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White LED: The Future Lamp

LED stands for Light Emitting Diode. An LED is a semiconductor chip that converts electrical energy into light. The conversion of energy into light happens on the quantum level within the molecular makeup of the semiconductor chip. The process begins with the chip acting as a diode with two terminals, a P (Positive hole carrier) and N (Negative electron) region in its basic structure, which allow the chip to conduct in one direction for operation. In addition, there are added chemical layers called epitaxy layers that enhance the ability of the device to emit light (Photons). As electrical energy passes through the P and N regions of the LED, electrons move to higher energy levels called band gap potentials. To meet the conservation of energy law, the electron's excess energy, gained while moving energy levels, will then produce a photon that our eye will perceive as light. At this point, the band gap potentials equal the energy of the photon created when the electron that was moving energy levels comes back to the ground state.

The colour of the light emitted directly relates to the size of the band gap potentials or the amount of energy the photons produce. Since different colours occur at different band gap potentials, or energy levels, this explains why different colour LEDs exhibit different forward voltages to operate. Recent advances in LED technology have led to brighter LEDs due to higher quantum efficiencies and higher chip extraction efficiencies. Another recent development of a blue color LED has led to RGB (Red Green Blue) white lighting as well as Phosphor on Blue to form white LEDs. The technique of Phosphor coating on Blue has shown that in the near future, white lighting from solid-state sources is a possibility, which has led to a lot of excitement.


Bright LEDs For Outdoor Applications
The first LEDs bright enough for use in outdoor applications were made of aluminium-gallium arsenide (AlGaAs). These red LEDs appeared as high mount-stop lights on automobiles and in a limited number of traffic lights. The recent advent of efficient green, blue and white LEDs may lead to more applications. Aluminium-gallium-indium phosphide (AlGaInP) and indium-gallium-nitride (InGaN) LEDs have succeeded AlGaAs as the brightest available LEDs. AlGaInP LEDs range in color from red to amber and produce about 3 lumens with efficacies greater than 20 lumens per electrical watt, although green and yellow AlGaInP LEDs have much lower efficacies. Hewlett-Packard plans to release AlGaInP LEDs with a light output of more than 10 lumens per LED.

 

 

 

In addition to providing an accurate input current limit, theLT1618 can also be used to provide a regulated outputcurrent for current-source applications. White LED driversare one such application for which the LT1618 isideally suited. With an input voltage range of 1.6V to 18V,the LT1618 can provide LED drive from a variety of inputsources, including two or more alkaline cells, or one ormore Li-Ion cells. The circuit in Figure 7 is capable ofdriving six white LEDs from a single Li-Ion cell. LEDbrightness can be adjusted using a pulse width modulated(PWM) signal, as shown, or by using a DC voltage to drivethe IADJ pin directly, without the R3/C3 lowpass filter. Ifbrightness control is not needed, simply tie the IADJ pin toground. Typicaloutput voltage with the LEDs shown isaround 22V, and the R1, R2 output divider sets themaximum output voltage to around 26V to protect theLT1618 if the LEDs are disconnected. The LT1618’s
constant-current loop regulates 50mV across the 2.49W
sense resistor, setting the LED current to 20mA.

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BiCMOS Technology

The need for high-performance, low-power, and low-cost systems for network transport and wireless communications is driving silicon technology toward higher speed, higher integration, and more functionality. Further more, this integration of RF and analog mixed-signal circuits into high-performance digital signal-processing (DSP) systems must be done with minimum cost overhead to be commercially viable. While some analog and RF designs have been attempted in mainstream digital-only complimentary metal-oxide semiconductor (CMOS) technologies, almost all designs that require stringent RF performance use bipolar or semiconductor technology. Silicon integrated circuit (IC) products that, at present, require modern bipolar or BiCMOS silicon technology in wired application space include the essential optical network (SONET) and synchronous digital hierarchy (SDH) operating at 10 Gb/s and higher. The viability of a mixed digital/analog. RF chip depends on the cost of making the silicon with the required elements; in practice, it must approximate the cost of the CMOS wafer, Cycle times for processing the wafer should not significantly exceed cycle times for a digital CMOS wafer. Yields of the SOC chip must be similar to those of a multi-chip implementation. Much of this article will examine process techniques that achieve the objectives of low cost, rapid cycle time, and solid yield