LEDs have been manufactured since the early 1970s, but in recent years advances in this technology have accelerated notably. The maximum available brightness for LEDs has increased by several orders of magnitude, starting with less than 0.01 lumen at 20 mA in the early 1970s and reaching more than 1 lumen now. With these improvements, target applications for LEDs have also broadened significantly. At luminosity levels above 0.3 lumens or so, LEDs start to become practical replacements for incandescent lamps. As such they provide a number of benefits, not the least of which is their great reliability. In general, today's LEDs will long outlive any of the systems in which they are used.
Nowhere is this characteristic more desirable than in automotive systems, where reliability is a matter both of safety and of avoiding expensive repairs. So it is not surprising that the automotive industry has moved relatively quickly to embrace LED technology wherever practicable and is, in fact, driving further innovation in the LED field because of particular automotive requirements.
Evolution of LED brightness
Between 1970 and 1995, LEDs evolved gradually to offer higher levels of brightness. Since the mid-1990s, however, the pace of innovation has accelerated, with the invention of blue and white LEDs and a doubling of average brightness for the devices.
Improvements in LED brightness are mainly due to advances in substrate materials. Beginning with the first gallium arsenide (GaAs) phosphide products, the industry turned to nitrogen-doped GaAsP and GaP to achieve the first yellow and green LEDs in the late 1970s, and then used single- and double-hetero GaAlAs to achieve luminosities of over 0.1 lm in the early 1990s. Since the early 1990s, various combinations of indium and gallium have served as the substrates for newer, brighter LEDs in colors including blue.
Despite these advances, several problems remained, including the fact that the substrate tends to absorb much of the light generated by the LED. Several approaches have been taken to work around this issue. Lumileds, for its part, attacked the problem by using a patented, transparent AlInGaP substrate. Another approach was to add a Bragg reflector grating layer above the substrate. This provides twice the brightness of LEDs with an absorbent substrate, but any light that is emitted at a 90° angle is lost. Vishay has improved on this solution with an organic mirror adhesion (OMA) technology, in which a mirror surface is grown on a silicon substrate. All the light that goes down to the mirror comes out of the front of the device, thus achieving the same level of brightness as is achieved with the transparent substrate approach, for approximately a four-fold improvement over standard LEDs.
LEDs don't just need to be bright; they also need to work efficiently. This means not only converting electric power into light with minimal losses, but also controlling the effect of the heat generated by the current running through the device. A huge challenge of LEDs is that their junction temperature, which goes up when more current is applied, has a direct effect on the wavelength of the light they produce. Simply increasing the forward current can thus merely change the color of the device without making it any brighter. Thus, die and packaging technologies are needed that allow more brightness with less current.
Vishay has addressed this problem with a series of low-profile PLCC packages that dissipate heat very efficiently and thus reduce the problem of color change brought on by higher current levels. The exclusive PLCC-3, which is pad-compatible with the industry-standard PLCC-4, provides a large metallic area through which heat can be dissipated and thus accommodates very bright LEDs powered by forward currents up to 50 mA using standard dies. Offered in a PLCC-3 package with a low 270 k/W thermal resistance rating, devices in the new TLMx320x series can be driven with higher currents to enable twice the brightness of comparable LEDs in the PLCC-2 package.