc

PIN diode:Here additional high resistance layer is added between P and N

layers as shown

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This coat reduces the diffusion time and hence increases the response time.

Low capacitance being major feature of PIN photodiodes, it offers large bandwidth. Hence it is useful for high speed photometry and optical communication applications.

Schottky diode:In these, there is a thin gold coating on the N-type material to ensure

the Schottky effect. This diode has better performance under UV radiation.

Avalanche Photodiode:These are high speed high sensitivity photodiodes which makes use

of internal gain mechanism by applying higher reverse voltages as compared to PIN diodes.

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APDs are constructed with uniform junction which exhibits avalanche effect at about 30V to 200V.

Electron-hole pair generated due to incident light are accelerated applied electric field. This ensures that, new electrons are moved to conduction band.

Thereby current increases rapidly.APD is also fast responsive similar to PIN photodiode.Offer higher SNR and hence used in sensitive applications like long

distance OFC communication

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Circuit symbol and V-I Characteristics:

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Here curve 1 shows the graph of dark state. This is similar to conventional diode.

When light falls on diode, curve shifts downwards as shown.

If the terminals of the photodiode are shorted, a photocurrent proportional to light intensity will flow from anode to cathode.

If the circuit is open, open circuit +ve voltage is seen at the anode.

From the graph it is clear that, ISC is linear w.r.t light intensity and VOC is logarithmically varying w.r.t light intensity.

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Two operating modes:Photovoltaic mode Photoconductive mode

In photovoltaic mode, no bias voltage is applied, due to the incident light a forward voltage is produced across the diode.

In photoconductive mode, reverse bias voltage is applied across the diode.

This widens the depletion region and hence speeds up the response time.

For applications with bandwidth less than 10kHz, photovoltaic mode is used and for all other applications photoconductive mode is used.

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Application circuit:In photovoltaic mode, photodiodes are used for low speed

applications or for detecting low light intensities.Following are the two application circuits for this operation

The output voltages of these are given by Idet X R and Idet X R

where Idet is the current through diode. The second circuit offers better linearity than circuit (a). This is due

to the fact that, for better linearity, is to be as small as possible across photodioe. (Rf/ A)

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Following are the circuits for PD working in photoconductive mode.

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Output voltage is obtained in the same manner as discussed earlier.

APDs are also connected in the same manner except that they need higher reverse voltage.

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