Optoelectronic Devices ELEX Notes PDF

# Optoelectronic Devices ## Optoelectronic Devices - The electronic technology in which optical radiation is emitted, modified, or converted (as in electrical-to-optical or optical-to-electrical). ## Related Technologies - **Photonics** - science and technology concerned with the the behavior of p...

# Optoelectronic Devices ## Optoelectronic Devices - The electronic technology in which optical radiation is emitted, modified, or converted (as in electrical-to-optical or optical-to-electrical). ## Related Technologies - **Photonics** - science and technology concerned with the the behavior of photons. - **Electronics** - science and technology concerned with the behavior of electrons. ## Types of Devices - **Photodiodes (PDs)** - optical radiation is converted to an electrical signal. - **Light-Emitting Diodes (LEDs)** - electrical energy is converted to an optical signal. - **Laser Diodes (LDs)** - electrical energy is converted to optical energy in laser form. ## Application Examples - Fiber optic communications - Image processing - Optical sensing ## Other Definitions - **Photon** - a quantum of electromagnetic energy with no mass, no charge, and energy $hc/λ$. - **Light** - electromagnetic radiation in the ultraviolet, visible, and infrared bands or optical range. - **Electromagnetic (EM) Spectrum** - radiation of all frequencies or wavelengths including electrical power transmission, radio frequencies, optical frequencies, and high-energy rays. Wavelength (in vacuum) or frequency f are related by $λf = c$, where c is the speed of light in vacuum. - **Radiation** - energy emitted or propagated as waves and energy quanta. - **Radiometry** - the measurement of radiant EM energy at specific wavelength ranges. # Electromagnetic Energy ## Electromagnetic Energy - The propagation of electromagnetic energy may be characterized by the vacuum wavelength $λ$, frequency $f = ω/2π$, or quantum energy $E_p$. - These waves travel with a phase velocity of $v_p$. - Material influences can be described by the index of refraction or refractive index n for light propagation. ## Relationships - The wavelength and frequency are related as: $λf = c$, where c is the speed of light in vacuum. - The quantum energy is: $E_p = hf = hc/λ$, where h is Planck's constant. - The phase velocity is: $v_p = c/n$, where n is the refractive index (n is unitless and is equal to or greater than 1). ## Divisions of the Electromagnetic Spectrum - **Radio Frequency (RF)** - electromagnetic radiation band with frequencies between about 10 kHz and 300,000 MHz. - **Shortwave Spectrum** - EM band with wavelengths between about 200 meters and 20 meters; includes the middle bands of radio frequencies. - **Microwave Spectrum** - EM band with wavelengths between about 1 meters and 1 millimeters; includes the upper bands of radio frequencies. - **Light** - electromagnetic radiation in the ultraviolet, visible, and infrared bands or optical range with wavelengths between about 1 nm and 10^5 nm. - **X-rays** - electromagnetic radiation with wavelengths between about 10 nm and 0.01 nm; usually described as high-energy photons. # Optics: The Nature of Light ## Light - Light refers to radiation in ultraviolet, visible, and infrared portions of the electromagnetic spectrum. - The wavelength $λ$ is associated with color in the visible portion of the spectrum. - The effective wavelength inside a material, as well as the phase velocity of light, is then decreased by the refractive index. - The effective wavelength and phase velocity are then $λ/n$ and $c/n$. ## Preferred Designations - **Wavelength**: vacuum wavelength rather than frequency *f* or energy. - **Semiconductor Applications**: photon energy $E_p$. ## Optical Spectrum - Optical wavelengths extend beyond what the eye can detect and include wavelengths between about 1 nm and 10^5 nm which interact with materials in similar ways. - The physical mechanisms behind many optical sources are electronic and molecular transition and the methods of beam control often depend on reflection and refraction from interfaces. - Light is subdivided into the ultraviolet, visible, and infrared bands. - There are not precise wavelength divisions between bands. | Light Bands | Lower Wavelength Limit | Upper Wavelength Limit | |---|---|---| | Infrared (IR) | about 700 nm | about 10^5 nm | | Visible | about 450 nm | about 700 nm | | Ultraviolet (UV) | about 1 nm | about 450 nm | # Light Analysis ## Wave-Photon Duality - Light is unusual in that it readily displays both wave-like and quantum