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LED Circuit and Device Physics 120 Pts Objective: Meas Applied Quantum Mechanics for Electrical Engineers Workshop III – LED Circuit and Device Physics 120 pts Objective: Measure the voltage drop across an LED in a simple circuit, demonstrate that the voltage drop is proportional to the band gap of the semiconductor material, and analyze the behavior of the circuit in terms of the device physics of the PN junction. Background: A light emitting diode (LED) is a device that is designed by fabricating a PN junction or diode as illustrated in Figure 1. Figure 1. Schematic of LED and illustration of carrier movement resulting in emission of light. Image: http://blog.ucsusa.org/wp-content/uploads/2014/10/UCS-LED-how-it-works-graphic.gif A simple PN junction can be fabricated by growing a p-type material on top of an n-type material, or by doping an n-type material with sufficient p-type dopants so that the surface becomes p-type. The energy band diagram for a PN Junction under forward bias conditions is illustrated in Figure 2, along with a schematic of the circuit under forward bias. Figure 2. LED circuit and PN Junction energy band diagram illustrating carrier movement under forward bias. Image: http://www.vit.ac.in/onlinelab/m9.aspx The application of a forward bias to the circuit results in a decrease in the barrier to electron movement from the n-side to the p-side. Likewise, the barrier of hole movement from the p- side to the n-side is also reduced. Thus, majority carriers from each side are able to cross the energy barrier. At the junction or interface where the p-side joins the n-side, there is a large concentration of holes and electrons. This results in a high recombination rate and the emission of photons. The energy of the emitted photons will be equal to the band gap of the semiconductor (i.e. the band gap of the semiconductor used to fabricate the PN junction). This process is called electroluminescence. When the LED is placed in a circuit, a resistor is added to reduce the circuit current below the maximum allowed LED current based on manufacturer specifications (see Table 1 for red, yellow, green and blue LEDs). The potential drop across the LED in a simple circuit is equal to the forward bias when the p-side is connected to the high potential side of the power supply, and the n-side is connected to the low potential side. The remaining potential in the circuit is dropped across the resistor. The energy of the photons emitted can be calculated as a function of the radiation wavelength as follows: ℎ푐 퐸 = Eq. 1 휆 where h = Planck’s constant = 6.63 x 10-34 J s c = speed of light = 3 x 108 m/s λ = wavelength of radiation in units of length Materials: 1 multimeter, 1 breadboard, 1 9V battery, 2 kΩ resistors, 1 battery connection, 1 connector, 1 red LED, 1 yellow LED, 1 green LED and 1 blue LED. There are several semiconductor materials that have band baps within the visible spectrum (400 nm – 700 nm). Table 1 includes 4 types of LEDs along with their corresponding wavelength (λ) in nm, maximum forward bias current in mA and part number (the LEDs for this workshop were purchased from Jameco Electronics). Table 1 - LED Specifications Material Color Wavelength, nm Max. Forward Part No. Current, mA AlGaAs Red 660 30 333973 GaAsP Yellow 585 20 34825 GaP Green 565 30 34761 GaN Blue 430 30 137411 LED Circuit: An LED circuit is shown in Figure 3. The resistor limits the maximum current (Imax) in the circuit and protects the LED. The maximum current flowing through the circuit can be estimated by 푉푏푎푡푡푒푟푦 퐼푚푎푥 = 푅1 Eq. 2 which assumes there is no diode in the circuit. When an LED is added to the circuit, there is a potential drop across the LED that controls the circuit current. The current through the diode is equal to the current through the resistor which is equal to (푉푏푎푡푡푒푟푦− 푉푑푖표푑푒) 퐼푑표푑푒 = 퐼푟푒푠푠푡표푟 = Eq. 3 푅1 R1 Figure 3 – Simple LED circuit Image: https://ccrma.stanford.edu/courses/250a/0708/lectures/electronics/ Step 1: Calculate energy associated with electromagnetic radiation Calculate the energy of the photons (in units of eV) for each LED as a result of band-to-band recombination for each of the LEDs shown in Table 1. Use equation 1 and how all calculations and unit conversions for each LED and add your answers to Table 2 below. [20 pts] AlGaAs (Red) – GaAsP (Yellow) - GaP (Green) – GaN (Blue) – Table 2 - Wavelength and photon energy for Red, Yellow, Green, and Blue LEDs Material Color Wavelength, nm Photon Energy, eV AlGaAs Red 660 GaAsP Yellow 585 GaP Green 565 GaN Blue 430 Step 2: Determine maximum current specifications Use equation 2 to estimate the minimum resistance () required in order to protect all the LEDs based on the current specifications listed in Table 1. Measure the actual voltage value of the 9V battery and use this value in your calculations. Only 1 calculation is required for this step, since you need to determine the minimum resistance in order to protect all the LEDs. [10 pts] Step 3: Construct LED Circuit and measure Vdiode and Idiode Record the measured resistance of a 1000 Ω the resistor in Table 3 below. Construct an LED circuit as illustrated in Figure 3 using a 1000 resistor for each of the LEDs. Connect the positive battery lead to the resistor and connect the resistor to the anode or positive end of the LED (longer lead). Connect the negative battery lead to the cathode side of the LED (shorter lead). Measure the voltage across each LED in parallel and measure the diode current in series. Note: Do not measure the diode current in parallel across the resistor. Measure and record the battery voltage before each of your LED measurements. Include each measurement in Table 3. [30 pts] Table 3 – Measured Diode Voltage and Current Material Color Wavelength, nm Vbattery Vdiode Idiode Resistance of 1kΩ : _____ _____ V mA AlGaAs Red 660 GaAsP Yellow 585 GaP Green 565 GaN Blue 430 Step 4: Calculate Idiode Using equation 3, calculate the theoretical diode current for each LED using the measured voltage of the battery, measured resistance of the resistor and the measured voltage across each LED, and compare your answers to the measured diode current using equation 4. Show all calculations for each LED and place your answers in Table 4. [30 pts] (퐼푑푖표푑푒푡ℎ푒표푟푒푡푐푎푙 − 퐼푑푖표푑푒푚푒푎푠푢푟푒푑) Eq. 4 % 퐷푖푓푓푒푟푒푛푐푒 = ∗ 100 퐼푑푖표푑푒푡ℎ푒표푟푒푡푐푎푙 AlGaAs (Red) – GaAsP (Yellow) - GaP (Green) – GaN (Blue) – Table 4 – Comparison of measured diode current to theoretical diode current Material Color Wavelength, nm Idiode, mA Idiode, mA % measured calculated Difference AlGaAs Red 660 GaAsP Yellow 585 GaP Green 565 GaN Blue 430 Step 5: Analysis A. Explain the difference between the measured and theoretical diode current. [5 pts] B. Reverse the LED leads in your circuit. What do you observe? Can you measure a current? [5 pts] C. Explain your results from part B in terms of the energy band diagram for a PN junction. See background explanation above. [10 pts] D. Add a second 1000 Ω resistor in parallel with the first which will decrease the equivalent resistance from 1000 Ω to 500 Ω (still above the minimum resistance required to protect the LEDs). Test the brightness of any of the LEDs with and without the second resistor. What is happening to the voltage drop across the LED? What is happening to the energy band diagram in Figure 2? [10 pts] .
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