Visible light emitting diodes (LEDs) combine high efficiency and long lifetimes. Today, they’re used in a wide range of applications. Extensive R&D efforts by manufacturers have led to the creation of devices with higher luminous flux, longer lifetimes, greater chromaticity, and more lumens per watt. Accurate and cost-effective testing is critical to ensure device reliability and quality.
LED testing involves different types of test sequences at various stages of production, such as during design R&D, on-wafer measurements during production, and final tests of packaged parts. Although testing LEDs typically involves both electrical and optical measurements, this article focuses on electrical characterization, including light measurement techniques where appropriate.
Figure 1 illustrates the electrical I-V curve of a typical diode. A complete test could include a multitude of voltage values versus current operating points, but a limited sample of points is generally sufficient to probe for the figures of merit.
Fig 1: Typical LED DC I-V curve and test points (not to scale).
Many tests require sourcing a known current and measuring a voltage; others require sourcing a voltage and measuring the resulting current. Therefore, high speed test instrumentation with integrated, synchronized sourcing and measuring capabilities is ideal for these types of tests.
Forward voltage test
In an LED test sequence, the forward voltage (VF) test verifies the forward operating voltage of the visible LED. When a forward current is applied to the diode, it begins to conduct. During the initial low current source values, the voltage drop across the diode increases rapidly, but the slope begins to level off as drive currents increase. The diode normally operates in this region of relatively constant voltage. It is also quite useful to test the diode under these operating conditions.
The VF test is performed by sourcing a known current and measuring the resulting voltage drop across the diode. Typical test currents range from tens of milliamps to amps, while the resulting voltage measurement is typically in the range of a few volts. Some manufacturers use the results of this test for binning purposes because the forward voltage is related to the chromaticity (the quality of color characterized by its dominant or complementary wavelength and purity taken together) of the LED.
Forward current biasing is also used for optical tests because electrical current flow is closely related to the amount of light emitted. Optical power can be measured by placing a photodiode or integrating sphere close to the device under test to capture the emitted photons. This light is then converted to a current, which is measured using an ammeter or one channel of a source-and-measure instrument.
In many test applications, the voltage and light output of the diode can be measured simultaneously using a fixed source current value. In addition, details such as spectral output can be determined from the same drive current value by using a spectrometer.
Reverse breakdown voltage test
Applying a negative bias current to the LED will allow probing for the reverse breakdown voltage (VR). The test current should be set to a level where the measured voltage value no longer increases significantly when the current is increased slightly more. At levels higher than this voltage, large increases in reverse bias current result in insignificant changes in reverse voltage.
The specification for this parameter is usually a minimum value. The VR test is performed by sourcing a low-level reverse bias current for a specified time, then measuring the voltage drop across the LED. The measurement result is typically in the range of tens of volts.
Leakage current tests
Normally, moderate voltage levels (volts to tens of volts) are used to measure a leakage current (IL). The leakage current test measures the low-level current that leaks across the LED when a reverse voltage less than breakdown is applied. It is a common practice for leakage measurements, and more generally for isolation measurements, to make sure only that a certain threshold is not exceeded in production.
There are two reasons for this. First, low current measurements require longer settling times, so they take longer to complete. Second, environmental interference and electrical noise exert greater influence on low-level signals, so extra care in shielding is required. This extra shielding complicates the test fixture and may interfere with automated handlers.