EE 5344 Introduction to MEMS CHAPTER 5 Radiation Sensors

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1 EE 5344 Introduction to MEMS CHAPTER 5 Radiation Sensors 5. Radiation Microsensors Radiation µ-sensors convert incident radiant signals into standard electrical out put signals. Radiant Signals Classification According to Weight 1. Electromagnetic 1. Baryons (heavy) 2. Neutrons 2. Mesons (medium) 3. Fast Electrons 3. Leptons (light) 4. Heavy-Charged Particles Electromagnetic Go over the EM Spectra. Fast Electrons Mass of an electron, but (+) or (-) charge, corresponding to a β-particle or a positron. + β β (Leptons) Neutrons Charge is zero. Symbol is n. (Baryon) Heavy Charged Particles α particle (2 protons + 2 neutrons, He ++ ) charge:+2 Proton (p) (charge +2) Both are Baryons. In Addition Mesons Such as Pions and Kaons-Charge can be positive, neutral, or negative π + π π - K + K K Classification of Radiation Sensors They are non-contacting, because they detect emission. 1 / Radiation Sensors

2 5.2 Nuclear Radiation Microsensors Measure nuclear particles such as α, and β particles as well as radiation such as γ-rays and x-rays Scintillation Counters (photons) pulses of light Incident nuclear radiation Photomultiplier tube Processor amplifier Scintillator (inorganic or organic crystal) Pulse height α E R (Energy of radiation) Pulse count rate α Φ R (Radiation flux) In addition to the materials listed in Table 6.3. ZnO (doped with gallium) and ZnS (doped with silver) are also used Solid-State Detectors Mostly semiconductor materials are used (CdTe, HgI 2, GaAs, Si, Ge). Electromagnetic energy interacts with semiconductors primarily through absorption processes. Absorption: relative change of irradiance or incident radiation intensity per unit path (distance into the material) δi R ( x) = αdx x: distance into the material α: absorption coefficient I R : Radiation intensity Solution: IR IR( x) = IR o exp( αx) The total absorption coefficient is the sum of three mechanisms: 2 / Radiation Sensors

3 1. Photoelectric effect. 2. Compton effect (scattering). 3. Pair production. Photoelectric Effect Dominates at low radiation energies (E<100 kev) and is important of detecting X-rays and γ-rays. The photon transfers all its energy to an electron knocking it off its lattice. The electron becomes totally unbound. photon with Energy hf e Vacuum level nucleus E b Free electron with energy E k E k +E b =hf E b =binding energy that depends on the material and the energy shell of the electron. Electron-hole pairs Increased conductivity Compton Scattering Dominant at intermediate energies. (100 KeV-1MeV) Important at the upper end of the X-ray spectrum. The photon gets scattered by ionizing collision, transferring only its partial energy to electrons. The scattered photon, can then participate in another Compton or a photoelectric event, until it loses all its energy. Compton effect does not involve a threshold as does the photoelectric effect. 3 / Radiation Sensors

4 Pair Production Pair production involves the conversion of electromagnetic energy into rest energy through a photon transformation into an electron and positron. m o = electron and positron rest mass. E = 2moc 2 = 1. 02MeV A positron has the same mass, but opposite charge of an electron and annihilates with an electron, releasing two 0.51 MeV X rays that may be absorbed through Compton scattering. Photoconducting Nuclear Sensors The incident photocurrent modulates the device conductance, which can be measured. I ph : incident photocurrent P opt : radiant power I ph qηp = hf opt P opt /hf = incident photons per unit time η : quantum efficiency However the dark conductivity of most semiconductors is already high. Therefore, it is hard to detect the incremental increase due to radiation. Solution: P-N Photodiode Nuclear Sensors Reverse biased p-n junction depletion region P : : n + + : + + : + + : + + : + + : ++: 4 / Radiation Sensors

5 The photons generate electron-hole pairs in the depletion region and therefore cause a significant increase in the reverse saturation current. 5.3 UV, Visible and NIR Radiation Microsensors Ultraviolet (UV): α 0.4 µm Visible (Vis): µm Infrared (IR): µm Near-IR mid-ir Far-IR µm 1.7µm-5µm 12µm-500µm 1. Photon Radiation Detectors 2. Thermal Radiation Detectors Long-IR 5µm-12µm Photon Radiation Detectors: Photo-conductive cells, photo diodes, photo-transistors Thermal Radiation Detectors: Pyroelectric sensors, bolometers. Fig 6.6 Range of Radiation µ-sensors Photo-conductive Cells Photo-conductive effect is utilized. When photons hit the semiconductor electron-hole pairs are generated. This significantly decreases the resistivity (increases the conductivity. Conductivity in dark: σ = q nµ + pµ ( ) n p carrier concentration mobilities Change in conductivity with light: σ = qn µ τ + µ τ t ( ) n p p N t = number of carriers (electron-hole pairs) generated per second per unit volume µ n, µ p = mobilities τ n,τ p = life-times of carriers 5 / Radiation Sensors