behavior. - Many of the properties of light can be adequately described by waves. - Notable exceptions include some cases of emission and absorption. - The concept of photons, which were part of the development of quantum mechanics, came from the photoelectric emission. - Laser diodes and photodiodes can only be explained using a quantum description of semiconductors and light. - The quantum energy $E_q = hf = hc/λ$, where h is Planck’s constant. | Wavelengths | Frequency | Quantum Energy | |---|---|---| | 10^5 nm | 2.998 x 10^12 Hz | 1.240 x 10^-02 eV | | 700 nm | 4.283 x 10^14 Hz | 1.771 eV | | 450 nm | 6.662 x 10^14 Hz | 2.755 eV | | 1 nm | 2.998 x 10^17 Hz | 1.240 x 10^3 eV | ## Analysis of Optical Phenomena - **Ray Optics** - a geometric representation of the behavior of light (also called geometrical optics) which corresponds to the limiting case of $λ → 0$. - **Electromagnetic (EM) Optics** - or physical optics, an electromagnetic representation of the behavior of light using Maxwell’s equations (limiting case as photon number approaches ∞). - **Quantum Optics** - the most general representation of the behavior of light in terms of photons, i.e. radiant energy packets, using quantum mechanics. ## Interaction with Semiconductors - **Absorption** - loss due to energy conversion as light passes through a material. Photons can be absorbed by electrons causing an upward band-to-band transition. - **Emission** - conversion of energy into light. Photons can be emitted as electrons undergo a downward band-to-band transition. # Semiconductor Optical Devices ## Photodetectors - **Semiconductor Structures** - incident photons are converted by semiconductor structures into usable electrons. - Solar cells convert light to electrical power. - Common devices for imaging and information detection are CCDs (charge coupled devices), photodiodes, and avalanche photodiodes. - The design of semiconductor photodetectors depends on the wavelength sensitivity of the materials. - Silicon and germanium are sensitive across the visible spectrum and near infrared spectrum. - The compound semiconductors InGaAsP and GaAs are sensitive in the upper visible and near infrared. - The peak quantum efficiency (electronic conductors generated per incident photon) of about 900 nm in silicon is near its upper cut-off wavelength of 1100 nm. - Detectors made of materials such as InGaAsP are optimized for important optical fiber wavelengths of 1300 nm and 1550 nm. | Wavelength | Material | |---|---| | Ultraviolet | Si | | Ultraviolet | Ge | | Visible | InGaAsP | | Visible | GaAs | | Near Infrared | InGaAsP | | Near Infrared | GaAs | ## Semiconductor Sources - **Light Emitting Diodes** and **Laser Diodes** - Light is emitted due to quantum transitions in gases, liquids, or solids. - Injection electroluminescence is the mechanism for light emission in semiconductor diodes. - Carriers make a downward transition from the conduction band to the valence band and emit light. - Laser sources are characterized by high coherence and directionality over an extremely narrow spectrum. # Light Absorption ## Attenuation - A media in which light is propagating may be lossy. - For a given wavelength, the irradiance will decrease at a rate proportional to the irradiance magnitude. - The resulting loss may be represented by an attenuation constant $α$, in units of inverse meter (1/m). - The attenuation constant is positive for lossy media, zero for lossless media, and negative for media with gain. ## Equation - For one-dimension, the defining differential equation for the irradiance (irradiance I with units of W/m²) amplitude is: $dI/dx = - α₁I$. - The solution is: $I = I_0 exp(- αx)$. - Where $I_0$ is the value of I at the position (x=0). # PN Photodiodes ## Photodiode - An optoelectronic device that is based on a semiconductor junction which absorbs light and converts the light input to a current. - Incident photons are absorbed through upward bandgap transitions. - Photon-induced carriers contribute to the drift current if absorbed within or at the edge of the transition region W. ## Photodiode Current - $I = I_0 [exp(qV/kT) - 1] - I_{Light}$. ## Photon-generated Current - $I_{Light} = nqP/hc$. - Where - $η$ = efficiency = carriers generated per incident photon - q = charge per carrier - P = optical power absorbed (J/s) - $hc/λ$ = energy per incident photon (J/photon) - note: $Pl/hc$ = incident photons per second ## Photoconductive Mode - The current-voltage behavior for reverse bias (negative V) depends on the incident light. - Reverse bias with $V << 0$, the current is