A research team at the University of Illinois at Urbana-Champaign has developed a transistor that emits infrared radiation and offers standard electrical output. The device, which produces modulated optical emission in phase with a base current at 1 MHz, suggests applications in displays, communications and optical integrated circuitry, and offers a challenge to optoelectronics to re-examine the fundamental speed limits of devices such as LEDs and laser diodes.InGaP/GaAs heterojunction bipolar transistors fabricated at the University of Illinois at Urbana-Champaign produce an infrared output as well as an electrical output. In experiments, the devices have displayed the ability to modulate their 885-nm output at speeds of up to 1 MHz. And their speed limits should be much higher. Nick Holonyak Jr., a professor in the department of electrical and computer engineering at the university, said that he and his collaborator Milton Feng had worked to understand why Feng's ultrafast 509-GHz heterojunction bipolar transistors displayed the performance that they did. They concluded that energy must have been being released from the active region of the devices without leading to heating. "In other words," Holonyak said, "there had to be -- for several reasons -- an AC photon signal."The researchers, along with graduate student Walid Hafez, set out to look for this photon signal. They fabricated InGaP/GaAs heterojunction bipolar transistors by metallorganic chemical vapor deposition that featured a 1 × 2-µm window in the base region. Using a multimode fiber probe connected to a silicon avalanche photodiode, they interrogated the aperture and found that 885-nm radiation indeed was generated in the graded base layer of the transistors as injected electrons and holes recombined. Because the infrared output tracks the electrical signals through the device, the researchers were able to modulate the optical output at speeds of up to 1 MHz. And the speed limits should be much higher.Holonyak characterized the work as most interesting for the questions it raises and the directions in which it might lead.For example, why should a bipolar device such as an LED or a laser diode be 10 times slower than a heterojunction bipolar transistor? Could light-emitting transistors be used to create optical interconnects that will replace the electrical wiring in circuitry?"Suddenly, we have much more work to pursue, many more questions to answer," he said.