UNIT V PHOTO ELECTRIC DEVICE 1/2Mv2

ASHWIN JS UNIT V PHOTO ELECTRIC DEVICE PHOTO EMISSIVITY

Using the experimental arrangement indicated in Fig. 19-1, the following characteristics of the photoemissive effect are obtained:

The photoelectrons liberated from the photosensitive surface possess a range of initial velocities. However, & definite negative potential when applied between the collector and the emitting surface will retard the fastest-moving electrons. This indicates that the emitted electrons are liberated with all velocities from zero to a definite maximum value. The maximum velocity of the emitted electrons is

2 1/2mv max =evr where Vr, is the retarding potential, in volts, necessary to reduce the photo current to zero. As the accelerating potential is increased, the number of electrons to the collector increases until saturation occurs. In Fig. 19-2a are plotted curves showing the variation of photocurrent I versus anode potential V with the light intensity j as a parameter. These curves indicate that V,, and hence Vmax, are independent of the light intensity.

2. If the photoelectric current is measured as a function of the anode potential for different light frequencies f and equal intensities of the incident light, the results obtained are essentially those illustrated in Fig. 19-26 It is observed that the greater the frequency of the incident light, the greater must be the retarding potential to reduce the photocurrent to zero. This means, of course, that the maximum velocity of emission of the photoelectrons increases with the frequency of the incident light. Experimentally, it is found that a linear relationship exists between V, and f.

The experimental facts 1 and 2 may be summarized in the statement that the maximum energy of the electrons liberated photoelectrically is independent of the light intensity but varies linearly with the frequency of the incident light.

3. If the saturation current is plotted as a function of the light intensity, we find that the photoelectric current is directly proportional to the intensity of the light ASHWIN JS

4. The foregoing photoelectric characteristics are practically independent of temperature, within wide ranges of temperature.

5. The electrons are emitted immediately upon the exposure of the surface to light. The time lag has been determined experimentally to be less than 3nsec.

6 Photoelectric cells are selective devices. This means that a given intensity of light of one wavelength, say red light, will not liberate the same number of electrons as an equal intensity of light of another wavelength. Say blue light. That is, the photoelectric yield, defined as the photocurrent (in amperes) per watt of incident light, depends upon the frequency of the light.

PHOTOELECTRIC THEORY

The foregoing experimental facts find their explanation in the electronic theory of metals and in the light-quantum hypothesis of Planck. As discussed in

Planck made the fundamental assumption that radiant energy is not continuous, but can exist only in discrete quantities called quanta, or photons. Bohr used the same theory of photons to explain the spectra of atoms . Einstein applied the same hypothesis to explain photo emission, as we now demonstrate. Planck's basic assumption is that, associated with light of frequency f (hertz) are a number of photons, each of which has an energy hf (joules), The greater the intensity of the light, the larger the number of photons present, but the energy of each photon remains unchanged. Of course, if the light beam is heterogeneous rather than monochromatic, the energy of the photons therewith associated will vary and will depend upon the frequency.

• According to Einstein, each photon of a light wave of frequency has the energy E is given by,

• E=hf

• where E= energy of photon(joule) • h= planks constant-6.626 x 10-34J-s • f= frequency of photon(Hz) • Einstein Equation 2 1/2mv  hf-Uw

ASHWIN JS LAW OF PHOTO ELECTRIC EFFECT

• Photoelectric effect is directly proportional to intensity. • If the frequency of the incident light is less than the threshold frequency then no electron ejected, no matter what the intensity . • The maximum kinetic energy of the electrons depend on the frequency of the incident light. • The electrons were emitted immediately - no time lag.

PHOTOTUBE

A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell'

OPERATING PRINCIPLES

Phototubes operate according to the photoelectric effect: Incoming photons strike a photocathode, knocking electrons out of its surface, which are attracted to an anode. Thus current is dependent on the frequency and intensity of incoming photons. Unlike photomultiplier tubes, no amplification takes place, so the current through the device is typically of the order of a few microamperes.