magnitude is proportional to the optical power with a small offset. $I = I_0 [exp(qV/kT) - 1] - I_{Light}$. $I~-I_0 - I_{Light}$. ## Maximizing Efficiency - Primary absorption in transition region - Large transition region for large absorption percentage - Little recombination in transition region (carriers must transit to drift current) (Note that photo-generated carriers must exit transition region before recombination. Need to have a small recombination lifetime, a small W, and/or a large electric field in W.) # Photodiode Types - PIN Structure ## PIN Photodiode - **Structure**: "i" intrinsic region (commonly p- or n-) between regions with "p" and "n" doping. - **Efficiency**: $η < 1$ ## Example Structure - A diagram of a PIN structure with labeled regions is shown in the image, including n+, i (p-), and pp+. ## Structural Design and Function - Thin n+ region to allow light to reach the transition region. - n+ region provide electrons as the primary carrier (faster than holes). - Transition region width matched to absorption needs (from α₁). - Transition region width primarily i-region for voltage insensitivity. - Large electric field throughout transition region. # Photodiode Types - Avalanche Structure ## Avalanche Photodiode (APD) - **Structure**: "p-"region for absorption and "p" region for avalanche multiplication between regions with "p+" and "n+" doping. - **Efficiency**: $η > 1$ (one photon gives many carriers by avalanche gain) ## Example Structure - A diagram of a Avalanche structure with labeled regions is shown in the image, including p+, p-, pn+, and n+. ## Design - Thin p+ region to allow light to reach the transition region. - n+ region provide electrons as the primary carrier (faster than holes). - p- transition region width matched to absorption needs (from α₁). - Separate p multiplication region with large electric field. # Light Emitting Diodes ## Emission - A process that converts energy into light. ## Injection Electroluminescence - A "direct-bandgap" semiconductor such as GaAs (Si and GE are indirect semiconductors and do not emit light). Injected carriers recombine with a light-emitting transition. The transition can be spontaneous or stimulated and occurs near the edges of the depletion region in a diode structure. ## Light Emitting Diode (LED) - An optoelectronic device that emits non-coherent optical radiation at a photon energy close to bandgap of the junction. - Structure: Typically a p+n or n+p diode such that the main transitions occur on the n-side or p-side respectively of the depletion region. - Operation: Forward-bias effect producing spontaneous emission. ## Example Band Structure for n+p diode (Forward Bias) - A table showing the band structure is shown, including the regions p-type & n-type. - The current is primarily electron flow and the main recombination region is the edge of the depletion region on the p-side.. - The optical output increases with forward-bias diode current. ## Heterostructures - The use of semiconductors with different bandgaps are often used such that the photons are emitted in a small bandgap semiconductor and exit the diode through a larger bandgap semiconductor. # Laser Diodes ## Laser - Light amplification by stimulated emission radiation. - A process that emits optical radiation which is coherent, highly directional, and nearly monochromatic. - The spectral purity of laser light is a key property, i.e. the output for a laser has an extremely small spectrum (wavelength spread). ## Lasing Operation - The key components of lasers are: - **Active Material** - a material in which energy is converted into light (A material in which energy is converted into light) - **Pumping Mechanism** - a mechanism to excite ions, electrons, or molecules so that a light-emitting transition is produced. - **Resonant Cavity** - a structure to produce optical feedback, i.e. light is amplified through stimulated emission verses spontaneous emission. ## Laser Diode (LD) - An optoelectronic device that is based on a semiconductor junction which emits optical laser radiation at a photon energy close to bandgap of the junction. The key components are: - **Active Material** - Direct-bandgap Semiconductors - **Pumping Mechanism** - Diode Junction Structure (Injection Electroluminescence) Heterostructures are common. - **Resonant Cavity** - A Waveguide Structure combined with End-face Mirrors ## Typical Output Characteristic (Forward Bias) - A graph is shown, showing the output power of a Laser as a function of diode current. - The Threshold current is the diode current needed to produce