Ultraviolet Photodetector (AlGaN, SiC, Ga2O3, Diamond)

1. Theory

• Definition: An ultraviolet (UV) photodetector converts light of 200 nm to 400 nm into an electrical signal in a wide-gap semiconductor: AlGaN (3.4 eV to 6.2 eV), 4H-SiC (3.2 eV), $\beta$-$\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ (4.9 eV), diamond (5.5 eV), and also GaN, ZnO, AlN and h-BN, in the form of a Schottky, p-i-n, metal-semiconductor-metal (MSM), photoconductor or avalanche device.
• Spectral selectivity: The gap sets a sharp cutoff, so that no filter is needed for rejecting the visible light: a gap above 4.42 eV (a cutoff below 280 nm, an aluminium fraction of about 0.4 in AlGaN) makes the detector solar-blind, because the ozone layer removes the sun’s radiation below about 280 nm.
• Gap type: GaN, AlN, AlGaN and ZnO have a direct gap and absorb in a thin layer, 4H-SiC, 6H-SiC and diamond have an indirect gap and need a thicker absorber for the same absorption.
• Low dark current: The intrinsic carrier density falls as $\exp (-E_g/2k_{B}T)$, so that the wide gap gives dark currents of femtoamperes to picoamperes in the p-i-n and Schottky devices and permits operation at 300 °C to 550 °C.
• Gain: Photoconductors and MSM devices of $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$, GaN and diamond show an EQE above 100% by trapping of one carrier, at the price of a response of milliseconds to seconds and of persistent photoconductivity; the avalanche devices of AlGaN reached a gain of 2500 at 62 V and of $1.2\times 10^4$ in a separate-absorption-multiplication structure.
• Device types: The Schottky diode (a responsivity of about 0.1 A/W at zero bias), the p-i-n photodiode (0.2 A/W, an EQE of 80% to 89%), the MSM detector, the photoconductor, the avalanche photodiode and the focal plane array on a readout circuit.
• Surface and substrate: Oxygen termination, interfaces, traps of the sapphire-grown films and the contact metals decide the dark current, the gain and the speed.
• Figures of merit: The responsivity, the EQE, the visible rejection ratio, the dark current, $D^{\ast }$, the gain, the speed and the maximum operating temperature.

2. Working Principle

• Photodiode: A photon above the gap creates a pair in the depleted layer, the field of the junction collects it with an internal efficiency close to one, and an EQE of 80% to 89% was reached at 275 nm in AlGaN p-i-n diodes.
• Schottky and MSM: The Schottky barrier on the wide-gap layer blocks the injection, a bias of 5 V to 20 V extends the depletion and raises the collection, and in an MSM with unequal barriers the zero-bias photocurrent is small and rises with the bias.
• Gain by trapping: In $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ and diamond one carrier is held in a trap while the other crosses the gap many times, so that the EQE exceeds 100% and the decay follows a stretched exponential with an activation energy of 58 meV to 360 meV.
• Avalanche: The multiplication in the high-field layer of AlGaN or 4H-SiC, up to a field close to the critical value of 3.0 MV/cm, gives 2500 at 62 V in AlGaN and $1.2\times 10^4$ in a separate-absorption-multiplication AlGaN structure, and a 4H-SiC diode with micro-holes reached 3.0 MV/cm at the hole edge at 40 V, far from the 1000 V of a photomultiplier.
• Temperature: The wide gap keeps the dark current low at high temperature, but the photocurrent also changes: it rises with temperature in GaN and AlGaN/GaN MSM detectors by thermal ionization and falls in 4H-SiC p-i-n diodes.

3. Applications

• Solar-blind detection: Flame, corona discharge and missile-plume detection, and UV communication and imaging in daylight, where an AlGaN or $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ detector responds to the ultraviolet and not to the visible light of the day.
• UV monitoring and spectroscopy: SiC photodiodes with an EQE of 50% to 80% at 270 nm to 290 nm for the UV index, sterilization monitoring and flame sensing, and diamond detectors for the vacuum ultraviolet of the sun.
• High-temperature sensing: SiC and III-nitride detectors operating at 300 °C to 550 °C in engines, in combustion monitoring and in space probes.
• Low-level detection: AlGaN avalanche detectors as a solid-state alternative to the photomultiplier at 200 nm to 400 nm, with a gain of 2500 at 62 V.