Vacuum devices have a near constant anode current for a given level of illumination relative to anode voltage. Gas filled devices are more sensitive but the frequency response to modulated illumination falls off at lower frequencies compared to the vacuum devices.

APPLICATIONS

One major application of the phototube was the reading of optical sound tracks for projected films. Phototubes were used in a variety of light-sensing applications until they were superseded by photoresistors and photodiodes

ASHWIN JS PHOTO MULTIPLIER TUBE

The photo multiplier tube consists of an evacuated glass envelope containing a photo cathode, an anode and several additional electrodes, termed Dynodes, each at a higher voltage, than the previous dynode. Figure 13.32 illustrates the principle of the photo multiplier. Electrons emitted by the cathode are attracted to the first dynode. Here a phenomenon known as secondary emission takes place. When electrons moving at a high velocity strike an appropriate material, the material emits a greater number of electrons than it was struck with. In this device, the high velocity is achieved by the use of a high voltage between the anode and the cathode.

The electrons emitted by the first dynode are then attracted to the second dynode, where the same action takes place again. Each dynode is at a higher voltage, in order to achieve the requisite electron velocity each time. Hence, secondary emission, and a resulting electron multiplication, occurs at each step, with an overall increase in electron flow that may be very great. Amplification of the original current by much as 105 — 109 is common. Luminons sensitivities range from lA per lumen or less, to over 2000 A per lumen. Typical anode current ratings range from a minimum of 100 μA to a maximum of 1 mA. The extreme luminous sensitivity possible with these devices is such that for a sensitivity of 100 A per lumen, only 10-5 lumen is needed to produce 1 mA of output current.

Magnetic fields affect the photo multiplier because some electrons may be deflected from their normal path between stages and therefore never reach a dynode or anode. Hence, the gain falls. To minimise this effect μ-metal magnetic shields are often placed around the photo multiplier tube.

PHOTOCONDUCTIVITY Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation

When light is absorbed by a material such as a semiconductor, the number of free electrons and electron holes increases and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.

Classic examples of photoconductive materials include : photocopying (xerography); lead sulfide, used in infrared detection applications

Application

When a photoconductive material is connected as part of a circuit, it functions as a resistor whose resistance depends on the light intensity. In this context, the material is called a photoresistor (also called light-dependent resistor or photoconductor). The most common ASHWIN JS application of photoresistors is as photodetectors, i.e. devices that measure light intensity. Photoresistors are not the only type of photodetector—other types include charge-coupled devices (CCDs), photodiodes and phototransistors—but they are among the most common. Some photodetector applications in which photoresistors are often used include camera light meters, street lights, clock radios, infrared detectors, nanophotonic systems and low-dimensional photo- sensors devices

PHOTOVOLTAIC EFFECT

The photovoltaic effect is the creation of voltage and electric current in a material upon exposure to light and is a physical and chemical phenomenon.

The photovoltaic effect is closely related to the photoelectric effect. In either case, light is absorbed, causing excitation of an electron or other charge carrier to a higher-energy state. The main distinction is that the term photoelectric effect is now usually used when the electron is ejected out of the material (usually into a vacuum) and photovoltaic effect used when the excited charge carrier is still contained within the material. In either case, an electric potential (or voltage) is produced by the separation of charges, and the light has to have a sufficient energy to overcome the potential barrier for excitation. The physical essence of the difference is usually that photoelectric emission separates the charges by ballistic conduction and photovoltaic emission separates them by diffusion, but some "hot carrier" photovoltaic device concepts blur this distinction.

When sunlight or other sufficiently energetic light is incident upon the photodiode, the electrons present in the valence band absorb energy and, being excited, jump to the conduction band and become free. These excited electrons diffuse, and some reach the rectifying junction (usually a diode p-n junction) where they are accelerated into the p-type semiconductor material by the built-in potential (Galvani potential). This generates a flow of electrical current electromotive force, and thus some of the light energy is converted into electric energy. The photovoltaic effect can also occur when two photons are absorbed simultaneously in a process called two-photon photovoltaic effect.