stimulated emission, i.e. it is the onset of lasing. - A laser diode is highly efficient in that it converts a high fraction of the electrical energy into useful optical output. # Transducers ## Transducer Definition - A transducer may be defined as any device that converts the energy from one form to another. - Most of the transducers either convert electrical energy in to mechanical displacement and convert some non electrical physical quantities like temperature, Light, Pressure, Force, Sound etc to an electrical signals.. - In an electronics instrument system the function of transducers is of two types: - To detect or sense the pressure, magnitude and change in physical quantity being measured.. - To produce a proportional electrical signal. ## Classification - The Classification of Transducers is done in many ways. - Some of the criteria for the classification are based on their area of application, Method of energy conversion, Nature of output signal, According to Electrical principles involved, Electrical parameter used, principle of operation, & Typical applications. ## Broad Classification of Transducers - **On the basis of transduction form used:** - Primary and secondary transducers - **Active and passive transducers:** - Transducers and inverse transducers. ## Primary and Secondary Transducers - **Primary transducer:** When the input signal is directly sensed by the transducer and physical phenomenon is converted into the electrical form directly then such a transducer is called the primary transducer. - **Secondary transducer:** When the input signal is sensed first by some detector or sensor and then its output being of some form other than input signals is given as input to a transducer for conversion into electrical form, then such a transducer falls in the category of secondary transducers. ## Active and Passive Transducers - **Active Transducers:** Self-generating type transducers i.e. the transducers, which develop their output the form of electrical voltage or current without any auxiliary source, are called the active transducers. Such transducers draw energy from the system under measurement. Normal such transducers give very small output and, therefore, use of amplifier becomes essential. - **Passive Transducers:** Transducers, in which electrical parameters i.e. resistance, inductance or capacitance changes with the change in input signal, are called the passive transducers. These transducers require external power source for energy conversion. In such transducer electrical parameters i.e. resistance, inductance or capacitance causes a change in voltages current or frequency of the external power source.. ## Transducers and Inverse Transducers - **Transducer:** A device that converts a non-electrical quantity into an electrical quantity. - **Inverse Transducer:** A device that converts an electrical quantity into a non-electrical quantity. It is a precision actuator having an electrical input and a low-power non-electrical output.. ## Examples of Inverse Transducers - Piezoelectric crystal - Transnational and angular moving-coil elements - Many data-indicating and recording devices are basically inverse transducers - An ammeter or voltmeter converts electric current into mechanical movement and the characteristics of such an instrument placed at the output of a measuring system are important. - A most useful application of inverse transducers is in feedback measuring systems. # Capacity Transducers ## Variable Capacitance Pressure Gage - **Principle of operation**: Distance between two parallel plates is varied by an externally applied force. - **Applications**: Displacement, pressure ## Capacitor Microphone - **Principle of operation**: Sound pressure varies the capacitance between a fixed plate and a movable diaphragm. - **Applications**: Speech, music, noise ## Dielectric Gage - **Principle of operation**: Variation in capacitance by changes in the dielectric. - **Applications**: Liquid level, thickness # Inductance Transducers ## Magnetic Circuit Transducer - **Principle of operation**: Self inductance or mutual inductance of ac-excited coil is varied by changes in the magnetic circuit. - **Applications**: Pressure, displacement ## Reluctance Pickup - **Principle of operation**: Reluctance of the magnetic circuit is varied by changing the position of the iron core of a coil. - **Applications**: Pressure, displacement, vibration, position ## Differential Transformer - **Principle of operation**: The differential voltage of two secondary windings of a transformer is varied by positioning the