Principles of Optical Detectors

  • The photodetector works on the principle of optical absorption. The main requirement of light detector or photodector is its fast response. For fiber optic communication purpose most suited photodetectors are PIN (p-type- Instrinsic-n-type) diodes and APD (Avalanche photodiodes)
  • The performance parameters of a photodetector are responsivity, quantum efficiency, response time and dark current.

Cut-off Wavelength (λc)

Any particular semiconductor can absorb photon over a limited wavelength range. The highest wavelength is known as cut-off wavelength (λc). The cut-off wavelength is determined by bandgap energy Eg of




Eg inelectron volts (eV) and

λc cut-off wavelength is in µm.

Typical value of λc for silicon is 1.06 µm and for germanium it is 1.6 µm.

Quantum Efficiency (η)

The quantum efficiency is define as the number of electron-hole carrier pair generated per incident photon of energy h v and is given as –


where,       Ip is average photocurrent.

Pin is average optical power incident on photodetector.

Absorption coefficient of material determines the quantum Quantum efficiency η < 1 as all the photons incident will not generate e-h pairs. It is normally expressed in percentage.

Detector Responsivity (

The responsivity of a photodetector is the ratio of the current output in amperes to the incident optical power in Responsivity is denoted by








Responsivity gives transfer characteristics of detector e. photo current per unit incident optical power.

Typical responsivities of pin photodiodes are – Silicon pin photodiode at 900 nm →0.65 A/W. Germanium pin photodiode at 1.3 µm →0.45 A/W. In GaAs pin photodiode at 1.3 µm →0.9 A/W.



r photodetectors are sued. As the intensity of optical signal at the receiver is very low, the detector has to meet high performance

  • The conversion efficiency must be high at the operating
  • The speed of response must be high enough to ensure that signal distortion does not
  • The detection process introduce the minimum amount of
  • It must be possible to operate continuously over a wide range of temperatures for many
  • The detector size must be compatible with the fiber dimensions.
  • At present, these requirements are met by reverse biased p-n In these devices, the semiconductor material absorbs a photon of light, which excites an electron from the valence band to the conduction band (opposite of photon emission). The photo generated electron leaves behind it a hole, and so each photon generates two charge carriers. The increases the material conductivity so call photoconductivity resulting in an increase in the diode current.
  • The diode equation is modified as –


Id is dark current i.e. current that flows when no signal is present. Is is photo generated current due to incident optical signal.

Fig. 3.2.1 shows a plot of this equation for varying amounts of incident optical power.

Three regions can be seen forward bias, reverse bias and avalanche

  1. Forward bias, region 1 : A change in incident power causes a change in terminal voltage, it is called as photovoltaic mode. If the diode is operated in this mode, the frequency response of the diode is poor and so photovoltaic operation is rarely used in optical.
  2. Reverse bias, region 2 : A change in optical power produces a proportional change in diode current, it is called as photoconductive mode of operation which most detectors Under these condition, the exponential term in equation 3.2.6 becomes insignificant and the reverse bias current is given by –
  3. Responsivity of photodiode is defined as the change in reverse bias current per unit change in optical powr, and so efficient detectors need large
  4. Avalanche breakdown, region 3 : When biased in this region, a photo generated electron-hole pair causes avalanche breakdown, resulting in large diode for a single incident Avalance photodiodes (APDs) operate in this region APDs exhibit carrier multiplication. They are usually very sensitive detectors. Unfortunately V-I characteristic is very steep in this region and so the bias voltage must be tightly controlled to prevent spontaneous breakdown.

PIN Photodiode

 PIN diode consists of an intrinsic semiconductor sandwiched between two heavily doped p-type and n-type semiconductors as shown in Fig. 3.2.2.


Sufficient reverse voltage is applied so as to keep intrinsic region free from carries, so its resistance is high, most of diode voltage appears across it, and the electrical forces are strong within it. The incident photons give up their energy and excite an electron from valance to conduction Thus a free electron hole pair is generated, these are called as photocarriers. These carriers are collected across the reverse biased junction resulting in rise in current in external circuit called photocurrent.

  • In the absence of light, PIN photodiodes behave electrically just like an ordinary rectifier If forward biased, they conduct large amount of current.
  • PIN detectors can be operated in two modes : Photovoltaic and photoconductive. In photovoltaic mode, no bias is applied to the detector. In this case the detector works very slow, and output is approximately logarithmic to the input light level. Real world fiber optic receivers never use the photovoltaic mode.
  • In photoconductive mode, the detector is reverse biased. The output in this case is a current that is very linear with the input light power.
  • The intrinsic region some what improves the sensitivity of the device. It does not provide internal The combination of different semiconductors operating at different wavelengths allows the selection of material capable of responding to the desired operating wavelength.