Besides the direct excitation of free electrons, a photovoltaic effect can also arise simply due to the heating caused by absorption of the light. The heating leads to increased temperature of the semiconductor material, which is accompanied by temperature gradients. These thermal gradients in turn may generate a voltage through the Seebeck effect. Whether direct excitation or thermal effects dominate the photovoltaic effect will depend on many material parameters.

In most photovoltaic applications the radiation is sunlight, and the devices are called solar cells. In the case of a semiconductor p-n (diode) junction solar cell, illuminating the material creates an electric current because excited electrons and the remaining holes are swept in different directions by the built-in electric field of the depletion region.

LIGHT EMITTING DIODE (LED)

The lighting emitting diode is a p-n junction diode. It is a specially doped diode and made up of a special type of semiconductors. When the light emits in the forward biased, then it is called as a light emitting diode. ASHWIN JS Types of Light Emitting Diodes  Gallium Arsenide (GaAs) – infra-red  Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange  Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow  Gallium Phosphide (GaP) – red, yellow and green  Aluminium Gallium Phosphide (AlGaP) – green

Working Principle of LED When Light Emitting Diode (LED) is forward biased, free electrons in the conduction band recombines with the holes in the valence band and releases energy in the form of light.

The process of emitting light in response to the strong electric field or flow of electric current is called electroluminescence.

The working principle of the Light emitting diode is based on the quantum theory. The quantum theory says that when the electron comes down from the higher energy level to the lower energy level then, the energy emits from the photon. The photon energy is equal to the energy gap between these two energy levels. If the PN-junction diode is in the forward biased, then the current flows through the diode.

The flow of current in the semiconductors is caused by the both flow of free electrons in the opposite direction of current and flow of electrons in the direction of the current. Hence there will be recombination due to the flow of these charge carriers.

The recombination indicates that the electrons in the conduction band jump down to the valence band. When the electrons jump from one band to another band the electrons will emit the electromagnetic energy in the form of photons and the photon energy is equal to the forbidden energy gap.

ASHWIN JS V-I Characteristics of LED

There are different types of light emitting diodes are available in the market and there are different LED characteristics which include the color light, or wavelength radiation, light intensity. The important characteristic of the LED is color. In the starting use of LED, there is the only red color. As the use of LED is increased with the help of the semiconductor process and doing the research on the new metals for LED, the different colors were formed.

The following graph shows the approximate curves between the forward voltage and the current. Each curve in the graph indicates the different colour.

PHOTODIODE

A photodiode is a p-n junction device that consumes light energy to generate electric current. It is also sometimes referred as photo-detector, photo-sensor, or light detector.

Photodiodes are specially designed to operate in reverse bias condition. Reverse bias means that the p-side of the photodiode is connected to the negative terminal of the battery and n-side is connected to the positive terminal of the battery.

The symbol of photodiode is similar to the normal p-n junction diode except that it contains arrows striking the diode. The arrows striking the diode represent light or photons.

How photodiode works?

A normal p-n junction diode allows a small amount of electric current under reverse bias condition. To increase the electric current under reverse bias condition, we need to generate more minority carriers. ASHWIN JS The external reverse voltage applied to the p-n junction diode will supply energy to the minority carriers but not increase the population of minority carriers.

However, a small number of minority carriers are generated due to external reverse bias voltage. The minority carriers generated at n-side or p-side will recombine in the same material before they cross the junction. As a result, no electric current flows due to these charge carriers. For example, the minority carriers generated in the p-type material experience a repulsive force from the external voltage and try to move towards n-side. However, before crossing the junction, the free electrons recombine with the holes within the same material. As a result, no electric current flows.

To overcome this problem, we need to apply external energy directly to the depletion region to generate more charge carriers.

A special type of diode called photodiode is designed to generate more number of charge carriers in depletion region. In photodiodes, we use light or photons as the external energy to generate charge carriers in depletion region.