magnetic core through an externally applied force. - **Applications**: Pressure, force, displacement, position ## Eddy Current Gage - **Principle of operation**: Inductance of a coil is varied by the proximity of an eddy current plate. - **Applications**: Displacement, thickness ## Magnetostriction Gage - **Principle of operation**: Magnetic properties are varied by pressure and stress. - **Applications**: Force, pressure, sound # Voltage and Current Transducers ## Hall Effect Pickup - **Principle of operation**: A potential difference is generated across a semiconductor plate (germanium) when magnetic flux interacts with an applied current. - **Applications**: Magnetic flux, Current ## Ionization Chamber - **Principle of operation**: Electron flow induced by ionization of gas due to radioactive radiation. - **Applications**: Particle counting, radiation ## Photoemissive Cell - **Principle of operation**: Electron emission due to incident radiation on photoemissive surface. - **Applications**: Light and radiation ## Photomultiplier Tube - **Principle of operation**: Secondary electron emission due to incident radiation on photosensitive cathode. - **Applications**: Light and radiation, photo-sensitive relays # Transducer Classification by Output - **Analog Transducer**: Converters input signal into the output signal of the form such as thermistor, strain gauge, LVDT, thermo-couple etc. - **Digital Transducer**: Converters input signal into the output signal of the form of pulse e.g. it gives discrete output. ## Advantages of Digital Transducers - They are becoming more and more popular now-a-days because of advantages associated with digital measuring instruments and also due to the effect that digital signals can be transmitted over a long distance without causing much distortion due to amplitude variation and phase shift. - A digital transducer is the same as an analog transducer combined with an ADC (analog-digital convertor). # Basic Requirements of a Transducer - Transducers are the input element in a measurement system, transforming some physical quantity to a proportional electrical signal. ## Factors Influencing The Choice of a Transducer 1. **Operating principle** 2. **Sensitivity** 3. **Operating Range** 4. **Accuracy** 5. **Cross Sensitivity** 6. **Errors** 7. **Transient and Frequency Response** 8. **Loading Effects** 9. **Environmental Compatibility** 10. **Insensitivity to Unwanted Signals** 11. **Usage and Ruggedness** 12. **Electrical aspects** 13. **Stability and Reliability** 14. **Static Characteristics** # Resistive Transducer ## Resistive Transducer Definition - The variable resistance transducers are one of the most commonly used types of transducers. - They are also called as resistive transducers or resistive sensors. - They can be used for measuring various physical quantities like temperature, pressure, displacement, force, vibrations etc. ## Application - Resistive transducers are usually used as the secondary transducers, where the output from the primary mechanical transducer acts as the input for the variable resistance transducer. - The output obtained from it is calibrated against the input quantity and it directly gives the value of the input. ## Principle of Working - The variable resistance transducer elements work on the principle that the resistance of the conductor is directly proportional to the length of the conductor and inversely proportional to the area of the conductor. - Thus, if L is the length of the conductor (in m) and A is its area (in m square), its resistance (in ohms) is given by: $R = PL/A$. - Where $P$ is called as resistivity of the material and it is constant for the materials and is measured in $ohm-m$. # Strain Gauges ## Strain Gauge Definition - Strain gauges are devices whose resistance changes under the application of force or strain. - They can be used for measurement of force, strain, stress, pressure, displacement, acceleration etc. ## Why Strain Gauges are Needed - It is often easy to measure the parameters like length, displacement, weight etc that can be felt easily by some senses. - However, it is very difficult to measure the dimensions like force, stress and strain that cannot be really sensed directly by any instrument. - For such cases special devices called strain gauges are very useful. ## Principle of Working - There are some materials whose resistance changes when strain is applied to them or when they are stretched and this change in resistance can be measured easily. - For applying the strain you need force, thus the change in resistance of