2.3 LED Optical and Electrical Characteristics
15
increases. If there is no protection circuit, the LED device will be burned down due
to the accumulation of heat resulted from the increase of the current. LED does not
emit light when it is reversely biased. However, the LED does have reverse current
which is very small. Usually, the reverse voltage of LED should not exceed 10 V, and
the maximum is no more than 15 V. If the reverse bias voltage exceeds this voltage,
the reverse breakdown will occur, which will lead to the damage of LED [13].
Current vs. voltage (I–V) characteristic is one of the most important parameters in
evaluating the electrical performance of LED. The following important parameters
can be obtained from the I-V characteristic curve:
Forward voltage V F : The voltage of the LED when it is loaded with 20 mA or
350 mA forward current.
Forward current I F : For low-power LED, its forward working current is generally 20 mA. For high-power chip, the forward working current should be determined
according to the size of the chip, which is generally 350 mA.
Reverse leakage current I R : It refers to the current of the LED when a 5 V
reverse bias voltage is applied to the device. Usually the smaller the reverse leakage
current is, the better the electrical performance of the LED is.
Reverse breakdown voltage V R : In general, the reverse voltage V R can be tested
when the reverse current is maintained at a specified value. The reverse current is
generally between 5 and 100 µA. Reverse breakdown voltage usually cannot exceed
20 V.
2.4 Principle of White LED
2.4.1 The Principle of Three Primary Colors and Addition
of Light
Human eye recognizes color according to the wavelength of light. Most of the colors
in the visible spectrum can be mixed by three basic colors in different proportions.
The three basic colors are red, green and blue. When these three kinds of light mix in
the same proportion and reach a certain intensity, they appear white (white light); if
the intensity of the three kinds of light is zero, it is black (dark), which is the principle
of additive method [14, 15]. Using the principle of additive method, white LED can
be obtained through different ways.
2.4.2 The Realization Method of White LED
White light-emitting diodes have the advantages of small size, high efficiency, long
life, energy saving and environmental protection, and will replace traditional light
sources in the future [16]. From the packaging level, the main fabrication methods
15
increases. If there is no protection circuit, the LED device will be burned down due
to the accumulation of heat resulted from the increase of the current. LED does not
emit light when it is reversely biased. However, the LED does have reverse current
which is very small. Usually, the reverse voltage of LED should not exceed 10 V, and
the maximum is no more than 15 V. If the reverse bias voltage exceeds this voltage,
the reverse breakdown will occur, which will lead to the damage of LED [13].
Current vs. voltage (I–V) characteristic is one of the most important parameters in
evaluating the electrical performance of LED. The following important parameters
can be obtained from the I-V characteristic curve:
Forward voltage V F : The voltage of the LED when it is loaded with 20 mA or
350 mA forward current.
Forward current I F : For low-power LED, its forward working current is generally 20 mA. For high-power chip, the forward working current should be determined
according to the size of the chip, which is generally 350 mA.
Reverse leakage current I R : It refers to the current of the LED when a 5 V
reverse bias voltage is applied to the device. Usually the smaller the reverse leakage
current is, the better the electrical performance of the LED is.
Reverse breakdown voltage V R : In general, the reverse voltage V R can be tested
when the reverse current is maintained at a specified value. The reverse current is
generally between 5 and 100 µA. Reverse breakdown voltage usually cannot exceed
20 V.
2.4 Principle of White LED
2.4.1 The Principle of Three Primary Colors and Addition
of Light
Human eye recognizes color according to the wavelength of light. Most of the colors
in the visible spectrum can be mixed by three basic colors in different proportions.
The three basic colors are red, green and blue. When these three kinds of light mix in
the same proportion and reach a certain intensity, they appear white (white light); if
the intensity of the three kinds of light is zero, it is black (dark), which is the principle
of additive method [14, 15]. Using the principle of additive method, white LED can
be obtained through different ways.
2.4.2 The Realization Method of White LED
White light-emitting diodes have the advantages of small size, high efficiency, long
life, energy saving and environmental protection, and will replace traditional light
sources in the future [16]. From the packaging level, the main fabrication methods