Surface Mount LEDs – Part 1: The Basics
This is Part 1 of a 4 Part Series
- Surface Mount LEDs – Part 1: The Basics (this article)
- Surface Mount LEDs – Part 2: How They are Made
- Surface Mount LEDs – Part 3: Optical Properties
- Surface Mount LEDs – Part 4: Applications
Introduced in the early 1960s, Light Emitting Diodes (LEDs) have advanced to become the light source of choice. In comparison to incandescent and fluorescent light sources they are more efficient and longer lasting. They are also smaller (as small as 0402 case size), and are especially well-suited to surface mount packaging, which enables them for use with low cost, automated assembly processes making them more cost efficient as well. LEDs are available providing light of pretty much any color across the visible spectrum, as well as numerous wavelengths in the IR and UV portions of the electromagnetic spectrum (EMS), giving designers much more latitude and flexibility than other technologies.
LEDs are two pole solid state devices. Like other diodes (excluding Zeners), they should only be operated with forward bias, as reverse bias can ruin them. When sufficient forward bias is imposed across an LED it begins to flow current. As part of this process electrons and holes combine, emitting photons with energies that are directly related to the band gap of the semiconductor material used to make the LED. This is known as electroluminescence. Most LEDs are made with direct band gap semiconductors, enabling light emission to occur efficiently, with little heat generation. Over time, as ever wider band gap semiconductor materials have been developed, LEDs emitting ever higher frequencies (well into the UV portion of the EMS) have been developed.
LEDs are nearly monochromatic, emitting light that is within a relatively small wavelength range depending upon the band gap of the associated semiconductor p-n junction. Wider band gaps produce higher energy (shorter wavelength) light such as blue, violet and UV, while narrower band gaps produce lower energy (longer wavelength) light such as red or infrared. The band gap also influences the forward bias necessary for current flow (light emission), with shorter wavelengths requiring more forward bias than longer wavelengths.
As with all diodes an LED is a non-ohmic device, meaning its current flow vs. voltage relationship is non-linear as shown below. LEDs are different however as breakdown voltage (VBreakdown) is generally less than that of a typical diode (~-5V vs. ~-50V). Also forward voltage (VForward) is generally more than that of a typical diode (~2.2 to 4V+ vs. ~0.3V for Germanium-based semiconductor devices or ~0.7V for Silicon-based semiconductor devices), increasing with increasing frequency of the light emitted, also shown below.
Thus, it is especially important that an LED not experience reverse bias, as breakdown occurs at relatively low voltage, and relatively low reverse current damages them. It is also important that VForward not exceed a value resulting in forward current (iForward) that exceeds specification. To avoid exceeding iForward limits, resistors are generally used in series with each LED, the resistor value selected such that it limits iForward appropriately based upon the voltage supplied.
Note from the figure that LEDs having relatively short wavelengths are generally used to achieve white light. This is accomplished using a phosphor covering the LED die that is stimulated by the relatively high energy, nearly monochromatic light from the LED. As a result of this stimulation, the phosphor fluoresces across the visible spectrum, emitting white light. Because of this conversion, white LEDs are generally not as efficient as other LEDs.
LEDs are produced using standard, well-developed and cost effective integrated circuit technology. Optics, such as coatings to transition from the high index of refraction LED die material to air, as well as lenses, etc., are added during the automated process, and are important in establishing the appropriate viewing angle, as well as in maximizing luminous efficiency, and in optimizing color mixing and light diffusion, etc. The resulting packaged LED devices are available in surface mount (SMT) configurations, in either single LED packages or multiple LED configurations that are useful for color (e.g., RGB) displays and the like. The SMT LED configuration simplifies design for mass assembly as well.
The high rate of advancement of LED lighting has resulted in surface mount LEDs being the clear choice for indicator lighting and other applications. They are smaller, longer lasting, more reliable and cheaper. They offer more design options and flexibility in achieving design goals. In surface mount form, they are easier to use in automated assembly processes. They are available in colors across the visible spectrum as well as IR and UV. Simply put, they have revolutionized the electronics, lighting and other industries…Let ’em shine, let ’em shine, let ’em shine!