Home Education What is Photodiode – How does a photodiode work

What is Photodiode – How does a photodiode work


Photodiode definition

Photodiode definition – a type of semiconductor diode that serves as a photo detector. The purpose of a photodiode is to convert power from photons of light into electric current. The generation of electric current is created when the power of photons of light is absorbed by the surface of a semiconductor. A negligible electric current is also generated without the participation of light sources. Increasing the absorption area of the photodiode increases the absorption of light but also increases the response time, i.e. the time to produce the intensity. Its electrical properties are dependent on the light radiation that is illuminating it. It is easy to recognize this component among other semiconductor diodes due to its characteristic housing with transparent window. Internal photodiode component construction is based on the p-n junction or p-i-n structure (p-i-n-with an intrinsic or undoped layer between two p-n doped semiconductor crystals). The photodiode is used widely due to its simplicity of operation and relatively low production cost.

photodiode symbol
Photodiode symbol

Photodiode working principle

Energy is delivered to the photodiode junction while it is being illuminated by photons (light) – injection of minority carriers takes place, wherein the pairs of hole and electron are generated. It causes the generation of current, which is proportional to the light flux that  “is put” on the photodiode (photoelectric effect). Total current that flows through the junction is the sum of two components: the saturation current (dark current) and current dependent from illumination intensity. The free electrons are attracted by the positive charged particles on the n-type area border, while the holes penetrate into the p-type area. 

Modes of operation

  • Photovoltaic mode (no-bias mode) – Photodiode acts as a source of electric current. While its junction is being illuminated by photons, it then creates EMF  (electromotive force), which means electric current. This phenomenon is called photovoltaic phenomenon, which is the basis for solar cells. We can say, that a typical solar cell is just a much bigger photodiode,
  • Photoconductive mode (reverse-bias mode) – Very often, reverse voltage is applied to a photodiode (reverse-bias of the cathode with respect to the anode) to widen the depletion layer, which decreases the junction’s capacitance. (If you are confused what I am talking about, check this article about Semiconductor Diode’s internal construction.)In case of absence of the component illumination, so called dark current flows, which has low values. In addition, reverse-bias also has influence on the dark current. After illuminating, the number of minority carriers raise, which causes increase of the reverse current.
photodiode characteristics
Photodiode characteristics

When observing current-voltage characteristics of photodiode, take a look at the waveforms in different quadrants of coordinate system. The third quarter characteristic shows the usage of photodiode as a photodetector (light sensor). It corresponds to the current saturation of the p-n junction. However, in the fourth quarter of characteristic, photodiode works as a light radiation converter – solar battery. If current doesn’t flow through illuminated diode, the amount of voltage that arises on the terminals is called photovoltaic voltage. Solar batteries are commonly used in electrical equipment power supply for example sanitation and battery charging when weather conditions or light are favourable for this purpose.

Photodiode structure

The figure below shows the construction details of the photodiode.

photodiode structure
Photodiode structure

PN connector of the device put inside a glass material. This is to enable light energy to pass through it. Considering that only the connector is exposed to radiation, the various other part of the glass material is painted black or is metallized.
The general unit has a really small dimension of virtually roughly 2.5 mm.
It deserves keeping in mind that the current flowing through the unit is consisted of microamperes and is determined by an ammeter.

How does a photodiode work

Mean that a semiconductor diode is lit up by electromagnetic radiation with an energy above the Eg band gap. When this radiation is absorbed in the area charge region of the junction and/or in the material adjacent to this area on either side of the junction, the resulting electron-hole sets are divided by the joint’s electric field. Minority carriers are of specific importance. These carriers move toward the joint as well as create an increase backwards current if the outer circuit of the joint is shorted. If the joint is open, a possible distinction appears at the ends of the joint. And this is the photovoltaic impact: when the joint is lit up, a source of current or voltage can be acquired, i.e., a source of electricity. On the other hand, the concentration of majority carriers is almost not transformed by absorption of light, due to the fact that the amount of carriers produced by light is a number of orders of size lower than the equilibrium concentration of these carriers.

