L02_Device_Lab_short_F2020(1)

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1 Materials Science for Engineers Laboratory #2 Properties of LED’s— Diode, LED, or Photovoltaic? Objective : Understand how pn junctions are used in a variety of applications from LEDs to diodes and solar cells. You will observe and measure the properties of several different band gap LED’s. Materials : LED’s (blue, green, amber, red), multimeter, 3V button battery (BR2325), clothespin, liquid nitrogen Safety : Do not stare long at any of the brightly lit LEDs. Wear safety glasses to protect from liquid nitrogen and stray UV radiation from LEDs. Procedure : Part 1: LED band gap and color Step 1: You should have a red, amber, green and blue LED. Check with 3V button battery by putting positive lead of LED to positive side of battery (see below for polarity of LED leads). See your TA if you need a particular color LED or if your button battery is dead. Figure 1. Schematic of an LED showing the positive and negative leads. https://www.fiberopticproducts.com/Led.ht14.gif NOTE: Room lights should be turned off (or mostly off) for the best results.

2 Figure 2. Schematic of a pn junction used for a LED (Callister). Forward bias of the junction causes an electron to cross into the p-side (a) where it is annihilated by an electron hole emitting a photon (b). The photon’s wavelength will depend on the band gap of the semiconductors involved. Step 2: Using the band gaps for common LED materials in Table 1, calculate the wavelength of the emitted photon and the expected color of the LED. Use the expression E g = hc/  where 1 eV = 1.6 x 10 -19 J Table 1 . Common LED materials and their band gaps. Arsenide (As) substituted for Phosphorous (P) lowers the band gap in GaP. material Band gap (eV) Wavelength of emitted photon (nm) Color of LED GaN 2.65 468.82 Blue GaP 2.19 567.29 Green GaP 0.85 As 0.15 2.11 588.8 Yellow GaP 0.65 As 0.35 2.03 612 Orange GaP 0.4 As 0.6 1.91 649 Red

3 Part 2: Spectrum of LEDs Step 1: Download the lspEVO app (~$3) from app store. Step 2 : Record a spectra for each of the 4 LEDs and insert in Table 2 below. Table 2 . Insert your spectra [crop it from the image above] for each LED LED Spectra Red Amber

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4 Green Blue Part 3: LED as a diode Forward bias the LED and you will see the color of the LED. Reverse bias (switch the polarity of the leads) of the LED and you will see no color. Figure 3: IV curve for a diode. https://learn.sparkfun.com/tutorials/diodes/real-diode- characteristics Part 4: LED as a photovoltaic Step 1: Attach a multimeter to one of the LED’s and turn to volts (DC)— V DC . You will see a jumpy voltage signal of a few hundred mV. [You might get a more stable reading if you change to a Volt readout by pressing “range” button once.] If you cover your hand over the LED, the voltage should drop dramatically. The LED is now behaving as a photovoltaic (i.e., a solar cell) upon exposure to room light.

5 Step 2: Now we will test all the LEDs photovoltaic properties, but instead of the room light, we will use the other 3 LEDs as light sources to try and generate a photovoltage. The classroom lights need to be turned off first. Hook up the multimeter to the red LED and turn to volts DC, then take the amber LED and attach to battery. Bring the two together and see if a voltage is generated and record in Table 3 below. Repeat for all the other colors and fill in the table below with the approximate mV generated. If voltage does not change, enter “X’ in the table. Table 3.Voltage generated by different colored LEDs. Room should be darkened Voltage (mV) generated as a function of the color of LED used to excite photovoltaic LED in first column. (Room lights must be turned off.) Photovoltaic LED ambient light Red Amber Green Blue Red 1600 na 1600 1400 130 Amber 1600 0 na 620 170 Green 2000 0 0 na 1100 Blue 2150 0 0 0 na Part 5: LED color as a function of temperature (amber/orange LED only) Obtain a cup of liquid nitrogen. Use a clothespin to attach the battery to an LED (Figure 3). Immerse just the bulb part of the LED into the liquid nitrogen and record your observations. Once color has stabilized remove the LED and confirm your observations as LED warms back up. Figure 3. Using a clothespin to hold battery and LED so that it can be immersed in liquid nitrogen. Do not immerse the battery! Table 4 . Observations when you immerse LED into liquid nitrogen LED Observations when immersed into (or removed from) liquid nitrogen Amber Amber-yellow-light green/yellow Slowly goes back to amber as it warms up

6 Figure 4. Band structure as a function of interatomic distance, a. (image: By Chetvorno - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=56983339 ) Results The results section should include, 1. The four tables above (1 - 4). 2. Any descriptive/qualitative results/observations. Please answer the following questions, 1. Forward bias the LED and you will see the color of the LED. Reverse bias (switch the polarity of the leads) of the LED and you will see no color. Use Figure 3 to explain what is happening. a. Forward bias leads to an LED, reverse bias causes the LED to turn into a solar cell. Figure 2 shows reversible effects of the LED. 2. Why do some LEDs generate a photovoltage in some LEDs but not in other LEDs (Table 2)? Which color LED generated photovoltage in the most LEDs and why? Which color LED generated a photovoltage in the fewest LEDs and why? Why does white light generate a voltage in all the LEDs? White light is a combination of all light. a. Due to the wavelengths. Blue LED generated photovoltage in more due to its low wavelength/high bandgap. Red LED fewest due to high wavelength/low bandgap. 3. Use Figure 4 to explain the color changes in the amber LED as you cool it down (or warm it up). a. Lower temperature leads to high bandgap which is lower wavelength. Green has higher wavelength than amber. 4. From Table 1, what materials do you think your LEDs are made from? a. Mostly gallium. nitrogen, phosphorous, and arsenide are doped 5. According to the spectra in Table 2 , what colors do each LED emit? Why don’t you see all of the colors? a. Red led emits red. Amber led emits blue and yellow. Green led emits green and yellow. Blue led emits purple, blue, orange and red,

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