the material can be calibrated to measure the applied force. - Thus, the devices whose resistance changes due to applied strain or applied force are called as the strain gauges. - When force is applied to any metallic wire its length increases due to the strain. - The more is the applied force, more is the strain and more is the increase in length of the wire. - If $L_1$ is the initial length of the wire and $L_2$ is the final length after application of the force, the strain is given as: $ε =(L_2-L_1)/L_1$. ## Additional Strain Gauge Information - Further, as the length of the stretched wire increases, its diameter decreases. - Now, we know that resistance of the conductor is the inverse function of the length. - As the length of the conductor increases its resistance decreases. - This change in resistance of the conductor can be measured easily and calibrated against the applied force. - Thus, strain gauges can be used to measure force and related parameters like displacement and stress. - The input and output relationship of the strain gauges can be expressed by the term gauge factor or gauge gradient, which is defined as the change in resistance R for the given value of applied strain $ε$. - Consider a wire strain gage, as illustrated above. The wire is composed of a uniform conductor of electric resistivity $Γ$ with length $l$ and cross-section area A. Its resistance R is a function of the geometry given by: $R = Γl/A$. - The resistance change rate is a combination effect of changes in length, cross-section area, and resistivity. $dR = Γ/A dl - Pl dA + l dΓ$. $dR/R = dl/l - dA/A + dΓ Γ$. - When the strain gage is attached and bonded well to the surface of an object, the two are considered to deform together. - The strain of the strain gage wire along the longitudinal direction is the same as the strain on the surface in the same direction. $ε₁ = dl/l$. - However, its cross-sectional area will also change due to the Poisson’s ratio.. - Suppose that the wire is cylindrical with initial radius r. The normal strain along the radial direction is: $ε_γ = dr/r = -νε₁ = -ν dl/l$. - The change rate of cross-section area is twice as the radial strain, when the strain is small. $dA/A = (1+ε_γ)²-1 = 2ε_γ + ε_γ² ≈ 2ε_γ$. $dA/A = -2ν dl/l$. - The resistance change rate becomes: $$dR/R = (dl/l - (dA/A) + (dΓ/Γ)$$ $$dR/R = (dl/l + 2ν dl/l) + (dΓ/Γ)$$ $$dR/R = (1+2ν) dl/l + (dΓ/Γ)$$ - For a given material, the sensitivity of resistance versus strain can be calibrated by the following equation. $S = dR/R$ $S = ε₁+ 2ν + dΓ/Γ$. - When the sensitivity factor S is given, (usually provided by strain gage vendors) the average strain at the point of attachment of the strain gage can be obtained by measuring the change in electric resistance of the strain gage. $ε₁ = dR/R/S = AR/SR$. ## Materials used for the Strain Gauges - Earlier wire types of strain gauges were used commonly, which are now being replaced by the metal foil types of gauges. - The metals can be easily cut into the zigzag foils for the formation of the strain gauges. - One of the most popular materials used for the strain gauges is the copper-nickel-manganese alloy. - Some semiconductor materials can also be used for making the strain gauges. ## Types of Strain Gauges based on principle of working 1. **Mechanical:** It is made up of two separate plastic layers. The bottom layer has a ruled scale on it and the top layer has a red arrow or pointer. One layer is glued to one side of the crack and one layer to the other.. As the crack opens, the layers slide very slowly past one another and the pointer moves over the scale. The red crosshairs move on the scale as the crack widens. Some mechanical strain gauges are even cruder than this. The piece of plastic or glass is sticked across a crack and observed its nature. 2. **Electrical:** The most common electrical strain gauges are thin, rectangular-shaped strips of foil with maze-like wiring patterns on them leading to a couple of electrical cables. When the material is strained, the foil strip is very slightly bent out of shape and the maze-like wires are either pulled apart (so their wires are stretched slightly thinner) or pushed together (so the wires are pushed together and become slightly thicker). Changing the width of a metal wire changes its electrical resistance.. This change in resistance is proportional to the stress applied. If the forces involved are small, the deformation is elastic and the strain gauge eventually returns to its original shape. 