LED innovator and Columbia University Professor Emeritus Gertrude Neumark Rothschild is the sole named inventor on U.S. Patent Nos. 4,904,618 and 5,252,499, directed to methods of making LEDs capable of emitting shorter wavelength (green or blue) light. Rothschild’s technology has had a major impact on LEDs by making production of green, blue and other short wavelength LEDs more economically viable. In 2005, she sued San Jose LED manufacturer Philips Lumileds in federal court in White Plains, New York for patent infringement.
Her patents address the problem of “doping” wide band gap semiconductor materials, an essential step in creating adequate conductance for the materials to function as LEDs. Doping means adding impurities to a semiconductor to increase the number of free charge carriers. When extra positive charge is added, the resulting semiconductor is called a p-type semiconductor; when negative charge is added, the result is an n-type semiconductor. In LEDs, a junction is formed between p-type and n-type semiconductors, and voltage is applied across the junction to cause movement of electrons from the negative to the positive material. As the electrons move from the “conduction band” to the “valence band” (electrons are confined to certain spaces called bands and can be made to travel from one band to another through a gap), the energy they lose is released in the form of light. The wavelength, or color, of the light depends on the width of the band gap.
Semiconductors with wide band gaps are more difficult to dope because they are more likely to become “compensated,” meaning that the added charge carriers are not accepted and semiconductor resistivity increases. Rothschild’s patents are directed to methods of doping wide band semiconductors and reducing compensation by introducing extra dopant in concentration with extra compensating species, in some cases atomic hydrogen, and subsequently removing the compensating species. This technique leaves a greater level of dopant in the host crystal, resulting in high conductivity.
Rothschild recently reached a settlement agreement with Philips. The terms of the settlement have not been made public, so we don’t know how favorable it is to either side. But one interesting tidbit about the case suggests that Rothschild may have done well. In July of last year, the court granted in part Rothschild’s motion for reconsideration of its claim construction opinion (claim construction is where the court interprets the legal scope of the patent terms; this step is critical in infringement cases and can spur settlement). In the court’s original claim construction opinion, it construed the term “doping . . . with . . . atomic hydrogen” to mean “incorporating atomic hydrogen not produced by disintegration of ambient gases.” However, during the court hearing on the subject, Judge William C. Conner’s initial reaction to the term was that it didn’t matter how the atomic hydrogen was produced. Judge Conner found that his limiting construction of the term, based on statements by the patentee during the application process, was in error and revised it to mean “doping with atomic hydrogen (from any source).” This broadening of the claim term may have helped Rothschild’s infringement case and encouraged Philips to settle.