Spectral Range and Solar-Blind Condition

Name	Formula	Related Formulae	Variables/Constants
Cutoff wavelength and solar-blind condition	${\lambda }_c\left(\mathrm{nm}\right)=\frac{1239.8}{E_g\left(\mathrm{eV}\right)}$	$E_g\left({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}\right)=x{E}_{AlN}+\left(1-x\right)E_{GaN}-bx\left(1-x\right)$ $E_g>4.42\mathrm{eV}\iff {\lambda }_c<280\mathrm{nm}$	${\lambda }_c:cutoffwavelength$ $E_g:gap$ $x:aluminiumfraction$ $\begin{array}{r}b:bowingparameterofthealloy,the\\ solar-blindconditionexcludesthe\\ radiationofthesunthatreachesthe\\ ground\end{array}$ $\begin{array}{r}(GaN3.4eVgives365nm,4H-SiC3.2eV\\ 387nm,\beta -G{a}_2{O}_{3}4.9eV253nm,diamond5.5\\ eV225nmandAlN6.2eV200nm,computed;\\ x=0.36,0.41and0.45forb=0,0.5and\\ 1eV(assumed),computed,againstx\approx 0.4in\\ thesource)\end{array}$

Responsivity, Rejection and Gain

Name	Formula	Related Formulae	Variables/Constants
Responsivity and external quantum efficiency	$\mathfrak{R}=EQE\frac{q\lambda }{hc}=\frac{EQE\cdot \lambda \left(\mathrm{nm}\right)}{1239.8}A/W$	$EQE=\left(1-R\right){\eta }_{abs}{\eta }_{c}G$ ${\mathfrak{R}}_{APD}=M{\mathfrak{R}}_0$	$\mathfrak{R}:responsivity$ $EQE:externalquantumefficiency$ $R:reflectance$ ${\eta }_{abs}:absorbedfraction$ ${\eta }_c:collectionefficiency$ $G:photoconductivegain$ $M:avalanchemultiplication$ ${\mathfrak{R}}_0:responsivityatunitygain$ $\begin{array}{r}(70mA/Wat272nmis32\%,176mA/Wat\\ 275nm79\%,125mA/Wat290nm53.4\%and\\ 180mA/Wat274nm81.4\%,computed;1.19\\ A/Wat290nmisEQE=5.09,computed)\end{array}$
Visible rejection ratio and photo-to-dark current ratio	$RR=\frac{\mathfrak{R}\left({\lambda }_{UV}\right)}{\mathfrak{R}\left({\lambda }_{vis}\right)}$	$PDCR=\frac{I_{ph}-I_d}{I_d},I_{ph}-I_d=\mathfrak{R}{P}_{opt}S$ $R{R}_{dB}=20{\log }_{10}\left(RR\right)$	$RR:rejectionratiooftheresponsivities$ $\begin{array}{r}{\lambda }_{UV},{\lambda }_{vis}:thetwowavelengthsofthe\\ comparison(theydifferbetweenpapers)\end{array}$ $PDCR:photo-to-darkcurrentratio$ $P_{opt}:opticalpowerdensity$ $S:area$ $\begin{array}{r}(0.2A/Wat220nmand0.0019A/Wat400\\ nmgiveRR=105,computed;0.70A/W,400\\ \mu W/c{\mathrm{m}}^2,0.008\mathrm{c}{\mathrm{m}}^{2}and3\times 10^{-10}AgivePDCR=\\ 7.5\times 10^3,computed,against7137printed)\end{array}$
Photoconductive gain and persistent photoconductivity	$G=\frac{\tau }{t_{tr}},t_{tr}=\frac{L^2}{\mu V}$	$I\left(t\right)=I_0\exp \left[-{\left(\frac{t}{\tau }\right)}^{\beta }\right],0<\beta <1$ $\tau ={\tau }_0\exp \left(\frac{\Delta E}{k_{B}T}\right)$	$G:gain$ $\tau :lifetimeofthetrappedcarrier$ $t_{tr}:transittime$ $L:electrodespacing$ $\mu :mobility$ $V:bias$ $\beta :stretchingexponentofthedecay$ $\Delta E:carriercapturebarrier$ $k_B:Boltzmannconstant$ $\begin{array}{r}(\Delta E=132meVto360meVforthe\\ nitrides;\mathfrak{R}=0.70A/Wat255nmisEQE=\\ 3.40,computed;\tau changesby3.6to33\\ between300Kand400Kforthese\\ barriers,computed)\end{array}$

