Tunneling Could Be Key to Modulating Bandwidth of Transistor Laser

Two recent studies could impact the fundamental modulation bandwidth of the transistor laser and increase its capacity for data transfer in optical and 5G wireless communications.

Electrical engineers Nick Holonyak, Jr. and Milton Feng at the University of Illinois invented the transistor laser in 2004, based on their discovery that a transistor could be modulated to be a signal and a device that could harness the physics between electrons and light. The transistor laser is a three-port device that incorporates quantum wells in the base and an optical cavity for energy-efficient, high-speed data transfer. The engineers are now exploring how optical absorption can be enhanced, using an approach they have called the Feng-Holonyak Intra-Cavity Photon-Assisted Tunneling (FH-ICPAT).

Milton Feng and Nick Holonyak Jr. invented the transistor laser in 2004 and continue to develop the technology for energy-efficient high-speed data transfer in optical and 5G wireless communications. Courtesy of University of Illinois.

"The fastest way for current to switch in a semiconductor material is for the electrons to jump between bands in the material in a process called tunneling," said Feng. "Light photons help shuttle the electrons across, a process called intra-cavity photon-assisted tunneling, making the device much faster."

The research team explained the principles of operation for tunneling modulation of a quantum well transistor laser with current amplification and optical output via intra-cavity photon-assisted tunneling in two recent papers.

"The tunneling gain mechanism is the result of the unique transistor laser base transport properties under the influence of FH-ICPAT and base dielectric relaxation, which yields fast carrier base transport and faster recombination than the original Bardeen transistor," said researcher Curtis Wang. "The voltage and current dependence of the tunneling current gain and optical modulation have been revealed in detail. Although the analysis is carried out for the transistor laser intra-cavity photon-assisted tunneling, the operation mechanism should apply in general to tunneling collector transistors of various design configurations."

In a companion paper, the researchers explained how optical absorption and modulation in a p-n junction diode for a direct-gap semiconductor could be enhanced by photon-assisted tunneling in the presence of an optical cavity and photon-field in a transistor laser.

"In the transistor laser, the coherent photons generated at the base quantum well interact with the collector field and 'assist' optical cavity electron tunneling from the valence band of the base to the energy state of conduction band of the collector," Feng said. "The stimulated light output can be modulated by either base current injection via stimulated optical generation or base-collector junction bias via optical absorption.

"In this work, we studied the intra-cavity coherent photon intensity on photon-assisted tunneling in the transistor laser and realize photon field-dependent optical absorption. This FH-ICPAT in a transistor laser is the unique property of voltage (field) modulation and the basis for ultrahigh-speed direct laser modulation and switching.

"We remain indebted to John Bardeen, our mentor, for his lifelong continuing interest in the transistor (parallel to the BCS theory), the effect of the electron and the hole (e-h) in helping to originate the diode laser and LED, and in addition now leading to the e-h recombination (electrical and optical) transistor laser," Feng added.

The research was published in the Journal of Applied Physics (doi: 10.1063/1.4967922) and (doi: 10.1063/1.4942222).