3. **Piezoelectric:** Some materials such as quartz crystals and various types of ceramics are effectively "natural" strain gauges. When pushed and pulled, they generate tiny electrical voltages between their opposite faces. This phenomenon is called piezoelectricity. By measuring the voltage from a piezoelectric sensor we can easily calculate the strain. Piezoelectric strain gauges are the most sensitive and reliable devices. ## Electrical Strain Gauge - A strain gauge takes advantage of the physical property of electrical conductance. - It does not depend on merely the electrical conductivity of a conductor, but also the conductor’s geometry. - When an electrical conductor is stretched within the limits of its elasticity such that it does not break or permanently deform, it will become narrower and longer. - Similarly, when it is compressed, it will broaden and shorten. - The change in the resistance is due to variation in the length and cross sectional area of gauge wire ## Gauge factor - The characteristics of the strain gauges are described in terms of its sensitivity (gauge factor). - Gauge factor is defined as unit change in resistance for per unit change in length of strain gauge wire given as: $G.F. = (AR/RG)/ε$ - Where, - $AR$ - the change in resistance caused by strain, - $RG$ - is the resistance of the undeformed gauge, and - $ε$ - Strain. ## Types of strain gauge based on construction - **Optical sensors** are sensitive and accurate, but are delicate and not very popular in industrial applications. - They use interference fringes produced by optical flats to measure strain. - Optical sensors operate best under laboratory conditions. - **The photoelectric gauge** uses a light beam, two fine gratings, and a photocell detector to generate an electrical current that is proportional to strain. - The gage length of these devices can be as short as 1/16 inch, but they are costly and delicate. - **Semiconductor strain gauges:** For measurements of small strain, semiconductor strain gauges, so called piezo-resistors, are often preferred over foil gauges. - Semiconductor strain gauges depend on the piezo-resistive effects of silicon or germanium and measure the change in resistance with stress as opposed to strain. - The semiconductor bonded strain gauge is a wafer with the resistance element diffused into a substrate of silicon. - The wafer element usually is not provided with a backing, and bonding it to the strained surface requires great care as only a thin layer of epoxy is used to attach it. - The size is much smaller and the cost much lower than for a metallic foil sensor. - The same epoxies that are used to attach foil gages are used to bond semiconductor gages. - The advantages are higher unit resistance and sensitivity whereas, greater sensitivity to temperature variations and tendency to drift are disadvantages in comparison to metallic foil sensors. - Another disadvantage of semiconductor strain gages is that the resistance-to-strain relationship is nonlinear. - With software compensation this can be avoided. - **Thin-film strain gauge:** These gauges eliminate the need for adhesive bonding. The gauge is produced by first depositing an electrical insulation (typically a ceramic) onto the stressed metal surface, and then depositing the strain gauge onto this insulation layer. Vacuum deposition or sputtering techniques are used to bond the materials molecularly. Because the thin-film gauge is molecularly bonded to the specimen, the installation is much more stable and the resistance values experience less drift. Another advantage is that the stressed force detector can be a metallic diaphragm or beam with a deposited layer of ceramic insulation. - **Diffused semiconductor strain gauges:** This is a further improvement in strain gage technology as they eliminate the need for bonding agents. - By eliminating bonding agents, errors due to creep and hysteresis also are eliminated. - The diffused semiconductor strain gage uses photolithography masking techniques and solid-state diffusion of boron to molecularly bond the resistance elements. - Electrical leads are directly attached to the pattern. - The diffused gauge is limited to moderate-temperature applications and requires temperature compensation. - Diffused semiconductors often are used as sensing elements in pressure transducers. - They are small, inexpensive, accurate and repeatable, provide a wide pressure range, and generate a strong output signal. - Their limitations include sensitivity to ambient temperature variations, which can be compensated for in intelligent transmitter designs. ## Types of strain gauge based on