JP2002094120A - Method of selecting light emitting diodes and light emitting diode indicating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of selecting light emitting diodes which reliably discards light emitting diodes having negative resistance characteristics. SOLUTION: A second transistor 6 is set on for a predetermined speed time TR to apply a backward voltage VR to an LED 1, and then a first transistor 5 is set on for a predetermined time Tr to apply a forward voltage to the LED 1. When the forward voltage is applied to the LED 1, a forward current begins to rapidly flow and quickly start the LED 1 to light, if it has a normal p-n junction. But, if the junction structure is abnormal, a time is taken for starting to flow the forward current after applying the forward voltage to the LED 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for selecting light emitting diodes for selecting light emitting diodes having uniform characteristics, and a light emitting diode display.

[0002]

2. Description of the Related Art Recently, a large number of light emitting diodes (hereinafter, referred to as LEDs) have been developed due to high luminance and diversification of light emission colors.
An LED display device having a display screen in which EDs are arranged in a dot matrix is becoming widespread. Since this LED display device can realize a large display screen and has good visibility of an image even from a distance, it is installed, for example, along a road and used to display traffic information and the like.

[0003] An extremely large number (thousands to tens of thousands) of LEDs are used for the display screen of the LED display device, depending on its size. These LEDs need to have almost uniform characteristics. Even if each LED is turned on using the same power supply, if the characteristics of these LEDs are different, lighting spots will appear on the display screen, making it difficult to identify the displayed information, and as a display device. Function is impaired.

[0004] Therefore, it is necessary to select LEDs having substantially uniform characteristics before attaching each LED to the display screen. FIG. 6 illustrates a conventional device for sorting LEDs. In this device, the light emitting diode 101 is inserted between the current limiting resistor 102 and the transistor 103, the transistor 103 is turned on, and a forward current If flows from the battery 104 to the light emitting diode 101. If the light emitting diode 101 emits light, the light emitting diode 101 is selected as a good product. If the light emitting diode 101 does not emit light, the light emitting diode 101 is excluded as a defective product.

[0005]

The graph of FIG. 7 shows the voltage-current characteristic SE of a normal light emitting diode. When the forward voltage exceeds the turn-on voltage Vfo, a forward current flows and the light emitting diode emits light. I do.

Incidentally, among the mass-produced light emitting diodes, there is a negative resistance characteristic SE as shown in the graph of FIG.
Some have 1, SE2, and SE3. Here, the LED having the negative resistance characteristic SE3 has the largest negative resistance, and the turn-on voltage V03 at which the forward current starts to flow is higher than the voltage Vcc of the battery. In this case, the LED does not emit light,
LEDs can be excluded as defective.

However, LEDs having negative resistance characteristics SE1 and SE2 have relatively small negative resistance, and turn-on voltages V01 and V02 at which forward current starts to flow are lower than battery voltage Vcc. For this reason, when a forward voltage is applied to the LED, a forward current flows through the LED, causing the LED to emit light, and an LED that is originally defective is selected as a non-defective product. In addition, the characteristics of such an LED gradually change during use, and the negative resistance gradually increases, and after a lapse of time, the turn-on voltages V01 and V02 become similar to the negative resistance characteristic SE3. Becomes higher than the battery voltage Vcc,
The ED does not emit light. Or, even if it emits light,
The time from the application of the forward voltage to the emission of light was very long. LED can be dynamically lit, L
In the case where the drive voltage of the ED is pulse width modulated, if it takes time to emit light, the luminous intensity of the LED decreases when the voltage application time of the LED is shortened.

When LEDs having such a negative resistance characteristic are mixed with a large number of LEDs on a display screen, luminance unevenness occurs on the display screen, and the quality of a displayed image is significantly deteriorated.