Dark Current, Noise and Speed

Name	Formula	Related Formulae	Variables/Constants
Intrinsic carriers and the wide-gap advantage	$n_i=\sqrt{N_c{N}_v}\exp \left(-\frac{E_g}{2k_{B}T}\right)$	$\frac{n_i\left(E_g\right)}{n_i\left(E_{Si}\right)}\approx \exp \left(-\frac{E_g-E_{Si}}{2k_{B}T}\right)$ $J_{gen}=\frac{q{n}_{i}W}{2{\tau }_g}$	$n_i:intrinsiccarrierdensity$ $N_c,N_v:effectivedensitiesofstates$ $E_g:gap$ $E_{Si}:1.12eV$ $\begin{array}{r}{J}_{gen}:generationcurrentdensityofthe\\ depletionregion\end{array}$ $W:itswidth$ $\begin{array}{r}{\tau }_g:generationlifetime(thefactor2\\ isforamid-gaptrapwithequalcapture\\ lifetimes,asonthePNandPIN\\ Photodiodepages;sometextsabsorbit\\ into{\tau }_g),therationeglectstheprefactors\end{array}$ $k_B:Boltzmannconstant$ $\begin{array}{r}(at300Ktheratiotosiliconis3\times 10^{-18}\\ for4H-SiC,7\times 10^{-20}forGaNand2\times 10^{-32}for\\ G{a}_2{O}_3,and4H-SiCrisesby5\times 10^{15}between\\ 300Kand723K,computed)\end{array}$
Shot-noise-limited detectivity	$D_{shot}^{\ast }=\frac{\mathfrak{R}}{\sqrt{2q{J}_d}}$	$D^{\ast }=\frac{\mathfrak{R}}{\sqrt{2q{J}_d+4k_{B}T/\left(R_{0}A\right)}}$ $NEP=\frac{\sqrt{2q{J}_{d}A}}{\mathfrak{R}}$	$\begin{array}{r}{D}_{shot}^{\ast }:detectivitylimitedbytheshot\\ noiseofthedarkcurrent\end{array}$ $J_d:darkcurrentdensity$ $R_{0}A:resistance-areaproduct$ $\begin{array}{r}A:area,theresultisanupperboundfor\\ adetectorwithoutgainandwithout\\ flickernoise\end{array}$ $k_B:Boltzmannconstant$ $\begin{array}{r}(\mathfrak{R}=0.18A/Wand{J}_d=0.2nA/\mathrm{c}{\mathrm{m}}^{2}give\\ 2.3\times 10^{13};0.159A/Wand1.5\times 10^{-11}A/\mathrm{c}{\mathrm{m}}^{2}give\\ 7.3\times 10^{13};0.2A/Wand5.6\times 10^{-10}A/\mathrm{c}{\mathrm{m}}^{2}give\\ 1.5\times 10^{13},computed)\end{array}$
Bandwidth and response time	$f_{-3dB}=\frac{0.35}{t_R}$	$f_{RC}=\frac{1}{2\pi RC}$ $\frac{1}{f_{-3dB}^2}\approx \frac{1}{f_t^2}+\frac{1}{f_{RC}^2}$	$t_R:10\%to90\%risetime$ $R:loadandseriesresistance$ $C:capacitance$ $f_t:transit-limitedcutoff$ $\begin{array}{r}(ariseof594\mu \mathrm{s}gives589HzforaSiC\\ MSM,2\mu \mathrm{s}gives175kHzfordiamond,110\\ msgives3.2HzforAlN,computed;4.1GHz\\ wasreportedforanAlGaNSchottkydiode)\end{array}$