For a photovoltaic phenomenon to take place, the list below conditions have to be satisfied.

  1. Redundant positive and negative charge carriers should be generated in the semiconductor under the influence of radiation;
  2. Redundant carriers of different indicators should be divided by some electrostatic inhomogeneity. Charge separation in a photodiode can take place when an electrostatic prospective distinction is produced, e.g., as exists at a p-n junction, at a metal-semiconductor contact, or at a semiconductor heterojunction.
  3. The created free carrier has to keep its mobility enough time to reach the inhomogeneity creating the charge splitting up.

Now think about a p-n junction at thermodynamic balance. There is always some current of majority carriers, called injection currents of electrons Ini and holes Ipi, flowing with the junction, which have the ability to conquer the potential barrier at the joint. In the opposite direction flows the thermal generation current of minority carriers: electrons Ingi holes Ipg. The number below programs the bandgap version of the p-n joint as well as shows the direction of these currents. At balance, the two currents equilibrium each other as well as the resultant current is zero. When a photon with energy more than the energy of the excited gap falls on the junction then, as already stated, the concentration of minority carriers strongly enhances. The supposed photogenerated current appears. Depending upon exactly how the junction is loaded, various sensations happen in a lit up solar battery. We will certainly think about 2 extreme cases.

photodiode electrons
P-N junction and majority and minority carrier currents.

If the junction is shorted, which is equivalent to the external circuit voltage being zero (Uz= 0) then the possible obstacle at the junction does not transform. In this situation, the injection current thickness coincide as in the unshorted joint. These currents stabilize the thermal generation currents yet the photogeneration currents remain out of balance. These are represented by the flux of electrons from p to n and holes from n to p, as revealed by the green arrows in the figure listed below.

photodiode Optical generation currents
Optical generation currents

Since the photodiode is short-circuited, we say that short-circuit photocurrent Isc flows. The density of the short-circuit photocurrent is expressed by the formula:

Isc= q Nph(Eg)

Where Nph is the variety of photons with energy equal to Eg. The variety of photons with a particular energy is equal to the proportion of the spectral glowing flux P lambda to the photon energy hc/lambda. Considering that the variety of photons is proportional to the radiation flux, the short-circuit current is additionally proportional to the incident radiation flux.
If the photodiode is open, then the resultant currents are photogenerated currents: electrons flow from p to n and holes from n to p. Therefore, the n-type region charges negatively and the p-type area charges positively. This polarization of the junction areas is equivalent to polarization in the transmission instructions. The value of this polarization voltage is called the bifurcation photocurrent, Voc. This circumstance is highlighted in the figure listed below, which reveals the bandgap model of a bifurcated photodiode.

Band model of a bifurcated photodiode
Band model of a bifurcated photodiode.

Lowering the potential barrier at the p-n junction causes the injection current to increase. At equilibrium, this injection current is balanced by photogeneration currents. The dark current flowing across a p-n junction polarized by a voltage Voc, is expressed by Eq:

Id= Io[exp(qVoc/kT)-1]

This current balances the maximum photogenerated current in the open illuminated p-n junction, i.e.

Isc–Id= 0

By substituting the value of Isc for Id, the following relationship is obtained:

photodiode formula

Since Isc ~ Pʎ, , the bias voltage depends logarithmically on the flux of radiation incident on the battery.

If the photodiode is packed with resistance RL, after that the current flowing with the battery is less than the short-circuit current as well as the voltage is less than the open-circuit voltage. The loaded photodiode can be treated as a current source. The electrical equivalent diagram of the battery is shown in the following figure According to this representation and also Kirchoff’s I law for node A:

IL+ I = Id

Hence the current flowing through the load:

I = Id-IL= -( IL-Id)

At a constant value of IL, an increase in the load resistance RL from 0 to ∞, causes Voc to increase and therefore the barrier height at the interface to decrease. As a result, the dark current Id decreases and at the same time the current flowing through the load decreases.