In an LED having a negative resistance characteristic,
It is presumed that the PN junction is formed instead of the PN junction due to the disorder of the carrier concentration near the PN junction. In particular, in the case of a gallium phosphide LED whose emission color is green as described in JP-A-7-326792, the PN
Since the luminous efficiency increases as the carrier concentration of the N layer of the junction decreases, the carrier concentration is set as low as possible, so that the carrier concentration is likely to be disordered. For example, FIG.
As shown in FIG.
Even if a normal PN junction having the characteristic SE as shown in FIG. 9B can be obtained in the area A of ３, the remaining area B of about ３ as shown in FIG. PN of negative resistance characteristic SE3
It becomes a PN junction. The LED cut out from the region B has the negative resistance characteristic SE3, and is therefore excluded as a defective product.

In a region C between the regions A and B, FIG.
As shown in (d), the inner N layer 111 and P layer 112 corresponding to the base of the PNPN junction become extremely thin (N
(The thickness of the layer 111 and the P layer 112 is shorter than the diffusion length of minority carriers.) As shown in FIG. 9E, the N layer 111 and the P layer 112 inside the PNPN junction are partially missing. In the case of such a junction structure, even a slight leak current lowers the turn-on voltage and turns on the LED. Since the LED cut out from the region C has the negative resistance characteristics SE1 and SE2 and the turn-on voltages V01 and V02, the LED is selected as a non-defective product as described above. Either it disappears or the time until light emission becomes very long.

In view of the above, the present invention has been made in view of the above-mentioned conventional problems, and a method of selecting a light emitting diode capable of reliably excluding a light emitting diode having a negative resistance characteristic. It is an object of the present invention to provide a light-emitting diode display device having a display screen composed of the same.

[0012]

In order to solve the above-mentioned problems, a method for selecting a light emitting diode according to the present invention comprises applying a pulsed reverse voltage and a forward voltage to a light emitting diode sequentially, and The delay time from when the application starts to when the current flows into the light emitting diode is measured, and the light emitting diode is selected based on the delay time.

According to the present invention, the light emitting diodes are selected based on the delay time from the start of the application of the forward voltage to the time at which the current flows to the light emitting diodes.

Here, in the case of an LED having a normal PN junction shown in FIG. 9B cut out from a region A of the wafer 10 shown in FIG. 9A, FIGS. As shown in the graph of (c), the forward current starts to flow and the light emission starts until 0.1 microsecond elapses after the application of the forward voltage. On the other hand, the LE of the joint structure shown in FIG. 9D or FIG.
In the case of D, as shown in the graphs of FIGS. 4A, 4B, and 4C, the forward current starts to flow and the light emission starts 0.4 microsecond after the forward voltage is applied. Begins. This is because, immediately after the application of the forward voltage, the turn-on voltage of the LED is higher than the voltage Vcc of the battery, and no forward current flows, but due to the leakage current, the inside of the PNPN junction shown in FIG. It is considered that carriers gradually accumulate in the N layer 111 and the P layer 112, and the turn-on voltage gradually decreases to become equal to or lower than the battery voltage Vcc. At this time, a forward current starts to flow rapidly. Can be

Therefore, comparing the LED having the normal PN junction shown in FIG. 9B with the LED having the junction structure shown in FIG. 9D or FIG. The time from the application of the directional voltage to the time when the forward current flows and emits light is much longer than that of the normal LED.

Further, in the case of an LED having a normal PN junction shown in FIG. 9B, immediately after applying a reverse voltage,
Even if a forward voltage is applied, a forward current If flows immediately. On the other hand, in the case of the LED having the junction structure shown in FIG. 9D or FIG.
Since minority carriers in the N layer 111 and the P layer 112 inside the N junction completely flow out, when a forward voltage is applied, carrier accumulation starts from a state of 0 carriers, Turn-on voltage is battery voltage V
A long time is definitely spent before it goes below cc.

That is, the forward voltage is applied after the application of the reverse voltage, whereby the delay between the time when the forward voltage is applied and the time when the forward current flows and emits light is different from that of the normal LED. The difference between the LEDs is surely generated, the normal LEDs are reliably extracted, and the defective LEDs are reliably excluded.