Avalanche Ultraviolet Detector

Name	Formula	Related Formulae	Variables/Constants
Avalanche multiplication and comparison with a photomultiplier	$M=\frac{I_{ph}\left(V\right)}{I_{ph}\left(V_0\right)},{\mathfrak{R}}_{APD}=M{\mathfrak{R}}_0$	$M=\frac{1}{1-{\int }_0^W\alpha dx}$ $E_{max}\approx E_{crit}\mathrm{at}\mathrm{breakdown}$	$\begin{array}{r}M:multiplicationrelativetothe\\ photocurrentatthereferencebias{V}_0(a\\ lowbiaswherethegainistakenasunity)\end{array}$ $\begin{array}{r}\alpha :ionizationcoefficient(equalfor\\ electronsandholesinthefirstform)\end{array}$ $W:widthofthemultiplicationlayer$ $E_{max}:peakfield$ $E_{crit}:criticalfield$ $\begin{array}{r}(M=2500at-62Vwith79.8mA/Wat270\\ nmgives200A/W,computed,against600\\ A/Wofaphotomultiplierabove1000V;3.0\\ MV/cmattheedgeofa4\mu \mathrm{m}holeof4H-SiC\\ at40V)\end{array}$

Important Questions

Ques: Is the aluminium fraction of about 0.4 for a solar-blind AlGaN detector consistent with the gaps of GaN and AlN?

The condition is $E_g>4.42$ eV, a cutoff below 280.5 nm (computed). A linear interpolation between 3.4 eV and 6.2 eV gives $x=0.36$, and a bowing term $-bx(1-x)$ raises it to 0.41 for $b=0.5$ eV and 0.45 for $b=1$ eV (computed, the bowing is assumed and not from the sources). The source states only ‘about 0.4’ and gives no gap curve, so the two agree within the unknown bowing. The measured cutoffs of the Schottky diodes, 274 nm and about 266 nm, and the peak response at 270 nm to 290 nm of the p-i-n and avalanche devices are all near the 280 nm edge, so the composition is set by the cutoff that is wanted, not by the formula. Reference: Cai et al., Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays, Light Sci. Appl. 10, 94 (2021); Miller et al., High-temperature Ultraviolet Photodetectors: A Review, arXiv:1809.07396.

Ques: Are the quoted responsivities and quantum efficiencies of the UV diodes consistent?

In the main, yes. 70 mA/W at 272 nm gives 32%, 176 mA/W at 275 nm 79% (80% printed), 125 mA/W at 290 nm 53.4% (53.4% printed), 180 mA/W at 274 nm 81.4% (81.5% printed) and 79.8 mA/W at 270 nm 36.6% (37% printed), all computed with $\mathfrak{R}=EQE\lambda /1239.8$. The exception is 192 mA/W at 5 V, which gives 86.6% (computed) against 89% printed, possibly a different wavelength. For the 4H-SiC micro-hole diode 0.159 A/W at 290 nm is 68.0%, and the relative increase over the reference is 10.5%, not the 6.5% printed, which is the difference in percentage points. At 40 V the responsivity of 1.19 A/W is an EQE of 5.09, i.e. gain. Reference: Cai et al., Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays, Light Sci. Appl. 10, 94 (2021); Zhou et al., 4H-SiC UV avalanche photodiode, IEEE Electron Device Lett. 39, 1724 (2018), as reported in Semiconductor Today, Nov/Dec 2018; Fu et al., Local Avalanche Effect of 4H-SiC p-i-n Ultraviolet Photodiodes With Periodic Micro-Hole Arrays, IEEE Electron Device Lett. 43, 64 (2022); Laser Focus World, Solar-blind avalanche photodiode has gain of 2500 (Dec. 2010).

Ques: Does the $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ MSM detector have gain, and why is it so slow?

