Scientists break light modulation speed record

June 16th, 2009 - 12:21 pm ICT by ANI  

Washington, June 16 (ANI): A team of scientists has broken the light modulation speed record, that too twice, with a signal-processing modulation speed of 4.3 gigahertz, breaking the previous record of 1.7 gigahertz held by a light-emitting diode.

The team, comprising of researchers at the University of Illinois and at U. of I. licensee Quantum Electro Opto Systems in Melaka, Malaysia, constructed a light-emitting transistor to set the new record.

By internally connecting the base and collector of a light-emitting transistor, they created a new form of light-emitting diode, which modulates at up to 7 gigahertz, breaking the speed record once again.

“Simple in design and construction, the tilted-charge light-emitting diode offers an attractive alternative for use in high-speed signal processing, optical communication systems and integrated optoelectronics,” said Nick Holonyak Jr., a John Bardeen Chair Professor of Electrical and Computer Engineering and Physics at Illinois.

The modulation speed of either a light-emitting diode or a light-emitting transistor is limited by the rate at which electrons and holes (the minus and plus charges - the carriers of current) recombine.

The recombination lifetime is important in determining device speed.

With a usual “slow” recombination process, the speed of a light-emitting diode is limited to approximately 1.7 gigahertz, which corresponds to a carrier lifetime of 100 picoseconds.

For more than 40 years, scientists thought breaking the 100-picosecond barrier was impossible.

To achieve high recombination speeds, an extremely high injection level and a very high charge population are required in light-emitting diodes.

These conditions are not necessary in transistors, however.

“Unlike a diode, a transistor does not store charge,” said Milton Feng, the Holonyak Chair Professor of Electrical and Computer Engineering.

“Charges are delivered to the transistor’s quantum well active region, where they either recombine almost instantly, or they are kept moving on out of the device. The charges do not become stacked-up, waiting to recombine with their oppositely charged twins,” he added.

To increase the modulation speed of their light-emitting transistor, the researchers reduced the emitter size, increased the so-called collector thickness (the third terminal region), and utilized a special internal common collector design.

These changes resulted in a faster signal at a very low current level, and at low heat dissipation.

Having a “fast” recombination process, the modulation speed of the light-emitting transistor was measured at 4.3 gigahertz, which corresponds to a recombination lifetime of 37 picoseconds, well under the “100-picosecond barrier.”

The tilted-charge light-emitting diode achieved a record-breaking modulation speed of 7 gigahertz, corresponding to a recombination lifetime of 23 picoseconds. (ANI)

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