In the present invention, the forward voltage is
The turn-on voltage of the light emitting diode is set in a range from the turn-on voltage to another voltage higher than the turn-on voltage by +3 V, and the reverse voltage is set in a range from -5 V to the reverse breakdown voltage of the light-emitting diode.

Here, the closer the forward voltage is to the turn-on voltage of the normal LED, the more surely the defective LED can be distinguished. That is, even if the negative resistance of the LED is small, no forward current flows and no light is emitted. However, even in the case of a normal LED, it is necessary to supply a current of several tens mA in order to detect a forward current, and to detect a light output, it is necessary to supply a similar current. For these reasons, it is preferable to set the forward voltage within a range from the turn-on voltage to another voltage higher than the turn-on voltage by +3 V.

As the reverse voltage is increased, FIG.
Although the time required to drive out minority carriers of the N layer 111 and the P layer 112 shown in (d) or (e) can be shortened, the reverse voltage cannot be made higher than the reverse breakdown voltage. Therefore, it is preferable to set the reverse voltage within a range from -5 V to the reverse breakdown voltage.

Further, in the present invention, the application time of the forward voltage is set within a range from 0.5 microsecond to 5 microseconds, and the application time of the reverse voltage is set to 5 microseconds or more. You.

As the time during which the forward current flows through the LED is shortened, the forward current does not flow and the LED does not emit light even if the negative resistance of the LED is small. it can. However, normal LED
Even before the forward current flows and emits light,
A time of about 0.1 microsecond may be spent. For these reasons, it is preferable to set the application time of the forward voltage within the range of 0.5 microsecond to 5 microseconds.

The longer the application time of the reverse voltage is, the more the N layer 111 and the P layer 1 shown in FIG.
Although the twelve minority carriers can be reliably removed, if the length is too long, the inspection time per LED becomes longer. For this reason, it is preferable to appropriately set the application time of the reverse voltage within a range of 5 microseconds or more in which the minority carriers can be sufficiently expelled.

Further, in the present invention, the delay time is set to 0.1.
Light emitting diodes longer than 2 microseconds are removed as defective.

As described above, in the case of the LED having the normal PN junction shown in FIG. 9B, the forward current If is applied until 0.1 microsecond elapses from the time when the forward voltage is applied. Flows and light is emitted. In contrast, FIG.
In the case of the LED having the junction structure shown in (d) or (e), a forward current starts to flow and light emission starts 0.4 microseconds after the forward voltage is applied. For these reasons, it is preferable to remove a light emitting diode having a delay time of 0.2 microsecond or more as a defective product.

Alternatively, the method for selecting a light emitting diode according to the present invention comprises the steps of sequentially applying a reverse voltage and a forward voltage, each having a pulse shape, each having a fixed pulse width, to the light emitting diode. The light output of the light emitting diode is detected, and the light emitting diode is selected based on the light output.

As described above, the difference between the normal LED and the defective LED occurs between the time when the forward voltage is applied and the time when the forward current flows and emits light. The light emitting diodes can be selected based on the average light output of the light emitting diodes to which the pulsed forward voltage is applied.

On the other hand, the light-emitting diode display device of the present invention has a plurality of light-emitting diodes selected by the above-described selection method arranged in a dot matrix.

In such a light-emitting diode display device, since the characteristics of the respective light-emitting diodes are uniform, even if the driving voltage application time to each light-emitting diode is extremely short, no luminance unevenness occurs on the display screen, and the display is not affected. Very high image quality can be maintained over a long period of time.

[0030]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a light emitting diode sorting apparatus to which one embodiment of the sorting method of the present invention is applied. In this device, a light emitting diode (LED) 1 to be inspected can be detachably connected,
1 is connected to the midpoint of the forward voltage power supply 2 and the reverse voltage power supply 3, and the anode of the LED 1 is connected to the midpoint of the first and second transistors 5 and 6 via the current limiting resistor 4. .

When the first transistor 5 is turned on, the forward voltage Vf obtained by dividing the voltage Vcc of the forward voltage power supply 2 becomes L
The forward current If is applied to the ED1, and the forward current If flows through the LED1. When the second transistor 6 is turned on, the reverse voltage VR of the reverse voltage source 3 is applied to the LED 1.