Types of photodiodes

The working operation of all types of photodiodes is same. Different types of photodiodes are developed based on specific application. For example, PIN photodiodes are developed to increase the response speed. PIN photodiodes are used where high response speed is needed.

The different types of photodiodes are  PN junction photodiode  PIN photodiode  Avalanche photodiode Among all the three photodiodes, PN junction and PIN photodiodes are most widely used. PN junction photodiode

PN junction photodiodes are the first form of photodiodes. They are the most widely used photodiodes before the development of PIN photodiodes. PN junction photodiode is also simply referred as photodiode. Nowadays, PN junction photodiodes are not widely used.

When external light energy is supplied to the p-n junction photodiode, the valence electrons in the depletion region gains energy.

If the light energy applied to the photodiode is greater the band-gap of semiconductor material, the valence electrons gain enough energy and break bonding with the parent atom. The valence ASHWIN JS electron which breaks bonding with the parent atom will become free electron. Free electrons moves freely from one place to another place by carrying the electric current.

When the valence electron leave the valence shell an empty space is created in the valence shell at which valence electron left. This empty space in the valence shell is called a hole. Thus, both free electrons and holes are generated as pairs. The mechanism of generating electron-hole pair by using light energy is known as the inner photoelectric effect.

The minority carriers in the depletion region experience force due to the depletion regional and the external electric field. For example, free electrons in the depletion region experience repulsive and attractive force from the negative and positive ions present at the edge of depletion region at p-side and n-side. As a result, free electrons move towards the n region. When the free electrons reaches n region, they are attracted towards the positive terminals of the battery. In the similar way, holes move in opposite direction.

The strong depletion region electric field and the external electric field increase the drift velocity of the free electrons. Because of this high drift velocity, the minority carriers (free electrons and holes) generated in the depletion region will cross the p-n junction before they recombine with atoms. As a result, the minority carrier current increases.

When no light is applied to the reverse bias photodiode, it carries a small reverse current due to external voltage. This small electric current under the absence of light is called dark current. It is denoted by I λ.

In a photodiode, reverse current is independent of reverse bias voltage. Reverse current is mostly depends on the light intensity.

In photodiodes, most of the electric current is carried by the charge carriers generated in the depletion region because the charge carriers in depletion region has high drift velocity and low recombination rate whereas the charge carriers in n-side or p-side has low drift velocity and high recombination rate. The electric current generated in the photodiode due to the application of light is called photocurrent. ASHWIN JS The total current through the photodiode is the sum of the dark current and the photocurrent. The dark current must be reduced to increase the sensitivity of the device.

The electric current flowing through a photodiode is directly proportional to the incident number of photons.

The VI characteristics of a photo diode is shown in the figure

Application of photodiode

 The photodiode is used in optical communication system.  The photodiode is used in automotive devices.  The photodiode is used in medical devices.  It is used in solar cell panels.  The Photodiode are used in consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.  It is used for exact measurement of the intensity of light in science & industry.  It is used in character recognition circuit.  It is used in camera light meters, and street lights.  It is used in demodulation.  The photodiode is used in logic circuit.  It is used in photo detection circuits PIN PHOTODIODE

PIN photodiodes are developed from the PN junction photodiodes. The operation of PIN photodiode is similar to the PN junction photodiode except that the PIN photodiode is manufactured differently to improve its performance.

The PIN photodiode is developed to increase the minority carrier current and response speed.

PIN photodiodes generate more electric current than the PN junction photodiodes with the same amount of light energy. ASHWIN JS Layers of PIN photodiode

A PN junction photodiode is made of two layers namely p-type and n-type semiconductor whereas PIN photodiode is made of three layers namely p-type, n-type and intrinsic semiconductor.

In PIN photodiode, an addition layer called intrinsic semiconductor is placed between the p-type and n-type semiconductor to increase the minority carrier current.

P-type semiconductor

If trivalent impurities are added to the intrinsic semiconductor, a p-type semiconductor is formed.