photodiode electric circuit

Photodiode dark resistance

It is true that there is constantly some minority charge carriers in a semiconductor crystal also under very dark conditions – these minority charge carriers in a semiconductor crystal existing because of inevitable contaminations and all-natural thermal excitation of the crystal. Thus, even in darkness, there would certainly be a little as well as continuous reverse saturation current in the diode. This current is taken care of for the photodiode and also the current is called dark current. The ratio of the optimum robust reverse voltage to the dark current of the photodiode is called the dark resistance of that diode.

photodiode dark current

When we use light to a diode, the reverse current will certainly enhance. This connection is linear. The value of the reverse current is straight proportional to the intensity of the event light.
If we boost the intensity of light, approximately a particular value of reverse current. The current will certainly not raise as the light intensity rises. We call this maximum value of reverse current as saturation current of photodiode.

photodiode dark current

Types of Photodiodes

PN photodiode

A p-n junction made of photon-absorbing material and negatively polarized can function as a photodetector. The current flowing in the negatively biased direction, which is small under unilluminated problems, depends on the variety of photons soaked up after lighting. Photons are absorbed throughout the semiconductor with an absorption coefficient α. Each absorbed photon creates an electron-hole set. The fate of the produced carriers is differed. There is a strong electric field in the depleted area, the produced electrons relocate in the direction of the “n” region, holes towards the “p” area(there is no recombination in the diminished area).

The equation explaining the p-n joint current is the popular diode current formula supplemented with a component relying on the number of soaked up photons. The saturation current iS is the dark current for the quit direction. The iFD current is composed by a well-known equation, however the quantum performance is tiny in this case since a considerable fraction of the created carriers do not take part in the current.

The response time is relatively long, carriers from the depleted layer are removed quickly, but the diffusion time of carriers in adjacent areas increases the duration of the current pulse.

A p-n joint as a photodetector. (A) An adversely polarized p-n joint and also the depleted region where electron-hole pairs are generated. (B) Current i(U) characteristics for an adversely polarized p-n junction for different lighting conditions.

A brightened p-n joint can operate under numerous power problems. Lighting of the joint causes the appearance of the photovoltaic voltage UBF at its terminals. When the terminals of the joint are shorted, the result of lighting is the look of photovoltaic current iBF. Regular operating conditions for a photodetector are obtained by using a huge barrier voltage UB. The electric field is solid, the width of the depleted layer increases, the time for carriers to flow with the depleted layer is little, so the quantum efficiency increases as well as the capacitance of the diode lowers.

PIN Photodiode

In a PIN photodiode, a layer of weakly doped semiconductor “i” is sandwiched between layers of p and also n semiconductors that are transparent to photons. The “i” region is strongly depleted under barrier polarization. In this area, there is a strong electric field that increases the carriers generated by photon absorption. Area “i” along with the diffusion areas on both sides inhabit a significant size d, a significant part of the carriers created by photons join the current, the quantum effectiveness η rises. The length of location “i” is a trade-off in between increasing performance η and raising carrier flow time, which reduces the operating bandwidth of the photodiode. The selection of products for p-i-n diodes is important for their proper operation. The material of which the “i” layer is made should absorb photons from a band of certain wavelengths. The width of the energy space Eg= (EC-EV) between the valence and also conduction bands ought to be a little smaller than the energy of the absorbed photons. It ought to be born in mind that the smaller the Eg worth, the larger the photodiode dark current caused by thermal effects. From this perspective, germanium is seldom used specifically as a result of its big dark current. The material easily made use of in photodiodes created for the 2nd and also 3rd optical fiber windows (1.3-1.7 μm) is In0.53 Ga0.47 As. In the heterojunction structure, the surrounding “p “and also “n” areas are made based on indium phosphide InP.
The most effective outcomes were gotten for the double heterojunction. The layers “p “as well as “n” are made from InP, which for the bandwidth of 1.2-1.6 um is transparent, the weakly doped layer is constructed from In0.53 Ga0.47 As, which causes absorption to occur just in the “i” layer.

photodiode pin structure
A p-i-n photodiode. (A) Simplified structure of a negatively polarized p-i-n photodiode. B) Electric field distribution in a negatively polarized p-i-n photodiode. (C) Structure of a p-i-n diode brightened from the substrate side.