Here, the second transistor 6 is turned on for a preset second time TR, a reverse voltage VR is applied to the LED 1, and the first transistor 5 is subsequently turned on for the preset first time Tf. Turn on to apply a forward voltage to LED1. It is preferable to set the second time TR to about five times the first time Tf.

For example, when the LED 1 has a normal PN junction shown in FIG. 9B cut out from the region A of the wafer 10 shown in FIG. 9A, when a forward voltage is applied to the LED 1, The forward current If flows quickly, and LED1
Emit light quickly.

On the other hand, when the LED 1 has the junction structure shown in FIG. 9D or 9 E cut out from the region C, the forward current flows after the forward voltage is applied to the LED 1. It takes time to get started.

Therefore, the delay time Td from the time when the forward voltage is applied to the LED 1 to the time when the forward current starts to flow is measured, and if this delay time Td is sufficiently short, L
ED1 is extracted as a non-defective product. If the delay time Td is long, LED1 is excluded as a defective product.

Before applying the forward voltage, the reverse voltage V
R is applied when the LED 1 is defective and the N layer 1 inside the PNPN junction shown in FIG.
This is to drive out the carriers of the P layer 11 and the P layer 112.
If carriers are accumulated here, the turn-on voltage of LED1 decreases (negative resistance decreases) and LE1
Since the forward current starts flowing through D1 relatively quickly, the delay time Td becomes short, and it becomes difficult to distinguish the non-defective product. When the reverse voltage VR is applied, carriers are expelled.
It takes time for the turn-on voltage of LED1 to rise to the original voltage (negative resistance increases to the original resistance value), and for the forward current to start flowing, the delay time Td becomes longer, and Can be easily distinguished.

In order to measure the delay time Td, for example, when one of the first and second transistors 5 and 6 is turned on, clock pulse counting is started, and the forward current of the LED 1 to be inspected is measured. When the clock rises, a logic circuit that terminates the counting of clock pulses and outputs the counted value may be configured.

Actually, the forward voltage Vf is set to 3.0 V, and the reverse voltage VR of the reverse voltage source 3 is set to -10.
V was set. In addition, the first time Tf for turning on the first transistor 5 was set to 0.8 microseconds, and the second time TR for turning on the second transistor 6 was set to 5 microseconds. In this case, the LED 1 to be inspected includes
A voltage having a pulse waveform as shown in FIG. 2 is applied.

Under these conditions, from the start of the application of the forward voltage of the LED 1 (the start of the first time Tf),
The delay time Td until the forward current starts to flow is measured. When the LED 1 has a normal PN junction shown in FIG. 9B, as shown in the graphs of FIG. 3A, FIG. 3B and FIG. The delay time Td from the start of the flow and the start of light emission is 0.1 microsecond or less, and the forward current immediately rises to 20 mA defined by the voltage Vcc of the forward voltage power supply 2 and the resistance of the resistor 4. On the other hand, when the LED has the bonding structure shown in FIG. 9D or FIG.
As shown in the graphs (b) and (c), the delay time Td from the start of the application of the forward voltage to the start of the flow of the forward current and the start of light emission is 0.4 microsecond or more.

Therefore, if LEDs having a delay time Td of 0.2 microseconds or more are regarded as defectives and removed, LEDs having a negative resistance are not included in the LEDs selected as non-defectives.

For confirmation, current-voltage characteristics were measured for non-defective LEDs having the characteristics shown in the graphs of FIGS. 3A, 3B and 3C, and the voltage-current characteristics SE shown in the graph of FIG. 7 were obtained. was gotten. 4 (a) and 4 (b)
And defective LED having the characteristics shown in the graph of (c)
When the current-voltage characteristics were measured, negative resistance characteristics SE1 shown in the graph of FIG. 8 were obtained.

When a large number of LEDs were inspected, these LEDs had various delay times Td, and some of them had a delay time Td of 0.8 microsecond.

