1. OPTI 202L Lab #12 – p-n Junctions: Photodiodes, Solar Cells LED’s, and Laser Diodes Dr. Mike Nofziger Professor College of Optical Sciences University of Arizona Dr. Mike Nofziger 2014

2. Lecture #12 Outline: ● Photodiodes—Physical Construction —Basic Properties —Basic Physics —Current Voltage Characteristics —Use in an Electrical Circuit ● Solar Cells—Basic Properties ● LED’s—Basic Properties Dr. Mike Nofziger 2014 Lecture 12

3. Photodiodes—Physical Construction: A 40 Gb/s “Optical Receiver” !! “Official Symbol” Dr. Mike Nofziger 2014 Lecture 12

4. Photodiodes—Basic Properties: Dr. Mike Nofziger 2014 Lecture 12 ● p-n junction (p-side ≡ “anode”, n-side ≡ “ cathode”) ● Built-in electric field (depletion region) separates the electrons and holes (electrons → p-side, holes → n-side) ● Photons absorbed (ideally in or near the depletion region) create electron-hole pairs ● Built-in electric field separates the electrons and holes before they recombine, producing a photocurrent (electrons → n-side, holes → p-side) ● I-V curve is very non-linear ● The photocurrent is linear with photon flux over 7-decades! ● Most common semiconductor material used to make photodiodes (for detection of visible light) is Silicon (Si).

5. Photodiodes—Basic Physics: Dr. Mike Nofziger 2014 Lecture 12

6. Photodiodes—Current-Voltage Characteristics: Dr. Mike Nofziger 2014 Lecture 12 The “Shockley diode equation” I o is the reverse saturation current V is the voltage across the junction Photocurrent generated by irradiance E e (W/m 2 ) Photocurrent generated by optical power ϕ e (W)

7. Photodiodes—Current-Voltage Characteristics: Dr. Mike Nofziger 2014 Lecture 12

8. Photodiodes—Use in an Electrical Circuit: Dr. Mike Nofziger 2014 Lecture 12 Operated at V = 0 “zero-bias”: Output is very linear over 7-decades of flux Operated at –V “reverse-bias”: Capacitance decreases, speed increases Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux: NOT the preferred way to operate a photodiode!

9. Basics of Solar Cells: ● A solar cell is a Photovoltaic (“PV”) detector: - is made of Silicon (not silicone!!) - absorbs light from ≈ 350nm – 1100nm - the absorption of light “frees up” electrons - This creates a voltage at the terminals of the cell (the “Open-Circuit” voltage) - If the cell’s terminals are shorted, the maximum current will flow (the “Short-Circuit” current) - If a load resistor is connected to the cell, a current will flow (the “Photocurrent”) Dr. Mike Nofziger 2014 Lecture 12

10. Basics of Solar Cells: ● The Power (Watts) that the cell can produce is given by: ● Because of internal resistance in the cell, the maximum power you can generate is across a load resistance equal to the internal resistance. www.keithley.com www.keithley.com Dr. Mike Nofziger 2014 Lecture 12

11. LED – Light Emitting Diode – solid-state, semiconductor p-n junction – GaAs, AlGaAs, GaP, AlGaInP, GaN, InGaN,etc. – 3mm, 5mm, or 8mm dia. plastic packages “Sources of Light” Low-Power LED’s  1-20mA of electrical current  “on/off indicators” High-power LED’s  500mA to >1A  Lumileds, Osram, Cree  Lighting!!  Replace incandescent bulbs Dr. Mike Nofziger 2014 Lecture 12

12. “Sources of Light” LED – Light Emitting Diode Dr. Mike Nofziger 2014 Lecture 12

13. “Sources of Light” White-Light LED’s: General Approaches Reference Dr. Mike Nofziger 2014 Lecture 12

14. “Sources of Light” White-Light LED’s (3 phosphors, “tri-phosphor”) – AlGaInN LED is the source of light – emits light in the near-UV spectral region – 380-430 nm – 3 phosphor coatings on top – Europium-based red and blue phosphors – Copper and aluminum doped zinc sulfide green phosphor – Color-mixing more easily controlled to produce a pure white light. – Example: CREECREE Reference Dr. Mike Nofziger 2014 Lecture 12

15. “Sources of Light” White-Light LED’s (1 phosphor) – GaN semiconductor LED is the source of light – emits blue light 450 – 470 nm – Yellow phosphor coating on top – Cerium-doped YAG: Y 3 Al 5 O 12 :Ce 3+ – light is emitted by scintillation (no afterglow) Example: CREECREE Dr. Mike Nofziger 2014 Lecture 12

16. “Sources of Light” White-Light LED’s: TRENDS Reference Dr. Mike Nofziger 2014 Lecture 12

17. “Sources of Light” White-Light LED’s: Applications vs. Dr. Mike Nofziger 2014 Lecture 12

18. “Sources of Light” White-Light LED’s: Applications Uses a CREE XM-L 1000 lumen LED Dr. Mike Nofziger 2014 Lecture 12

19. Laser Diodes—LI Curve Dr. Mike Nofziger 2014 Lecture 12