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Experiments with Light-Emitting Diodes

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Experiments with Light-Emitting Diodes Experiments with light-emitting diodes Yaakov Kraftmakhera) Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel (Received 15 February 2011; accepted 16 May 2011) The radiant and luminous power spectra, efficiency, and luminous efficacy of commercially available light-emitting diodes (LEDs) are measured. The output radiant power is determined with a silicon photodiode from its typical spectral response. A calculation of the radiant power spectra and the luminous power spectra is demonstrated. The frequency response of the LEDs is determined in the range 10–107 Hz. For the white LED, the frequency response of the primary blue emission and the green-yellow phosphorescence is measured separately, and the phosphorescence time constant is estimated. The ratio h/e is estimated using the emission wavelengths and the “turn- on” voltages. VC 2011 American Association of Physics Teachers. [DOI: 10.1119/1.3599072] I. INTRODUCTION II. THE SETUP A light-emitting diode (LED) is a semiconductor device Three color LEDs and one white LED from HuiYuan Opto- with a p-n junction that emits photons when electric cur- Electronic20 were used in the experiments: LB-P200R1C-H3 rent passes through it.1–3 The semiconductor crystal is (red), LB-P200Y1C-H3 (yellow), LB-P200B2C-H3 (blue), doped to fabricate an n-type region and a p-type region, and LB-P20WC3-60 (white). The input current of the LEDs one above the other. Forward electrical bias across the indicated by the supplier is 0.75 A. In the experiments we will LED causes the holes and electrons to be injected from discuss, the maximum input current is 0.1 A, and the input opposite sides of the p-n junction into the active area, power does not exceed 0.3 W. where their recombination results in emission of photons. Two types of white LEDs are available. One type com- The energy of the emitted photons is approximately the bines two or three LEDs of appropriate colors. With such a band-gap energy of the semiconductor. The band-gap device, a spectrum similar to that of daylight is achievable. energy of ternary and quaternary semiconductor com- Modifications of the light from “warm white” to “cool pounds can be adjusted in a certain range by varying their daylight” are possible by varying the contributions of the composition. components. In the other type a suitable phosphor is posi- The first LEDs were homojunction diodes in which the tioned onto a blue LED, so that the output light contains blue material of the core layer and that of the surrounding clad light and a Stokes-shifted phosphorescence band. The white layers are identical. Then heterostructures with layers having LED we used is of the second type. a varying band-gap and refractive index were recognized as An important characteristic of a lighting source is the advantageous. Contemporary LEDs are more complicated color temperature (see, for example, Ref. 21). The color tem- double heterostructure diodes. perature does not mean that the spectrum of a light source is LEDs are efficient light sources for many applications, similar to that of a blackbody of equal temperature. Fluores- including indicators, large-area displays, and opto-couplers. cent lamps and LEDs are designed to emit only visible light. Holonyak4 pointed out that the LED is an ultimate light Therefore, their spectra differ radically from those of thermal source. Mayer5 has considered the current status and pro- sources governed by Planck’s distribution, which unavoid- spective of solid-state lighting, where the LED is an excel- ably include intensive infrared emission. The spectrum of a lent alternative to incandescent and fluorescent light bulbs. lighting source can be characterized by the blue-to-red ratio, LEDs are easily modulated sources and are widely used in which can be made to be equal to that of a blackbody at a optical communications with optical fibers.6 The possibility given temperature. This ratio determines the color tempera- to modulate LEDs in a broad frequency band is crucial for ture of the source. The green band in the spectrum is needed simultaneously transmitting many television or audio pro- for attaining high luminous efficacy of lighting sources (see grams through a single optical fiber. To correctly reproduce the following). For the white LED we used, the supplier the programs, the amplitude modulation characteristic should claims that the color temperature is in the range 6000–7000 be linear. K. This value is typical for “cool daylight” lamps. For “warm LEDs can be used to demonstrate their basic properties7–15 white” lamps, the color temperature is in the range 2700– and as auxiliary tools for many experiments and demonstra- 3300 K. tions.16–19 The