At 255 nm, 10 V, 400 $\mu W/c{\mathrm{m}}^2$ and 0.008 $\mathrm{c}{\mathrm{m}}^2$ the incident power is 3.2 μW, so that 0.70 A/W gives 2.24 μA, a ratio to the dark current of $3\times 10^{-10}$ A of $7.5\times 10^3$ (computed), close to the printed 7137 (0.67 A/W would give it). The EQE is $0.70\times 1239.8/255=3.40$ (computed), above one, which implies gain, although the source reports neither gain nor EQE. The rise has components of 590 ms and 2.4 s at room temperature, and the activation energies of 73 meV and 205 meV (rise) and 58 meV and 168 meV (decay) are attributed to self-trapped holes, an interpretation of the authors. The gain and the speed are connected, as for a photoconductor. Reference: Tak et al., High-Temperature Photocurrent Mechanism of β-Ga2O3 Based MSM Solar-Blind Photodetectors, arXiv:1812.07197; Miller et al., High-temperature Ultraviolet Photodetectors: A Review, arXiv:1809.07396.

Ques: How does the bias change the zero-bias and rejection performance of the epitaxial $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ MSM detector?

The 150 nm film had a zero-bias responsivity of 1.4 mA/W at 255 nm, an EQE of 0.68% (computed; 0.5% printed) and a rejection of 255 nm against 450 nm of about $10^2$. At 5 V the peak responsivity was 9.4 A/W in the forward and 1.1 A/W in the reverse direction, an EQE of 46 and 5.3 (computed), and the rejection rose to about $10^5$. The asymmetry presumably comes from the unequal contacts of the asymmetric device (barriers of 0.71 eV and 0.82 eV), and the rise of both quantities with bias is consistent with gain; the response is slow in all the devices, a rise of 1.9 s to 2.5 s and a fall of 0.4 s to 0.9 s, even at zero bias. The printed dark currents disagree between the abstract, the text (100 nA and 1.7 nA) and the table (7.3 nA). Reference: Pratiyush et al., MBE grown Self-Powered β-Ga2O3 MSM Deep-UV Photodetector, arXiv:1802.01574.

Ques: What can be said about the detectivity of the diamond detector?

The source gives 0.2 A/W at 220 nm and 30 V, an EQE of 113% (computed), a rejection of 105 against 400 nm (0.2/0.0019, computed), a dark current of 1.12 pA over 0.2 $\mathrm{m}{\mathrm{m}}^2$ and a rise of 20 μs with a decay of 1000 μs. The detectivity is printed as $6.57\times 10^{10}$ per watt without the bandwidth and the area, so it cannot be compared. The dark current density is $5.6\times 10^{-10}A/c{\mathrm{m}}^2$ (computed), which with 0.2 A/W gives a shot-noise-limited upper bound of $1.5\times 10^{13}\mathrm{cm}\mathrm{H}{\mathrm{z}}^{1/2}/W$ (computed). With gain and flicker noise the real value is lower, and the contacts of tungsten and palladium are called ohmic without a measurement of the contact resistance. Reference: Liu et al., Fabrication of UV Photodetector on TiO2/Diamond Film, Sci. Rep. 5, 14420 (2015).

Ques: Why does the dark current of the SiC and GaN detectors grow slowly with temperature, and what does the photocurrent do?

The intrinsic density of 4H-SiC rises by $5\times 10^{15}$ between 300 K and 723 K (computed, prefactors neglected), yet the PDCR of the Cr/Pd MSM detector fell from $1.3\times 10^5$ at room temperature to 0.62 at 450 °C, a factor $2\times 10^5$ (computed). The dark current therefore did not follow the intrinsic generation, which would have buried the signal long before, and another mechanism such as leakage sets it; the source gives none. The photocurrent differs by device: the 4H-SiC p-i-n diode fell from 0.12 to 0.046 A/W at 550 °C, while GaN and AlGaN/GaN MSM detectors rose from about 8 to 13 A/W and from 1 to 18 A/W by thermal ionization, so that the responsivity of these gain devices is not constant with temperature, and the review lists the SiC p-n diode with 0.15 A/W to 0.2 A/W from room temperature to 300 °C as temperature-independent. Reference: Miller et al., High-temperature Ultraviolet Photodetectors: A Review, arXiv:1809.07396.

Ques: Can an AlGaN or SiC avalanche detector replace a photomultiplier tube?