A streamlined structure of a PIN photodiode is revealed above. When the photodiode is negatively polarized as well as not lit up, a small dark current IS flows in the photodiode circuit, similar to that for a p-n diode. A strong electric field exists in the “i” area, which quickly eliminates electrons and holes created during photon absorption. The level of sensitivity of the PIN photodiode is well explained by the relation.

Two factors are introduced in the relationship above. The factor (1-R) shows the effect of the representation result of photons from the surface area of the semiconductor material of the photodiode. The representation effect can substantially decrease the sensitivity worth of RFD. As a result, in several styles, an unique anti-reflection layer is used on the photodiode surface area to match the wave impedances of vacuum and also semiconductor material. The quantum efficiency increases when the anti-reflection layer coating is used.
The second factor (1-e-αd) accounts for the fact that not all photons are absorbed in the “i” layer, suggesting a substantial reduction in sensitivity. Increasing the thickness d of layer “i” improves the level of sensitivity however raises the carrier flow time, which limits the operating bandwidth of the photodiode.
The optical radiation is fed to the photodiode through an optical fiber. The delivery structure can be made in various ways. The optical signal streams almost losslessly into the InGaAs layer where the generation of electron-hole sets occurs.
The photodiode can additionally be brightened from the opposite. Here the output electrical signal is fed directly right into the microwave microstrip line. Such remedies are utilized in broadband optical receivers.
Unique photodiode structures have actually also been constructed in which the “p “as well as “n “layers are made in the form of Bragg mirrors, comparable to those in VCSEL surface exhaust lasers. The photon-absorbing “i” layer between the Bragg mirrors kinds a powerful structure. The photodiode thus ends up being a selective device, favoring a specific wavelength λ.

Avalanche photodiode

In avalanche photodiode, an additional “p” region is introduced into the p-i-n diode structure. With barrier polarization, typically tens of volts, there is a strong electric field in this area. Electrons flowing in this area are accelerated, gain energy and generate successive electron-hole pairs. The process of collision ionization, avalanche multiplication occurs and as a result the diode current increases many times (M times). The diode current increases exponentially with increasing U until avalanche breakdown. Typical, practically achievable values of M reach up to 100.

Avalanche diodes were at first conveniently made use of in optical links as a result of their high sensitivity worths. Nonetheless, their drawbacks have actually considerably limited their applications. The negative aspects of avalanche photodiode consist of:

  • large polarization voltages,
  • decrease of operating bandwidth by about √, a hundredfold rise in sensitivity is spent for by a tenfold reduction in operating data transfer,
  • strong dependence of sensitivity on temperature level,
  • high noise.

It additionally turned out that as a result of the large noise added by the avalanche diode, it is simpler to obtain appropriate level of sensitivity of the optical receiver using transistor amplifiers. Consequently, avalanche photodiodes are made use of just in unique optical link systems.

avalanche diode structure
Illustration of the operation of an avalanche photodiode. (A) Simplified structure of avalanche photodiode with p region of current multiplication. (B) Electric field distribution for barrier polarization. (C) Structure of a planar avalanche photodiode.

Silicon-based avalanche photodiodes run in a wide transmission capacity of 450-1000 nm. Their optimum sensitivity array is in the 600-850 nm band. The gain factor of these diodes is relatively large, reaching the value of M= 103. For longer wavelengths avalanche photodiodes are made on the basis of InGaAs. Their gain factor does not normally exceed 102. Avalanche photodiode are utilized in fiber optic communication systems, as well as in optical sensor systems.