In p-type semiconductors, the number of free electrons in the conduction band is lesser than the number of holes in the valence band. Therefore, holes are the majority charge carriers and free electrons are the minority charge carriers. In p-type semiconductors, holes carry most of the electric current.

N-type semiconductor

If pentavalent impurities are added to the intrinsic semiconductor, an n-type semiconductor is formed.

In n-type semiconductors, the number of free electrons in the conduction band is greater than the number of holes in the valence band. Therefore, free electrons are the majority charge carriers and holes are the minority charge carriers. In n-type semiconductors, free electrons carry most of the electric current.

Intrinsic semiconductor

Intrinsic semiconductors are the pure form of semiconductors. In intrinsic semiconductor, the number of free electrons in the conduction band is equal to the number of holes in the valence band. Therefore, intrinsic semiconductor has no charge carriers to conduct electric current.

However, at room temperature a small number of charge carriers are generated. These small number of charge carriers will carry electric current.

ASHWIN JS PIN photodiode operation

A PIN photodiode is made of p region and n region separated by a highly resistive intrinsic layer. The intrinsic layer is placed between the p region and n region to increase the width of depletion region.

The p-type and n-type semiconductors are heavily doped. Therefore, the p region and n region of the PIN photodiode has large number of charge carriers to carry electric current. However, these charge carriers will not carry electric current under reverse bias condition.

On the other hand, intrinsic semiconductor is an undoped semiconductor material. Therefore, the intrinsic region does not have charge carriers to conduct electric current.

Under reverse bias condition, the majority charge carriers in n region and p region moves away from the junction. As a result, the width of depletion region becomes very wide. Therefore, majority carriers will not carry electric current under reverse bias condition.

However, the minority carriers will carry electric current because they experience repulsive force from the external electric field.

In PIN photodiode, the charge carriers generated in the depletion region carry most of the electric current. The charge carriers generated in the p region or n region carry only a small electric current.

When light or photon energy is applied to the PIN diode, most part of the energy is observed by the intrinsic or depletion region because of the wide depletion width. As a result, a large number of electron-hole pairs are generated.

Free electrons generated in the intrinsic region move towards n-side whereas holes generated in the intrinsic region move towards p-side. The free electrons and holes moved from one region to another region carry electric current.

When free electrons and holes reach n region and p region, they are attracted to towards the positive and negative terminals of the battery.

ASHWIN JS The population of minority carriers in PIN photodiode is very large compared to the PN junction photodiode. Therefore, PIN photodiode carry large minority carrier current than PN junction photodiode.

When forward bias voltage is applied to the PIN photodiode, it behaves like a resistor.

We know that capacitance is directly proportional to the size of electrodes and inversely proportional to the distance between electrodes. In PIN photodiode, the p region and n region acts as electrodes and intrinsic region acts as dielectric.

The separation distance between p region and n region in PIN photodiode is very large because of the wide depletion width. Therefore, PIN photodiode has low capacitance compared to the PN junction photodiode.

In PIN photodiode, most of the electric current is carried by the charge carriers generated in the depletion region. The charge carriers generated in p region or n region carry only a small electric current. Therefore, increasing the width of depletion region increases the minority carrier electric current.

Advantages of PIN photodiode

1. Wide bandwidth

2. High quantum efficiency

3. High response speed

AVALANCHE PHOTODIODE

Avalanche photodiode is a photo detector in which more electron-hole pairs are generated due to impact ionisation. It is like P-N photodiode or PIN photodiode where electron-hole pairs are generated due to absorption of photons but in addition to this avalanche photodiode uses the impact ionisation principle for increasing magnitude of photocurrent.

Construction of Avalanche Photodiode

It has four regions N+ region, P region, an intrinsic layer and P+ region. The N+ and P+ region are heavily doped and the intrinsic layer is lightly doped. Its construction can be understood more clearly with the help of the below diagram.

