Dissertation - FTP Directory Listing - University of Surrey

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V G < V FB V FB < V G < V T V T < V GAccumulation Depletion InversionFigure 2.4 Showing different operating modes of the MOS capacitorAccumulation mode: this occurs when the gate voltage V G is less than the flat band voltage V FB (flatband voltage refers to the voltage separating the accumulation and depletion regimes). For a P typephotogate accumulation usually occurs for negative voltages where the negative charge on the gateattracts holes to the oxide-semiconductor interface.Depletion mode: this occurs when the gate voltage is greater than the flatband voltage but less thanthe threshold voltage of the capacitor (the threshold voltage is the gate voltage at which the densityof electrons in the inversion layer is equal to the density of holes in the substrate). Here a negativecharge builds up in the semiconductor. Initially this charge is due to the depletion of thesemiconductor starting from the oxide-semiconductor interface. As the gate voltage is increased thewidth of the depletion region increases.Inversion mode: As the potential across the semiconductor increases another type of chargeemerges at the oxide-semiconductor interface: this charge is due to minority carriers which form an‘inversion’ layer. As the gate voltage is further increased the depletion region width barely increasessince the charge in the inversion layer increases exponentially with the surface potential cancellingincreases in the field.The depletion region forms a potential well in the substrate and when incident photons generate e-hpairs the e are collected in the potential well until the electric field is cancelled out and can hold no8
more. Thermally generated electrons are also attracted to the potential well and cause unwantedsignal known as dark current which limits the length of exposure times. The main drawbacks of thephotogate lie in the polysilicon gate which absorbs a fraction of the incident light and also the factthe charge needs to be transferred to another readout element increasing noise and readout time.2.3. Image sensorsImage sensors consist of a closely spaced 2-dimensional array of photo detectors linked to timingand readout circuits. The two modern types of image sensors CCDs and CMOS sensors differ mainlyin the method of readout of the array. The CCD was invented in 1969 at Bell Laboratories. It wasoriginally designed as a new type of semiconductor memory for computers, allowing charge packetsto be stored and transferred across a substrate. However it was realised it could also be very usefulas solid state camera as the device was light sensitive. By 1975 the first CCD camera with imagequality sharp enough for television broadcast was demonstrated. In the years following CCDs tookoff in both commercial and scientific applications to become the dominant imaging technology.Around the same time as the invention of CCDs another imaging technology was being developedthat used the CMOS process. At the time CCDs produced superior image quality with the fabricationmethods available. It was only recently, since the 1990s, that CMOS image sensors become a viablealternative. Both types of sensor have their own advantages and disadvantages meaning differentapplications can be suited for one imaging technology or the other. However work by engineerscontinues to reduce weaknesses of both technologies broadening their range of applications.2.3.1. CCD image sensorsThe CCD image sensor consists of an array of closely spaced photogates on a single substrate. Across sectional diagram of a three phase CCD is shown in Figure 2.5 along with its timing diagram.The phase of a CCD refers to the number of clocking signals required to transfer a charge packetfrom one pixel to the next.9
Page 2 and 3: ABSTRACTThe characterisation of fou
Page 4 and 5: LIST OF ABBREVIATIONSAPSCCDCDSCMOSD
Page 6 and 7: 2.8.3. FORTIS .....................
Page 8 and 9: CHAPTER 4Figure 4.1 LED uniformity
Page 10 and 11: Over the next 30 years CCDs improve
Page 12 and 13: There are 3 main types of semicondu
Page 14 and 15: 2.2. Photodetectors2.2.1. The Photo
Page 18 and 19: Figure 2.5 Cross sectional diagram
Page 20 and 21: 2.3.1.2. Interline transferIn an in
Page 22 and 23: Figure 2.9 NMOS transistor operatin
Page 24 and 25: in three main categories of CMOS se
Page 26 and 27: transistor M SEL is turned on and t
Page 28 and 29: qK = ×cA SF(2.5)Where:K = conversi
Page 30 and 31: 2.4.6. Signal to noise ratioSNR is
Page 32 and 33: When h/ ≫ this equation reduces
Page 34 and 35: The gain functions in this relation
Page 36 and 37: Where: σ (DNTOTAL) is the total no
Page 38 and 39: The Aspect system is a complete tes
Page 40 and 41: The pixel architecture (Figure 2. .
Page 42 and 43: eset, hence reducing kT/C noise. In
Page 44 and 45: Figure 2.28 Readout structure of TE
Page 46 and 47: 3.2. Sensor characterisationThe Asp
Page 48 and 49: Once everything was setup and the c
Page 50 and 51: From this it can be seen that for e
Page 52 and 53: 4.2.2. A test structure results:The
Page 54 and 55: values is 493.7 DN. The noise of te
Page 56 and 57: 20,100e-. Finally Figure 4.9 shows
Page 58 and 59: This test structure demonstrates go
Page 60 and 61: 4.2.5. D test structure results:Fig
Page 62 and 63: Of the two variants it appears D2 p
Page 64 and 65: 100Number of pixels1012000 2200 240
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As can be seen the plot exhibits so
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100000uy - uy. dark [DN]10000100010
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the halogen lamp had to tested with
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have an effect it is not great enou
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10000010000uy - uy. dark [DN]100010
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temp.var - dark.temp.var [DN]500004
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effect of these changes can’t be
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100Number of pixels1015350 5400 545
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10000Linearityuy - uy. dark [DN]100
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lower than the standard diode. The
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6. REFERENCES1. http://cerncourier.
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7. APPENDICES7.1. Appendix A: EMVA
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7.2. Appendix B: Aspect quick start
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iii)iv)Once you have followed all o
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5. Running idVIEWi) Before running
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a) Taking Measurements with only on
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a) The EMVA Standard takes into con
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7.3. Appendix C: Per pixel analysis
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Plot the gradient of the PTC by dra
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The final tab which both the integr
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