Schottky barrier photodiodes

One of the extra popular kinds of photodetectors are MSM diodes, based on a metal-semiconductor-metal (MSM) framework, typically called Schottky barrier photodiodes. Two metal electrodes are made on the semiconductor material to develop 2 Schottky joints. Under operating conditions, a polarizing voltage is put on the electrodes. In a Schottky barrier photodiode, one of the p-n joint materials – usually j “p” – is changed by a steel. The steel layer is generally very thin as well as clear to optical radiation. In some layouts, the optical signal is fed from the opposite of the framework. In a metal-semiconductor joint, the depleted layer kinds near the surface area, therefore eliminating surface recombination. MSM diodes with planar framework with inter-finger electrodes have the smallest capacitance values Cj. With little series resistance RS, the moment constants RSCj are small and also for that reason MSM diodes have the highest operating regularities well over 100 GHz. The intermediate frequency in optical blending systems rises to 3000 GHz.

MSM photodiode with Schottky barrier. View of the metal electrodes of the photodiode.
MSM photodiode with Schottky barrier. View of the metal electrodes of the photodiode.

MSM photodiodes are the only competitors of p-i-n photodiodes. They are often used in planar photonic integrated circuits.

Photodiode Circuit

Photodiode amplifier Preamplifier

The main task of the preamplifier circuit in optical buffers is to convert the photodetector current signal right into a voltage signal of a minimum of 50 mV amplitude, which can be even more shaped by succeeding buffer phases. The most basic electronic component that can be utilized to convert a current signal to a voltage signal is a resistor. A photo modification in the current flowing through the resistor creates a modification in the voltage existing at the resistor, and this signal can consequently be amplified by a voltage amplifier. In the circuit shown right here, two variants of its application are possible, with a large value of Rb or with a tiny worth of Rb, specifically. In the first case, also a small change in current generated by the photodetector creates a big adjustment in voltage. This type of photodiode circuit has high level of sensitivity as well as reduced noise, however this goes to the price of minimized limiting frequency of the circuit, arising from the large worth of the time consistent RbCT. In extreme cases, the dark current of the photodiode can create the voltage to rise so high that the photodiode is permanently filled and also radiation detection is not feasible. With a tiny worth of resistor Rb, the scenario is turned around, the system will have a broad bandwidth, good vibrant residential properties while it will certainly be susceptible to noise as well as will have reduced level of sensitivity.

Preamplifier: a) open loop, b) transimpedance amplifier
Preamplifier: a) open loop, b) transimpedance amplifier

The compromise solution and also the one most often used in practice is the transimpedance amplifier circuit, TIA for short. In this case, the resistor is in the feedback loop, closing the voltage amplifier circuit. This photodiode circuit is considered to be the optimal solution, as it allows relatively high operating frequencies with a large gain factor.

photodiode preamplifier formula

As can be seen, the impact of the photodiode capacitance and also the input capacitance of the preamplifier circuit in the case of transimpedance amplifiers is K ura less (Ku-open loophole voltage gain). This permits the TIA circuit to have the same bandwidth with a resistance Rf much smaller sized than Rb. If Rf=Rb the transimpedance amplifier will have a broader bandwidth than an equivalent open-loop amplifier.
Because of the advantages offered, the TIA circuit was picked for more analysis of the optical buffer In a normal optical buffer circuit, the output signal from the preamplifier is fed to a voltage amplifier and/or comparator, which improves the edge quality and also establishes the amplitude, and afterwards to a logic circuit at the output of the barrier, which sets the digital signal to a predetermined requirement. These last stages of the optical buffer will certainly not be examined in the rest of the paper, because their style is of second importance and also the well-known solutions have acceptable criteria.

Integration with Laser Diode

When reviewing safety and security features, one have to mention the integrated photodiode. The figure below shows the interior framework of an actual laser diode. The figure plainly reveals that the little structure of the diode, is put on a reasonably large warm sink. A common laser diode radiates in 2 contrary directions. The back, weak flux falls on the ingrained photodiode. Readily offered laser diodes generally have three (seldom 4) leads. One encounters various plans of the laser diode and photodiode connections extra frequently. In the absence of a catalog, you can determine the pinout (discover the ends of the laser diode) experimentally making use of a 5V voltage source and a 450W resistor, taking the necessary precautions stated previously.

laser photodiode

The current of this built-in photodiode (in the reverse direction) is directly proportional to the emitted optical power.  This current at full output power is 0.02…2mA, depending on the type of element – consult catalogs for details.