As is apparent from FIGS. 3 and 4, almost at the same time as the forward current of the LED 1 starts flowing,
LED1 is starting to emit light. Comparing the characteristics shown in the graphs of FIGS. 3 (a), (b) and (c) with the characteristics shown in the graphs of FIGS. 4 (a), (b) and (c), a good LED
In contrast, the emission time of the defective LED is １ ／, and the average of the light output is also １ ／. For this reason, a good product and a defective product can be distinguished based on the light output of the LED.

Here, when the same forward current is applied to a large number of non-defective LEDs, the light output varies by ± 50%.
For this reason, 1/2 of the average light output of non-defective LEDs is set as the threshold.

The forward voltage Vf and the reverse voltage VR
, The first time Tf and the second time TR are set under the same conditions as described above, and if the light output of the LED is equal to or greater than the threshold, this is regarded as a non-defective product, and if the light output of the LED is less than the threshold, This is regarded as defective.

When the selection is performed based on the light output in this manner, the light output of the LED is received by the light receiving element, and the determination may be performed based on the output level of the light receiving element.
It is easy to realize a measuring device. However, when compared with measuring the delay time and making a determination based on the delay time, the measurement error is large, so it is necessary to provide a sufficient margin for the threshold value as the determination reference.

As described above, based on the delay time and the optical output, the LE
If D is selected, an LED with uniform characteristics can be obtained. FIG. 5 is a block diagram showing an embodiment of a light emitting diode display device configured using only LEDs having such characteristics.

As shown in FIG. 5, in this display device, 16 × 16 LEDs 1 are arranged vertically and horizontally,
Each transistor 11 is connected to one cathode,
The emitters of these transistors 11 are grounded.
The anode of each LED 1 is connected to each resistor 12, and a voltage of 3 V is applied to these resistors 12. Further, a shift register 13 is provided for each column. These registers 13 sequentially input, shift, and output serial bit signals. The shift register 13 in the first column is a shift register 13 corresponding to the 16 LEDs 1 in the first column.
Six bits are sequentially input and output while shifting, and the transistor 11 of each LED1 is turned on or off according to the value of each bit, thereby selectively emitting each LED1. Similarly, the shift register 1 of each other column
Reference numeral 3 sequentially inputs and shifts and outputs 16 bits corresponding to the 16 LEDs 1, and selectively causes each LED 1 to emit light according to the value of each bit.

Here, although the circuit is simplified, each transistor 11 is AC-driven by a respective drive circuit, whereby each LED 1 is turned on and off.

This display device is installed, for example, along a road, and displays traffic information and the like. From the viewpoint of a driver of an automobile running at high speed, it is necessary to refresh the display screen about 1000 times per second in order to eliminate the fluctuation of the image on the display device. Therefore, it is necessary to light LED1 once every millisecond. Actually, the lighting time differs between daytime and nighttime, and during the daytime, the LED is turned on almost continuously for 1 millisecond. On the other hand, in the nighttime, if the display screen becomes too bright, halation occurs. Therefore, the lighting time is shortened to about 1/200 in the daytime, and the brightness of the display screen is suppressed to about 1/200 in the daytime. . That is, LED1 is turned on for 5 microseconds, and then LED1 is turned off for 995 microseconds. By repeating such turning on and off, the average brightness of the display screen is reduced to about 1/200 of daytime. ing.

A non-defective LED selected by the selection method of the present invention does not have negative resistance and has almost no delay time from the point of application of voltage to lighting. For this reason, lighting spots do not occur on the display screen, and the displayed information can be easily identified.

On the other hand, if a defective LED having a negative resistance is mixed as in the conventional display device, the LED
When the lighting time of the LED is shortened, that is, when the voltage application time to the LED is shortened, it takes time until the forward current flows, and the light emission time is shortened. For example, if the lighting time of a defective LED having a negative resistance is reduced by 1 microsecond to the lighting time of a good LED of 5 microseconds, the average brightness of the defective LED is lower than that of the non-defective LED. Is reduced by 20% or more. As a result, lighting spots occur on the display screen, making it difficult to identify displayed information, and impairing the original function of the display device.