experiments described in the following can be The input electric current and power, radiant output considered as an addition to those published earlier.7–15 Im- power, and efficiency of the LEDs can be measured or calcu- portant topics are the determination of the output radiant lated, and then displayed versus the voltage applied to the power, the radiant and luminous power spectra, and the lumi- device or versus the current passing through it. With a data- nous efficacy of LEDs. For a white LED, the frequency acquisition system, the measurements are possible in a short response of the primary blue emission and of the green-yellow time. We use the ScienceWorkshop data-acquisition system phosphorescence is measured separately. The value of h/e is with DataStudio software from PASCO.22 The LED of inter- calculated from the emission wavelengths and the “turn-on” est is connected in series with a 10 X limiting resistor to the voltages. Signal generator in the ScienceWorkshop 750 Interface. The 825 Am. J. Phys. 79 (8), August 2011 http://aapt.org/ajp VC 2011 American Association of Physics Teachers 825 Fig. 1. General scheme of the experiments: (a) output radiant power and ef- ficiency; (b) emission spectra; and (c) frequency response. PD represents Fig. 2. (Color online) Input current and power consumed by the LEDs ver- photodiode. sus the applied voltage. Red (R), yellow (Y), blue (B), and white (W). The turn-on voltage increases with the energy of the emitted photons. Output voltage is the Positive ramp up voltage linearly increasing from zero to a maximum value set to achieve the maximum desired current through the LED. The period of spectral response of the photodiode taken from such a graph the Output voltage is 20 s. The Signal generator operates in can be represented as Auto mode: it starts to generate the Output voltage after start- R k I=P A=W 1:2 10À3 k 300 ; (1) ing a run. The option Automatic stop is used for automati- ð Þ¼ ð Þ¼ Â ð À Þ cally ending each run. where k is the wavelength in nanometers. With minor modi- The output radiant power of the LEDs is determined with fications, Eq. (1) holds for all silicon photodiodes. This a silicon photodiode by using its typical spectral response. approach is not as precise as a measurement with a sensor The radiant power spectra are obtained with a diffraction based on the thermal action of absorbed light. However, it is grating and converted to the luminous power spectra by much simpler and satisfactory for our purposes. using the standard luminosity function. The frequency DataStudio displays the output characteristics of an LED response of a LED is determined by sine-wave modulation versus the input current (Fig. 3). The output radiant power is of the feeding current. Setups used in the experiments are nearly proportional to the input current, and thus the rapid schematically shown in Fig. 1. increase of the input current or power indicates the threshold of the LED emission. III. MEASUREMENTS AND RESULTS For the color LEDs, the wavelengths for Eq. (1) are taken A. Radiant Power and Efficiency to be at the peaks of the radiant power spectra (see the fol- lowing). For the white LED, the mean wavelength is taken The input current i of the LEDs is measured directly as the as 550 nm. This simplification introduces an additional Output current of the Signal generator. DataStudio calcu- uncertainty to our results. For 100 mA input currents, the lates the voltage applied to the LED as the Output voltage output radiant power of the LEDs ranges from 13 mW (yel- minus the Output current times the resistance of the limiting low) to 53 mW (white). The efficiency is the ratio of the out- resistor, 10 X. The input electric power is calculated from put radiant power to the input electric power. For the LEDs the input current and applied voltage. Immediately after a tested, the efficiency ranges from 0.065 (yellow) to 0.19 run, DataStudio displays the input current and power versus (white). For input currents in the range of 10–40 mA, the ef- the applied voltage (see Fig. 2). ficiency of the white LED is even higher and is 0.21. To determine the output radiant power of an LED, its light In an ideal LED every electron-hole recombination pro- is directed onto a silicon photodiode (United Detector Tech- duces one output photon of energy nearly equal to the band- nology, PIN-10D) positioned adjacent to the LED. The sensi- gap energy. The external quantum efficiency of such an LED tive area of the photodiode is about 1 cm in diameter, so that thus equals unity. Similarly, an ideal photodiode produces the light from the LED is almost fully utilized. The Voltage one electron for every incident photon, and hence its external sensor acquires the voltage on a 100 X load resistor of the quantum efficiency also equals unity (see Ref.
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