A photodiode is used to keep the laser power at a given level.  It is enough to place such a photodiode in a feedback circuit to obtain a system maintaining constant optical output power. Virtually any BC transistor can be used.

Silicon photodiode spectral response

Silicon photodetectors with a design-defined spectral response are described. To this end, micromachining modern technologies generally as well as two properties of the integrated silicon photodetector in particular are made use of. First, the wavelength dependancy of the absorption coefficient is exploited. Second, the truth that the multilayer interference filter at the pn-junction is developed by processing a silicon wafer is exploited. The silicon complex refractive index, n * = n – jk, is wavelength reliant in the noticeable part of the spectrum because of an indirect band gap at 1.12 eV and the possibility of a direct transition at 3.4 eV, that makes the material highly absorb UV radiation and also acts practically like a transparent material for wavelengths over 800 nm. This mechanism allows the design of color sensors and also photodiodes with discerning response in the IR or UV array. The transmission of event light with a surface stack of thin films to volumetric silicon relies on the wavelength. The needed compatibility with conventional microelectronic processes in silicon limits the range of ideal materials to silicon-compatible materials traditionally utilized for integrated circuit fabrication. Precise data on: crystalline Si, thermally grown SiO2, LPCVD polysilicon, silicon nitride (low loss and stoichiometric) and also oxides (LTO, PSG, BSG, BPSG), PECVD oxynitrides as well as slim film metals are provided to boost the predictive quality of the simulation. For a full micro spectrometer, micromachining actions are typically utilized to fabricate the diffusion component. Devices operating in the visible or infrared spectral array based upon a Fabry-Perot grating or etalon are presented.

Photodiode array detector

The most sophisticated UV detector that is available today is the photodiode-array detector. The DAD permits simultaneous measurement of the entire spectrum of UV at a certain sampling rate, during the process of analysis of chromatographic data. The primary difference between traditional variable wavelength detectors and diode array device (DAD) is in the design of the monochromator and measuring chamber. In the DAD the monochromator is located behind the chamber that is used for measuring (so-called “reverse optics”). UV detectors are mainly of applications when organic solvents which absorb UV light are employed for separation. The main issue is making the solvent UV-compatible while satisfying the requirements for solubility for the polymers. However it is important to be aware that the smaller IDL value, compared to UV-DAD, can be achieved with UV detectors with a single wavelength registration.

photodiode array detector

Dual photodiode sensor

The representation of a unit pixel for a dual photodiode sensor in a range of different sensitivities is shown in the following two figures. The CMOS convolution process is responsible for producing the structure of a single pixel. Different dimensions of N+ doping regions have identical sensitivity. This occurs with a traditional photodiode as shown in Figure A below. This is due to the lack of microlens integration during the conventional CMOS process. Illumination of the entire area of the photodiode occurs without the light source collecting that light. In this case, the value of the light that passes to the dual photodiode sensor is related to the area of the single photodiode. The metal shielding provokes the production of a difference in sensitivity, as shown in Figure B. There is a reduction in light intensity for the photodiode with low sensitivity. Thanks to, the use of dual photodiode sensor in a structure with a metal shield, WDR is achieved in such a pixel structure. The production cost is reduced due to the absence of microlenses, whose manufacturing costs are higher than in the case of the production of the described structure.

dual photodiode sensor

Photodiode quantum efficiency

Quantum efficiency

Photodiode applications

Photodiodes are used in many industrial areas and devices, which include:

  • industrial automation,
  • optical switching devices,
  • measuring systems of electrical quantities,
  • measuring systems of non-electrical quantities,
  • photometry,
  • remote control systems,
  • high-speed Analog/Digital (A/D) converters.



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