6 then this change in the conductivity can be sensed as resistance change. ρl L R = = A Aσ L R = σ 2 σ A CdS is widely used for visible region. The lifetime of holes is short, τ p <<1 That it behaves like an-type semiconductor σ q N t µ n τ n Photodiodes pn photodiodes pin photodiodes Schottky photodiodes Avalanche photodiodes Photodiodes are more sensitive than photoconductors. They have a faster response. They have excellent linearity for UV-NIR region. a) pn type (Si or GaAsP) o SiO p Si n Si Can be operated in reverse bias or virtual earth. 6 / Radiation Sensors

7 I L R L A V out = AI L R L reverse bias - + R f I sc + - V out = -I sc R f Virtual earth Both operate on the principle that the reverse saturation current increases with light. I dark light V b) PIN Photodiodes (Si) 7 / Radiation Sensors

8 An insulator sandwiched between a p-layer and a n-layer. SiO 2 I P n + Insulator Layer is very thin. The insulator region adjusts to the depletion region. Otherwise works like a pn junction, but faster. c) Schottky Photodiode (GaAsP, GaP) A u n Schottky type photodiode forms a rectifying contact if the metal is chosen right and the doping is within a specified range. For Au metal, an n-type silicon is required. φ B = height of the Schottky barrier φ m = work function of the metal φ = work function in of the semiconductor S φ = φ φ B Then m S 8 / Radiation Sensors

9 J φ B qv = J o exp exp 1 kt kt d) Avalanche Photodiode When a PN junction is operated under a high reverse bias, accelerated electrons collide with electrons in the bands and knock them off, generating electron-holes pairs. Then these secondary electrons are available to knock more electrons off. This is called an avalanche breakdown. - + P e - h + h e - n The condition for avalanche to occur: depletion region 3 E k E G 2 Kinetic Energy of Electrons Band-Gap Energy Sensitive in the visible to near IR region Phototransistors Operation is similar to a pn junction diode. One of the 2 pn junctions is used. 5.4 Infrared Radiation Microsensors Definition of IR radiation and Planck s Law 8πhf12 hf / ( ) [ ] 1 12 kt P f12 = e 1 3 c 9 / Radiation Sensors

10 Different kind of IR detectors: Photon Photoconductive Photovoltaic Thermal Pyroelectric bolometric Photoconductive IR Sensors Thin films of PbS, PbSe, Hg 1-x Cd x Te(MCT) PbS 2.2 µm (room temperature) PbSe 3.8 µm (room temperature) Hg 1-x Cd x Te adjustable from 1 µm 16 µm (even higher ones are available now) (usually 77K) The sensitivity of PbS & PbSe devices are not high enough to be used in IR imaging applications. Table 6.7, page Photovoltaic IR Sensors: The operation principle is similar to operation of PN junction diodes used in UV-Visible region. Materials: Ge, InGaAs, InAs and InSb. Hg 1-x Cd x Te is also widely used in the U.S., although it is not mentioned in the book. Table 6.8, page Pyroelectric IR sensors The polarization of the molecules change due to change in the temperature of the material. This releases charge which can be detected as current. dt I p = pa dt p = pyroelectric coefficient A = electrode area (usually in capacitor configuration) I p = induced pyroelectric current. t= time T=temperature 10 / Radiation Sensors

11 Table of different pyroelectric materials and their coefficients. Voltage responsibility of a pyroelectric detector R V : R V = output voltage input IR radiation ηparω = κ 1+ ω 1+ ω 2 2 τ e 2 2 τ th ω: angular frequency of chopper A: device area R: electrical resistance τ e : electrical time constant τ th : thermal time constant η= absorption coefficient o η 1 τ e = RC e τ th = C th /κ C e : electrical capacitance C th : thermal capacitance log R V 1 τ th log ω 1 τ e Bolometric IR Sensors The resistance of thin film changes due to heat radiated by IR spectrum. Operation is similar to contacting thermoresistors, except radiation sensors. Both pyroelectric and bolometric devices have to be thermally isolated, which requires micromachining. Most commonly used bolometric materials are: a: Si, Vox also semiconducting yttrium barium copper oxide, YBaCuO 11 / Radiation Sensors

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