Characteristics of common PIN photodiodes

Sr. No.










µ m

0.4 – 1.1

0.8 – 1.8

1.0 – 1.7





0.4 – 0.6

0.5 – 0.7

0.6 – 0.9


Quantum efficiency



75 -90

50 – 55

60 – 70


Darl current



1 – 10

50 – 500

1 - 20


Rise time



0.5 – 1

0.1 – 0.5

0.02 – 0.5





0.3 – 0.6

0.5 – 3

1 – 10


Bias voltage



50 – 100

5 – 10

5 - 6

Depletion Layer Photocurrent

 Consider a reverse biased PIN

  • The total current density through depletion layer is –

Jtot = Jdr + Jdiff      


Jdr is drift current densioty due to carriers generated in depletion region.

Jdiff is diffusion current density due to carriers generated outside depletion region.

  • The drift current density is expressed as –


A is photodiode area.

f0 is incident photon flux per unit area.

  • The diffusion current density is expressed as –




Dp is hole diffusion coefficient.

Pn is hole concentration in n-type material. Pn0 is equilibrium hole density.

Substituting in equation 3.2.7, total current density through reverse biased depletion layer is –


Response Time

  • Factors that determine the response time of a photodiode are –
  1. Transit time of photocarriers within the depletion
  2. Diffusion time of photocarriers outside the depletion
  • RC time constant of diode and external
    • The transit time is given by –


  • The diffusion process is slow and diffusion times are less than carrier drift By considering the photodiode response time the effect of diffusion can be calculated. Fig. 3.2.4 shows the response time of photodiode which is not fully depleted.
  • The detector behaves as a simple low pass RC filter having passband of


RT, is combination input resistance of load and amplifier. CT is sum of photodiode and amplifier capacitance.

Example 3.2.5 : Compute the bandwidth of a photodetector having parameters as – Photodiode capacitance = 3 pF

Amplifier capacitance = 4 pF Load resistance = 50 Ω

Amplifier input resistance = 1 MΩ

Solution : Sum of photodiode and amplifier capacitance CT = 3 + 4 = 7 pF

Combination of load resistance and amplifier and input resistance RT = 50Ω || 1 MΩ ≈ 50 Ω

B = 454.95 MHz   

Avalanche Photodiode (APD)

  • When a p-n junction diode is applied with high reverse bias breakdown can occur by two separate mechanisms direct ionization of the lattice atoms, zener breakdown and high velocity carriers impact ionization of the lattice atoms called avalanche APDs uses the avalanche breakdown phenomena for its operation. The APD has its internal gain which increases its responsivity.
  • 3.2.5 shows the schematic structure of an APD. By virtue of the doping concentration and physical construction of the n+ p junction, the electric filed is high enough to cause impact ionization. Under normal operating bias, the I-layer (the p־ region) is completely depleted. This is known as reach through condition, hence APDs are also known as reach through APD or RAPDs.]

  • Similar to PIN photodiode, light absorption in APDs is most efficient in I-layer. In this region, the E-field separates the carriers and the electrons drift into the avalanche region where carrier multiplication occurs. If the APD is biased close to breakdown, it will result in reverse leakage current. Thus APDs are usually biased just below breakdown, with the bias voltage being tightly
  • The multiplication for all carriers generated in the photodiode is given as –



IM = Average value of total multiplied output current. IP = Primary unmultiplied photocurrent.

  • Responsivity of APD is given by –



R0 = Unity gain responsivity.

MSM Photodetector

  •  Metal-semiconductor-metal (MSM) photodetector uses a sandwiched semiconductor between two metals. The middle semiconductor layer acts as optical absorbing layer. A Schottky barrier is formed at each metal semiconductor interface (junction), which prevents flow of
  • When optical power is incident on it, the electron-hole pairs generated through photo absorption flow towards metal contacts and causes
  • MSM photodetectors are manufactured using different combinations of semiconductors such as – GaAs, InGaAs, InP, Each MSM photodetectors had distinct features e.g. responsivity, quantum efficiency, bandwidth etc.
  • With InAIAs based MSM photodetector, 92 % quantum efficiency can be obtained at 1.3 µm with low dark An inverted MSM photodetector shows high responsivity when illuminated from top.
  • A GaAs based device with travelling wave structure gives a bandwidth beyond 500 GHz.

Important Formulae for PIN and APD

PIN photodiode 1.



Recommended Questions

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State the significance of each parameter in the expression.

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Optical Detector

  1. With a proper sketch briefly explain the structure of PIN diode.
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  2. Deduce the expression for total current density for APD.
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