Although GaN light emitting diodes emit light, the light is quite dark. Nakamura decided to change from a pn junction structure to a double Hetero structure. After that, the development speed is rapidly increasing. The development was also smooth, and impurities were incorporated into the double heterostructure to form a luminescent center, achieving a brightness of 1 cd and finally being put into production. Applications such as signal lights have also begun to appear.
In April 1992, Nakamura, who returned from the American Society to study, developed a double heterostructure to study the formation of InGaN films (Table 1). If a double heterostructure is introduced into a GaN LED, the brightness should be greatly increased.
Note 1) The pn junction type light-emitting diodes have been completed at that time. The pn junction only joins the p-type and n-type semiconductors, and the structure is simple. A forward bias is applied to the pn junction to inject electrons which are generated when electrons are rejoined in a hole. In the case of a double heterostructure light-emitting diode, a light-emitting layer is sandwiched between a semiconductor layer having a larger energy gap than the light-emitting layer. In terms of bonding between the light-emitting layer and the surrounding semiconductor layer, both sides are hetero-bonded (joining between different materials). When a forward bias is applied to the pn junction, the injected carriers are not all migrated from the energy band to the energy band (re-bonding). Most of the carriers will flow out to the electrodes, contributing to the luminescence. In the case of a double heterojunction type light emitting diode, the energy gap of the light emitting layer is smaller than the surrounding area. Therefore, the carriers are closed in the light-emitting layer, and the probability of re-engagement is increased. Therefore, if a double heterostructure is used, the brightness can be mentioned to be higher than the pn junction.
Table 1: Blue LED development chronology (click to enlarge)
Since this period, Nakamura's research team has received funding and talent input. This is the embodiment of the president's determination to productize. For the research of GaN light-emitting diodes, Nichia has invested hundreds of millions of dollars. From the company's point of view, this decision is like jumping from the stage of the Kiyomizu-dera Temple in Kyoto. I finally waited for the day when the GaN LED was illuminated. So the company hopes to turn it into a bestseller as soon as possible. As the object of the decision to invest, the president’s expectations for it are self-evident.
However, this expectation has become an obstacle to the progress of Nakamura. The president thinks that even if it is a little dark, it doesn't matter, and he is anxious to productize the pn junction LED. Nakamura has seen the limitations of pn junction LEDs and hopes to push the research to a deeper level. Because he has the confidence to produce results in a short time.
In spite of opposition from all, launch a tough battle
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