Accurately Characterizing a Light Emitting Diode

Kegan Orlowski and John Noé

Laser Teaching Center
Department of Physics and Astronomy
Stony Brook University

In near future virtually all artificial light will come from light emitting diodes (LEDs), which have much higher energy efficiency and last far longer than current light sources, especially incandescent bulbs. LEDs are special semiconductor devices that produce light when a voltage is applied across them. The electrons in the semiconductor material are excited from an energy level called the valence band to the conduction band. The energy difference between the two bands is called the bandgap energy(Eg). The electrons emit photons with energy Eg as they fall from the conduction band to the valence band in a process called radiative recombination. The bandgap energy is determined by the materials the semiconductor is made of and in turn determines the wavelength of light produced. Already white LEDs light many of Stony Brook University's parking lots. There are two ways to produce white light with an LED. One method places red, green, and recently developed blue LEDs adjacent to each other to give a white appearance. The other approach is to wavelength-shift the light from a blue LED with the use of phosphors. The rapidly improving LED technology is destined to become the number one man-made light source within our lifetimes.

The overall goal of this research is to accurately characterize the electro-optical properties of light emitting diodes. The project is intended to complement and enhance lab assignments in the junior electronics lab, PHY335. As a first step we have measured the current-voltage(I-V) curve of a red LED over more than eight orders of magnitude, from less than one nanoampere to about 50 milliamps. The current through any diode rises exponentially with increasing forward voltage according to William Shockley's diode equation, I=Is[exp(eV/nkBT)-1]. Due to the exponential in this equation the current is very sensitive to both the applied voltage V and the diode junction temperature T. Here e is the charge on the electron, kB is Boltzmann's constant, and n is an "ideality constant" close to unity.

The low-current measurements were performed using a Datel DVC 8500 variable voltage source and a Keithley 486 Picoammeter. Above 2 mA the regular PHY335 setup with two voltmeters, variable resistors, and a fixed voltage supply was employed. Over much of the current range the measured I-V curve is in good agreement with the diode equation. However above about 1 mA the current did not rise as quickly as expected. A rise in temperature of the diode junction is believed to be the cause. We have been successful in modeling this effect numerically. Also, for very low currents we noticed that the observed current depended on the ambient light level. The final measurements were done in the dark. Future research could involve trying to better control the LED temperature, measurements of optical output power, and experiments with other types (colors) of LEDs.

We thank Chris Corder (Metcalf group) for the use of the picoammeter.

[1] "Net Lecturer"."Introduction to Electronic Devices". N.p., n.d., Web. 14 Apr. 2014.