Transistor Lasers: Opening New Optoelectronic Frontiers
by Milton Feng
Sunday, May 6, 2007| Chicago, Illinois
This talk is based on the work of Professor Milton Feng, the Nick Holonyak Jr. Chair Professor of Electrical and Computer Engineering, and of Professor Nick Holonyak, Jr., the John Bardeen Chair Professor of Electrical and Computer Engineering and Physics, both of the University of Illinois at Urbana-Champaign. The work is supported in part by a DARPA-UPI-HUNT program.
From the form of the transistor invented by Bardeen and Brattain in 1947, we realize that the “magic” of the transistor is intrinsically in its base. It is the base that potentially offers more, particularly when we arrive at the direct-gap, high-speed, high-current density heterojunction bipolar transistor (HBT), and realize that the base, although thin (10-100 nm), has room for more layering (in bandgap and doping) and can be modified.
Employing quantum-wells (QWs) and cavity reflection, we can re-invent the base region and its mechanics (its carrier recombination and transport fraction), reduce the current gain, and achieve stimulated recombination; i.e., realize a transistor laser—a novel device with an electrical input, an electrical output, and an optical output. The result is a unique transistor in form and operation, as well as a unique three-terminal laser. More structure is evident in the output collector current and voltage characteristics because of sensitivity to QW bandfilling, state change, spectral change, mode hopping, change in optical field strength, and the effect of photon-assisted collector tunneling. The quantum well base region and stimulated recombination (stimulated emission), besides yielding a transistor laser, changes the transistor into an active element that can be used for nonlinear and switching applications (both electrical and optical). With two electrical inputs on the base, we have demonstrated a new mixer with both electrical and optical output in the nonlinear operation region and signals that produce new waveforms in the linear operation region. The new device is amazing in the sense of opening new frontiers in optoelectronics integration.
Because the base transit time of a transistor with a 100 nm base thickness is ~2 ps based on a simple diffusion model, the base recombination lifetime can be determined as current gain times base transit time. It is clear the transistor with a short (thin) base, and hence, a smaller charge storage volume, can sustain high emitter current injection and fast depletion of the stored charge via both the recombination process in the base and the usual charge removal behavior of the collector. We have realized fast laser modulation as well as significant modification of the base minority carrier lifetime by the use of base quantum wells (or dots). The modulation speed of the transistor laser is determined by the recombination lifetime and photon lifetime. In a properly designed transistor laser, the direct modulation speed should approach the transistor speed (> 100 GHz), which, of course, is of special interest and an enticing prospect.
Milton Feng is the Nick Holonyak Jr. Chair Professor of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign. Prof. Feng was born in Taiwan, China. He received his BS degree in electrical engineering from Columbia University (New York) in 1973 and his MS and PhD degrees in electrical engineering from the University of Illinois, Urbana-Champaign, in 1976 and 1979, respectively.
From 1979 to 1983, he was head of the GaAs material and device group at Torrance Research Center, Hughes Aircraft Company, where he was in charge of ion implantation, AsCl3 VPE, MOCVD, and MBE technology. In 1983, he developed a direct ion-implanted low-noise and power MESFET and MMICs for X-band phase array radar application. Dr. Feng demonstrated the first 60-GHz GaAs amplifiers in 1983. From 1984 to 1986, he worked for Ford Microelectronics, Inc., in Colorado Springs, CO, where he managed the advanced digital integrated circuit development program in 1 K SRAM and 500 gate array.
Since 1991, Dr. Feng has been a professor of electrical and computer engineering and a research professor at the Microelectronics Laboratory at the University of Illinois. Prof. Feng invented the pseudomorphic HBT (PHBT), “pushed” the transistor speed boundary toward THz, and demonstrated InP PHBTs with the world’s fastest speed performance (> 800 GHz). Prof. Feng, along with Prof. N. Holonyak, Jr., demonstrated the first laser operation of a quantum-well-based light emitting transistor (QWLET), a transistor laser (TL). A transistor laser opens up a rich domain of integrated circuitry and high speed signal processing that involves both electrical and optical signals.
Prof. Feng has published over 175 papers, 170 conference talks, and been granted 12 U.S. patents in semiconductor microelectronics. He is an IEEE and OSA Fellow, and serves on many executive and strategy committees both in industry and at conferences. In 1989, he received the Ford Aerospace Corporate Technology Outstanding Principal Investigator Award for his contribution of advancing ion implantation GaAs and InGaAs MESFETs into manufacturable millimeter-wave ICs. In 1997, he received the IEEE David Sarnoff Award, and in 2000, he received the Pan Wen Yuan Outstanding Research Award in Microelectronics. In 2005, he was chosen as the first Holonyak Chair Professor of Electrical and Computer Engineering. In 2006, his transistor laser research paper was selected as one of the top five papers in the 43 year-history of Applied Physics Letters, and also was selected as one of the top 100 most important discoveries in 2005 by Discover magazine.