mounting - **Bonded strain gauge** - A bonded strain-gage element, consisting of a metallic wire, etched foil, vacuum-deposited film, or semiconductor bar, is cemented to the strained surface. - **Unbonded Strain Gauge** - The unbonded strain gage consists of a wire stretched between two points in an insulating medium such as air. One end of the wire is fixed and the other end is attached to a movable element. # Inductive Transducers ## Inductive Transducer Definition - The variable inductance transducers work generally upon of the following three principals: - Change of self inductance - Change of mutual inductance - Production of eddy current ## Linear Variable Differential Transformer – LVDT Transducer - The differential transformer transducer measures force in terms of the displacement of the ferromagnetic core of a transformer. - The basic construction of the LVDT is given in Figure 9. - The transformer consists of a single primary winding and two secondary windings which are placed on either side of the primary. - The secondaries have an equal number of turns but they are connected in series opposition so that the emfs induced in the coils OPPOSE each other. - The position of the movable core determines the flux linkage between the ac-excited primary winding and each of the two secondary winding. ## How the Core Position Affects the Output - Relative positions of the core generate the indicated output voltages as shown in Figure. - The linear characteristics impose limited core movements, which are typically up to 5 mm from the null position. - With the core in the center, (or reference position or Figure), the induced emfs in the secondaries are equal, and since they oppose each other, the output voltage will be 0 V. - When an externally applied force moves the core to the left-hand position, more magnetic flux links the left-hand coil than the right-hand coil and the Differential Output E0 = ES1 - ES2 Is in-phase with Ei as ES1 > ES2. - The induced emf of the left hand coil is therefore larger than the induced emf of the right-hand coil. - The magnitude of the output voltage is then equal to the difference between the two secondary voltages, and it is in phase with the voltage of the left-hand coil. - Similarly, when the core is forced to move to the right, more flux links the right-hand coil than the left-hand coil and the resultant output voltage is now in phase with the emf of the right-hand coil, while its magnitude again equals the difference between the two induced emfs. - Ideally the output voltage at the null position should be equal to zero. - In actual practice there exists a small voltage at the null position. This may be on account of presence of harmonics in the input supply voltage and also due to harmonics produced in the output voltage due to use of iron Displacement core. - There may be either an incomplete magnetic or electrical unbalance or both which result in a finite output voltage at the null position. - This finite residual voltage is generally less than 1% of the maximum output voltage in the linear range. - Other causes of residual voltage are stray magnetic fields and temperature effects. ## Advantages of LVDT - **High Range:** The LVDTs has a very high range for measurement of displacement This can be used for measurement of displacement ranging from 1.25 mm to 2.50 mm. - **Friction and Electrical Isolation** - **Immunity from External Effects** - **High input and high sensitivity** - **Ruggedness:** The transducer can usually tolerate high degree of shock and vibration - **Low Hysteresis** - **Low Power consumption** ## Disadvantages of LVDT - **Relatively large displacement are required for appreciable differential output** - **They are sensitivity to stray magnetic fields but shielding is possible** - **Many times, the transducer performance is affected by vibrations** - **The receiving instrument must be selected to operate on ac signal** - **The dynamic response is limited mechanically by the mass of the core and electrically by frequency of applied voltage. The frequency of the carrier of the carrier should be at least ten times the highest frequency component to be measured** - **Temperature affects the performance** ## Applications of LVDT - Acting as a secondary transducer it can be used as a device to measure force, weight and pressure etc. - The force measurement can be done by using a load cell as the primary transducer while fluid pressure can be measured by using Bourdon tube which acts as primary transducer. - The force or the pressure is converted into a voltage. - In