ASHWIN JS AVALANCHE PHOTODIODE

Working of Avalanche Photodiode

We have already discussed in Photodiodes and PIN Photodiodes that photons striking the surface of diodes contribute to the photocurrent. But in the case of avalanche diode, an additional factor is introduced to impact ionisation which increases photocurrent several times. This additional factor is called avalanche multiplication factor.

Impact ionisation is the process in which one energy carrier with sufficient high kinetic energy strikes bounded energy carrier and imparts its energy to it so that the bounded energy carrier can move freely. This leads to higher concentration of energy carriers and thus higher magnitude of current.

This phenomenon of impact implantation plays a significant role in increasing photocurrent. The current – gain bandwidth product of Avalanche photodiode is about 100 GHz. Thus, this type of photodiode can respond to light modulated at microwave frequencies.

Advantages and Disadvantages of Avalanche Photodiode

1. It can detect very weak signal due to high current-gain bandwidth product.

2. The construction is quite complicated i.e. care should be taken about the junction. The junction should be uniform and the guard ring is used to protect the diode from edge breakdown.

Applications of Avalanche photodiode

Due to its ability to detect low-level signals it is used in fibre optic communication Systems. A properly designed silicon avalanche photodiode can provide a response time of about 1ns. ASHWIN JS PHOTOTRANSISTOR

The phototransistor is a device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives.

Phototransistors and photodiodes can both be used for sensing light, but the phototransistor is more sensitive in view of the gain provided by the transistor. This makes phototransistors more suitable in a number of applications.

Phototransistor operation

The phototransistor uses the basic transistor concept as the basis of its operation. In fact a phototransistor can be made by exposing the semiconductor of an ordinary transistor to light. Very early photo transistors were made by not covering the plastic encapsulation of the transistor with black paint.

The photo transistor operates because light striking the semiconductor frees electronics / holes and causes current to flow in the base region.

Photo transistors are operated in their active regime, although the base connection is generally left open circuit or disconnected because it is often not required. The base of the photo transistor would only be used to bias the transistor so that additional collector current was flowing and this would mask any current flowing as a result of the photo-action. For operation the bias conditions are quite simple. The collector of an n-p-n transistor is made positive with respect to the emitter or negative for a p-n-p transistor.

The light enters the base region where it causes hole electron pairs to be generated. This generation mainly occurs in the reverse biased base-collector junction. The hole-electron pairs move under the influence of the electric field and provide the base current, causing electrons to be injected into the emitter. As a result the photodiode current is multiplied by the current gain β of the transistor.

The performance of the phototransistor can be superior to that of the photodiode for some applications in view of its gain. As a rough guide, where a photodiode may enable a current flow of around 1µA under typical room conditions, a phototransistor may allow a current of 100µA to flow. These are very rough approximations, but show the order of magnitude of the various values and comparisons.

Phototransistor structure

Although ordinary transistors exhibit the photosensitive effects if they are exposed to light, the structure of the phototransistor is specifically optimised for photo applications. The photo transistor has much larger base and collector areas than would be used for a normal transistor. These devices were generally made using diffusion or ion implantation.

ASHWIN JS Characteristics :  Low-cost visible and near-IR photo detection.  Available with gains from 100 to over 1500.  Moderately fast response times.  Available in a wide range of packages including epoxy-coated, transfer-molded and surface mounting technology.  Electrical characteristics similar to that of signal transistors.

Phototransistor applications

As a result of their ease of use and their applications, phototransistors are used in many applications.

 Opto-isolators - here the phototransistor is used as the light sensor, the light emitter being relatively close, but at a different potential. The physical gap between the light emitter and detector provides a considerable degree of electrical isolation.

 Position sensing - in this application the optoisolator can be used to detect the position of a moving element, often the moving element has a light or interrupts a beam of light which the phototransistor detects.

 Security systems - phototransistor can be used in many ways in security systems, often detecting whether a beam of light is present or has been broken by an intruder.

 Coin counters - phototransistor can be used in coin and other counting applications. A beam of light is interrupted each time a coin or other item passes a given point. The number of times the beam is interrupted equals the number of coins or objects to be counted.