Photodiode

Photodiode

• The photodiode is a semiconductor p‒n junction device whose region of operation is limited to the reverse biased region.

• Fig. 3.5.1 (a) shows the symbol of photodiode while Fig. 3.5.1 (b) shows the working principle of photodiode.

• The photodiode is connected in reverse biased condition. The depletion region width is large.

• Under normal condition, it carries small reverse current due to minority charge carriers.

• When light is incident through glass window on the p‒n junction, photons in the light bombard the p‒n junction and some energy is imparted to the valence electrons.

• Due to this, valence electrons are dislodged from the covalent bonds and become free electrons.

• Thus more electron‒hole pairs are generated,

• Thus total number of minority charge carriers increases and hence the reverse current increases.

1. Photodiode Characteristics

• The photodiode is designed such that it is sensitive to the light.

• When there is no light, the reverse biased photodiode carries a current which is very small and called by dark current. It is denoted as I_λ. It is purely due to thermally generated minority carriers.

• When light is allowed to fall on a p‒n junction through a small window, photons transfer energy to valence electrons to make them free. Hence reverse current increases. It is proportional to the light intensity.

• Fig. 3.5.2 shows the photodiode characteristics. Fig. 3.5.2 (a) shows the relation between reverse current and light intensity while Fig. 3.5.2 (b) shows relation between reverse voltage and reverse current at different light intensities.

• It can be seen that reverse current is not dependent on reverse voltage and totally depends on light intensity.

2. Use of Photodiode as Variable Resistance Device

• Consider a typical photodiode with dark current I_λ=20 μA at V_R= ‒2V

  Dark resistance = V_R/I_λ  = 2 / 20 μΑ = 100 kΩ

• If now photodiode is illuminated with 25000 Lm/m² (Lumens per square metres) then current changes to 350 μA at same reverse voltage.

        Illuminated resistance = 2 / 350 μΑ = 8.914 ΚΩ

• This shows that the photodiode can be used as a variable resistance device controlled by light intensity. It is also called photoconductive device.

• The response of photodiode is very fast hence change in resistance from high to low or otherwise is also very fast. Hence it can be used in variety of applications.

3. Why to be used in Reverse Biased?

• The reverse current without light in diode is in the range of μA. The change in this current due to the light is also in the range of μA. Thus such a change can be significantly observed in the reverse current.

• If the photodiode is forward biased, the current flowing through it is in mA. The applied forward biased voltage takes the control of the current instead of the light.

• The change in forward current due to light is negligible and can not be noticed.

• The resistance of forward biased diode is not affected by the light.

• Hence to have significant effect of light on the current and to operate photodiode as a variable Kresistance device, it is always connected in reverse biased condition.

4. Small Signal Model of Photodiode

• Fig. 3.5.3 shows the small signal model for photodiode.

• In Fig. 3.5.3 (a) a photodiode is represented by an ideal junction diode in parallel with a current source which is proportional to the light intensity.

• The model in the Fig. 3.5.3 (b) assumes that the diode is heavily reversed biased, and hence that the diode may be replaced by its reverse resistance R. This model also includes the effect of barrier capacitance C and the ohmic resistance r.

• In both the figures, the symbol L represents light flux in lumens, and K is a proportionality constant in the range 10 to 50 mA/lumen.

5. Advantages

1. Can be used as variable resistance device.

2. Highly sensitive to the light.

3. The speed of operation is very high. The switching of current and hence the resistance value from high to low or otherwise is very fast.

6. Disadvantages

1. The dark current I_λ is temperature dependent.

2. The overall photodiode characteristics are temperature dependent hence have poor temperature stability.

3. The current and change in current is in the range of μA which may not be sufficient to drive other circuits. Hence amplification is necessary.

7. Photodiode Applications

• The two commonly used systems using photodiode are alarm system and a counting ag system. Fig. 3.5.4 shows a photodiode employed in an alarm system.

• The reverse current I_λ continues to flow as long as light beam is incident on the photodiode. When the light is interrupted, the current I_λ drops to the dark current level. This initiates the alarm system sounding the alarm.

• Fig. 3.5.5 shows a photodiode used to count the items on a conveyor belt. As each item passes, the light beam is broken. Thus reverse current I_λ drops to the dark current level. This activates the counting mechanism and the counter is increased by one.

• Other applications of photodiode are

1. As a cell.

2. As a variable resistance device.

3. In light intensity meters.

4. In high‒speed logic circuits.

5. In fibre optic receivers.

Review Questions

1. Discuss the working principle of a photodiode.

2. What is photodiode ? Explain its construction and operation.

3. Explain the V‒I characteristics of photodiode.

4. State the advantages, disadvantages and applicaitons of photodiode.