INDUSTRY LEDs
AFTER PRICE, imperfect colour quality is the biggest criticism levelled at the LED light bulb. This downside stems from the way that the white light is generated: A GaN-based, blue-emitting chip pumps a yellow phosphor, and the mixing of these two colours produces white light. With this approach, the output does not feature a significant contribution from the red region of the visible spectrum.
A superior approach for making lighting products – that is also an option for solid- state projection displays – is to generate white light by mixing the emission from red, green and blue LEDs. Advantages of this approach are not limited to a higher colour-rendering index, and also include the opportunity for higher efficacy and flexible colour steering.
To produce a high-efficacy system with this form of colour mixing, efficient sources must be employed. Blue and red LED performance is already impressive, with recent improvements spurring peak power conversion efficiencies beyond 81 percent and 70 percent, respectively, but the green cousin is lagging far behind. This particular species suffers from a problem commonly known as the green gap.
Going green
The big challenge associated with trying to make an efficient green LED is that there is not an ideal, mature material system to work with. The III-N family that is used to create powerful blue LEDs is far less efficient at longer wavelengths, and a similar problem plagues the III-phosphides that are very efficient in
Figure 1: Efficacy of III–nitride (green data points) and III–phosphide (red data points) LEDs with different wavelengths (data taken from recent publications). The blue lines represent the CIE 1924 luminosity function multiplied by the corresponding value of the wall plug efficiency (WPE). Marked in yellow is the green–yellow range, which is not adequately covered by either the III–nitrides or the III–phosphides. This is the essence of the green gap problem
the red range; extend emission of this class of LED to shorter wavelengths and efficiency plummets. So, in short, both material systems present low efficiencies in the green-yellow spectral range (see Figure 1).
Applications for high efficiency green LEDs vary from projection systems to backlighting, video walls and solid-state lighting with a high colour- rendering index
With the III-phosphides, the falling efficiency as emission is propelled to the green is a fundamental limitation of the material system. Altering the composition of AlInGaP so that it emits in the green – rather than red, orange, or yellow – leads to insufficient carrier confinement, due to the relatively low band gap of this material system. This rules out efficient radiative recombination.
In comparison, for III-nitrides, the barriers to high efficiency may be very tough, but they are not insurmountable. With this material system, two factors are behind the decline in efficiency as emission stretches to the green: a fall in external quantum efficiency and a decrease in electrical efficiency.
The first weakness has its origins in the need to apply an extraordinarily high forward voltage to green LEDs. These devices feature extremely high internal piezoelectric fields. So, for a given current, the voltage that has to be applied to this type of LEDs is even higher than that for a blue variant, despite
October 2013
www.compoundsemiconductor.net 33
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142