Lecture 7 July 24, 2017 Detecting light (converting light to electrical signal) Photoconductor Photodiode Managing electrical signal Metal-oxide-semiconductor (MOS) capacitor Charge coupled device (CCD) MOS field-effect transistor (MOSFET) Complementary MOS (CMOS) Rainbow (Dr. Janet Pan) 1
Photoconductor Light variable resistor Photoconduct or Time Constant Spectral Band µm CdS 100ms 0.47 to 0.71 CdSe 10ms 0.6 to 0.77 + - PbS 400us 1 to 3 PbSe 10us 1.5 to 4 What are these materials? Semiconductor http://encyclopedia2.thefreedictionary.com/photoconductors 2
pn Junction under Voltage Bias A. I B. I exponential V V saturated C. I D. I linear V V 3
Pn Junction Photodiode No light incident (green curve) Light incident on a reversed-based pn junction (red curve) A. B. C. D. 4
Photodiode http://en.wikipedia.org/wiki/photodiode 5
PIN Photodiode More light -> more current Very fast: > 60 GHz +V Vout +V p i n Vout pin photodiode 6
Avalanche Photodiode Higher supply voltage Electrons knock other electrons loose 1 photon = many e- à internal gain Higher noise +V p n Vout 7
Current vs. Voltage of a pn Junction http://www.electronics-tutorials.ws/diode/diode_3.html 8
Photomultiplier Tube -0.3V -0.5V -0.7V -0.9V http:// www.olympusconfo cal.com/theory/ pmtintro.html Vout -0.4V -0.6V -0.8V -V Very sensitive (single photon) V 1000 volts Room light can burn it out 9
Photographic Film Grains AgBr n = 2.236 Protective Layer Base: Glass, Mylar, Acetate Expose Emulsion ( gelatin ) n = 1.53 Develop Fix 10
Digital Image Sensors 11
Q. If a negative bias is applied to the Gate in the structure below what do you expect to happen? - - - - - - - Oxide P-type» a) An electric field is formed pointing from the bottom of the page to the top cause holes to move towards the oxidesemiconductor interface. b) Electrons diffuse from the oxide to the conduction band of the p-type semiconductor. c) Holes move from the oxide to the semiconductor due to the electric field and current flows. 12
Q. What happens if a small positive voltage is applied to the gate? + + + + + Oxide P-type a) Electrons flow through the oxide» b) Hole are pushed away from the oxide semiconductor interface leaving ionized acceptors to balance the positive charge on the gate c) Holes are attracted to the positive charge giving rise to hole accumulation at the oxide-semi interface. 13
Charge Coupled Device (CCD) http://www.microscopyu.com/articles/digitalimaging/ccdintro.html 14
How a CCD works: Bucket Brigade 15
CMOS image sensor Complementary Metal Oxide Semiconductor Each pixel has a photodiode and an amplifier A New Digital Image Sensor Compatible with IC technology (which is based on CMOS) Lower power consumption and higher speed than CCD 16
MOSFET Metal Oxide Semiconductor Field Effect Transistor 17
N-channel MOSFET Is V(Threshold) A. < 0 B. = 0 Enhancement-mode Semicon not inverted under the oxide with V G = 0 Normally off C. > 0 18
P-channel MOSFET Is V(Threshold) A. < 0 B. = 0 Enhancement-mode Semicon not inverted under the oxide with V G = 0 Normally off C. > 0 19
CMOS Consider digital voltage signal to the input (1 s and 0 s). If V in is high (large positive, or 1), A. PMOS is on, NMOS is on B. PMOS is on, NMOS is off C. PMOS is off, NMOS is on D. PMOS is off, NMOS is off 20
CMOS If V in is low (0), A. PMOS is on, NMOS is on B. PMOS is on, NMOS is off C. PMOS is off, NMOS is on D. PMOS is off, NMOS is off 21
CMOS Inverter V in = 1 (high), NMOS is on PMOS is off à V out = 0 V in = 0 (low), NMOS is off PMOS is on à V out = 1 Almost all the time (except during switching between 0 and 1) à There is no current between V DD (supply voltage) and ground à Low power consumption 22 à CMOS is used in microprocessor chips, memory chips,
MOSFET and CMOS 23
Rainbow Observe Primary rainbow Bright inside primary rainbow Dimmer between primary and secondary rainbow Secondary rainbow colors are reversed Polarization of rainbow Lab demos - Dr. Janet Pan https://en.wikipedia.org/wiki/rainbow 24
Rainbow Observe Primary rainbow Bright inside primary rainbow Dimmer between primary and secondary rainbow Secondary rainbow colors are reversed Polarization of rainbow Lab demos - Dr. Janet Pan http://www.atoptics.co.uk/fz954.htm 25
Observing a Rainbow https://en.wikipedia.org/wiki/rainbow Sun behind observer Mist of water droplets Dark clouds opposite sun Lab demos - Dr. Janet Pan 26
Rainbow Calculation of the Descartes Angle https://en.wikipedia.org/wiki/rainbow Lab demos - Dr. Janet Pan 27
Secondary Rainbow Secondary Rainbow Primary Rainbow Lab demos - Dr. Janet Pan http://www.atoptics.co.uk/rainbows/orders.htm 28