A photomultiplier has a responsivity of 600 A/W, a gain near $10^6$ and a voltage above 1000 V. The AlGaN detector with $M=2500$ at -62 V and 79.8 mA/W at 270 nm (zero-bias EQE 37%) has 200 A/W (computed, taking the gain as relative to the zero-bias responsivity), three times less at one sixteenth of the voltage, with 20 nA of dark current at a gain of 10 for a diameter of 100 μm. A 4H-SiC detector of 800 μm diameter had a dark current of about 1 pA ($0.2nA/c{\mathrm{m}}^2$, computed) and an EQE of 81.5%. The gain is defined relative to a low bias, and a gain of more than $10^6$ at 365 nm is listed beside 4.2 at 140 V, so the sources must be compared with care; no excess noise is given. Reference: Laser Focus World, Solar-blind avalanche photodiode has gain of 2500 (Dec. 2010); Tut, GaN/AlGaN-based UV photodetectors with performances exceeding the PMTs, Ph.D. thesis, Bilkent University (2008); Zhou et al., 4H-SiC UV avalanche photodiode, IEEE Electron Device Lett. 39, 1724 (2018), as reported in Semiconductor Today, Nov/Dec 2018; Cai et al., Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays, Light Sci. Appl. 10, 94 (2021).

Ques: Why do the speeds of the UV detectors span ten orders of magnitude?

An AlGaN Schottky diode gave a pulse of 53 ps and a bandwidth of 4.1 GHz, a p-i-n diode 4.5 ns (77 MHz by $0.35/t_R$ if it is a rise time, computed), a 4H-SiC MSM 594 μs (589 Hz, computed), diamond 2 μs to 20 μs, AlN 110 ms and the $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ MSM 0.16 s to 3.2 s. The diodes are limited by transit and RC time, while the photoconductors are limited by the lifetime of the trapped carrier, which is the cost of the gain. The persistent photoconductivity of GaN decayed in 39 h in the solid device and in 24 s in a suspended one, a ratio of 5850 (computed), and follows $\tau ={\tau }_0\exp (\Delta E/k_{B}T)$ with 132 meV to 360 meV, so that the time changes by 3.6 to 33 between 300 K and 400 K (computed). Reference: Cai et al., Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays, Light Sci. Appl. 10, 94 (2021); Miller et al., High-temperature Ultraviolet Photodetectors: A Review, arXiv:1809.07396; Liu et al., Fabrication of UV Photodetector on TiO2/Diamond Film, Sci. Rep. 5, 14420 (2015).

Ques: Are the visible rejection ratios of different papers comparable?

No, because the wavelengths and the biases differ. The Schottky diodes quote $10^3$ to $10^4$ and the p-i-n diodes $10^6$, the MSM devices $10^4$ and above 8000 at 150 °C, a 4H-SiC avalanche detector more than $10^3$ for 274 nm against 400 nm, diamond 100 for 220 nm against 400 nm (18 to 25 for 340 nm), and the epitaxial $\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ detector $10^2$ at 0 V and $10^5$ at 5 V for 255 nm against 450 nm. A tail of the responsivity at 400 nm one decade lower changes the ratio by a factor ten, and in a device with gain the visible response is amplified as well. A comparison needs the same pair of wavelengths and the same bias, and 20 log gives twice the value in decibels of 10 log. Reference: Cai et al., Progress on AlGaN-based solar-blind ultraviolet photodetectors and focal plane arrays, Light Sci. Appl. 10, 94 (2021); Pratiyush et al., MBE grown Self-Powered β-Ga2O3 MSM Deep-UV Photodetector, arXiv:1802.01574; Liu et al., Fabrication of UV Photodetector on TiO2/Diamond Film, Sci. Rep. 5, 14420 (2015); Zhou et al., 4H-SiC UV avalanche photodiode, IEEE Electron Device Lett. 39, 1724 (2018), as reported in Semiconductor Today, Nov/Dec 2018.

Related pages in this vault: Metal-Semiconductor-Metal Photodetector (MSM), Avalanche Photodiode (APD), Colloidal Quantum-Dot Photodetector (CQD), Perovskite Photodetector (Photodiode and Photoconductor), Photoconductor, Photodetectors (PIN and Avalanche Photodiodes), Diodes (PN, PIN, Schottky, Zener), Ultraviolet LED (AlGaN Deep-UV LED), Photoluminescence Spectrometer.