Optical Dtectors

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

              Eg  in electron volts (eV) and
              λc cut-off wavelength is in m.
       Typical value of λc for silicon is 1.06 µm for germanium it is 1.6 µm.
1.2 Quantum Efficiency (η)
. The quantum efficiency is defined as the number of electron-hole carrier pair generated per incident photon energy hv and is given as –
       Where,     Ip is average photocurrent.
                       Pin is average optical power incident on photodetector.
. Absorption coefficient of material determines the quantum efficiency. Quantum efficiency  η < 1 as all the photons incident will not generate e-h pairs. It is normally expressed in percentage.
1.3 Detector Responsivity (ℜ)
. The responsivity of a photodetector is the ratio of the current output in amperes to the incident optical power in watts. Responsivity is denoted by ℜ.

. Responsivity gives transfer characteristics of detector i.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.

1.4 Working of Photodiodes
. In order toconvert the modulated light back into an electrical signal, photodiodes or photodetectors are used. As the intensity of optical signal at the receiver is very low, the detector has to meet high performance specifications.
- The conversion efficiency must be high at the operating wavelength.
- The speed of response must be high enough to ensure that signal distortion does not occur.
- The detection process introduce the minimum amount of noise.
- It must be possible to operate continuously over a wide range of temperatures for many years.
- The detector size must be compatible with the fiber dimensions.
. At present, these requirements are met by reverse biased p-n photodiodes. In these devices, the semiconductor materials 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. This increases the material conductivity so called 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. 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 breakdown.
i) 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 links.
ii) Reverse bias, region 2 : A change in optical power produces a proportional change in diode current, it is called as photovoltaic mode of operation which most detectors use. Under these condition, the exponential term in equation becomes insignificant and reverse bias current is given by –
       Idiode = (Id + Is)
. Responsivity of photodiode is defined as the change in reverse bias current per unit change in optical power, and so efficient detectors need large responsivities.
iii) 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 photon. Avalanche photodiodes (APDs) operate in this region. APDs exhibit carrier multiplication. They are usually very sensitive detectors. Unfortunately V-I characteristics is very steep in this region and so the bias voltage must be tightly controlled to prevent spontaneous breakdown.
2. PIN Photodiode
. PIN diode consists of an intrinsic semiconductor sandwiched between two heavily doped  p-type and n-type semiconductors as shown in Fig. 2.

. Sufficient reverse voltage is applied so as to keep intrinsic region free from carriers, 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 band. 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 diode. If forward biased, they conduct large amount of current. 
. PIN detectors can be operated in two modes : Photovoltiac 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 receiver never use the photvoltiac mode.
. In photoconductive mode, the detector is reverse biased. The amount 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 gain. The combination of different semiconductors operating at different wavelength allows the selection of material capable of responding to the desired operating wavelength.
Characteristics of common PIN photodiodes

2.1 Depletion Layer Photocurrent
. Consider a reverse biased PIN photodiode.

. The total current density through depletion layer is –

              Jdr is drift current density 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.
              Φ0 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, total current density through reverse biased depletion layer is –

2.2 Response Time
. Factors that determine the response time of a photodiode are –
i) Transit time of photocarriers within the depletion region.
ii) Diffusion time of photocarriers outside the depletion region.
iii) RC time constant of diode and external circuit.
. The transit time is given by -

. The diffusion process is slow and diffusion times are less than carriers drift time. By considering the photodiode response time the effect of diffusion can be calculated. Fig. 4 shows the response time of photodiode which is not fully depleted.

. The detector behave 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.

2.3 Avalanche Photodiode (APD)
 . When a p-n junction diode is applied with high reverse bias breakdown can occur by two separate mechanism direct ionization of the lattice atoms, zener breakdown and high velocity carriers causing impact ionization of the lattice atoms called avalanche breakdown. APDs uses the avalanche breakdown phenomena for its operation. The APD has its internal gain which increases its responsivity.
. Fig. 5 shows the schematic structure of an APD. By virtue of the doping concentration and physical construction of the n+ p junction, the electric field 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 electrons drift into the avalanche region where carriers multiplication occurs. If the APD is biased close to breakdown, it will result in reverse leakage current. Thus APDs are usually biased just breakdown, with the bias voltage being tightly controlled.
. 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 –

Characteristics of common APDs

2.4 Comparison of PIN and APD

2.5 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 electrons.
. When optical power is incident on it, the electron-hole pairs generated through photo absorption flow towards metal contacts and causes photocurrent.
. MSM photodetectros are manufactured using different combinations of semiconductors such as – GaAs, InGaAs, InP, InAlAs. Each MSM photodetectors has distinct features e.g. responsivity, quantum efficiency, bandwidth etc.
. With InAlAs based MSM photodetector, 92 % quantum efficiency can be obtained at 1.3 µm with low dark current. 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.

2.6 Important Formulae fir PIN and APD

PIN photodiode



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