In the case of an LED having a negative resistance,
After approximately 1000 hours of use, L
The turn-on voltage of the ED gradually increases to 3 V or more, the brightness gradually decreases, and eventually stops lighting.

Therefore, the selection method of the present invention, which excludes LEDs having negative resistance, is indispensable for improving the reliability of the light emitting diode display.

The present invention is not limited to the above embodiment, but can be variously modified. For example, it is preferable that the forward voltage Vf, the reverse voltage VR, the first time Tf, and the second time TR are appropriately changed and set according to the type and characteristics of the diode.

[0057]

As described above, according to the present invention, the forward voltage is applied after the application of the reverse voltage, whereby the time when the forward current flows and the light is emitted starts from the time when the forward voltage is applied. The difference between the normal LED and the defective LED is surely caused for the delay time until. For this reason,
Based on the delay time or the light output of the light emitting diode,
Normal LEDs can be reliably extracted, and defective LEDs can be reliably excluded.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a light emitting diode sorting apparatus to which an embodiment of a sorting method according to the present invention is applied.

FIG. 2 is a diagram showing a voltage waveform applied to a light emitting diode in the device of FIG.

3A shows a waveform of a forward voltage applied to a light emitting diode, FIG. 3B shows a current waveform flowing through a non-defective light emitting diode, and FIG. 3C shows an optical output waveform of a non-defective light emitting diode. Is shown.

4A shows a waveform of a forward voltage applied to a light emitting diode, FIG. 4B shows a waveform of a current flowing through a defective light emitting diode, and FIG. 4C shows an optical output of the defective light emitting diode. The waveform is shown.

FIG. 5 is a block diagram showing one embodiment of a light emitting diode display device of the present invention.

FIG. 6 is a circuit diagram showing a conventional device for selecting light emitting diodes.

FIG. 7 is a graph showing voltage-current characteristics of a normal light emitting diode.

FIG. 8 is a graph showing a negative resistance characteristic of a light emitting diode having a negative resistance.

9A is a plan view showing a semiconductor wafer from which a plurality of light emitting diodes are cut out, FIG. 9B is a cross-sectional view showing a region A of the wafer shown in FIG. 9A, and FIG. FIG. 2A is a cross-sectional view illustrating a region B of the wafer by cutting it.
(D) and (e) are cross-sectional views each showing a region C of the wafer shown in (a) by cutting.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 LED 2 forward voltage power supply 3 reverse voltage power supply 4 current limiting resistor 5 first transistor 6 second transistor 11 transistor 12 resistor 13 shift register

Claims (6)

[Claims] 1. A pulse-like reverse voltage and a forward voltage are sequentially applied to a light emitting diode, and a delay time from a start of application of the forward voltage to a time when a current flows into the light emitting diode is measured. A method for selecting a light emitting diode based on the method. 2. The forward voltage is set within a range from the turn-on voltage of the light emitting diode to another voltage + 3V higher than the turn-on voltage, and the reverse voltage is -5V.
The method for selecting a light emitting diode according to claim 1, wherein the temperature is set within a range up to the reverse breakdown voltage of the light emitting diode. 3. The application time of the forward voltage is set in a range from 0.5 microsecond to 5 microseconds, and the application time of the reverse voltage is set to 5 microseconds or more. The method for selecting a light emitting diode according to claim 1. 4. The method for selecting a light emitting diode according to claim 1, wherein a light emitting diode having a delay time of 0.2 microsecond or more is removed as a defective product. 5. The light-emitting diode has a pulse shape,
A reverse voltage and a forward voltage having respective constant pulse widths are sequentially applied, and when the forward voltage is applied, the light output of the light emitting diode is detected, and the light emitting diode is selected based on the light output. Method for selecting light emitting diodes. 6. A light emitting diode display device in which a plurality of light emitting diodes selected by the selecting method according to claim 1 are arranged in a dot matrix.