novel devices technology
T
he semiconductor industry rakes in billions and dollars from the manufacture of devices invented
way back in the middle of the twentieth century. With the benefit of hindsight it is clear that the most important of them all is the transistor, a device invented by John Bardeen and Walter Brattain in 1947, which has been the key building block in the development of microelectronics, integrated circuits, consumer electronics, and the computer industry. Not far behind the transistor is the visible LED, made for the first time by Nick Holonyak in 1962, and the laser diode, independently invented in that same year by Holonyak and Robert Hall. These two optoelectronic devices have provided a great foundation for revolutionizing display, lighting, and information technology.
Although the performance of all these devices has come on in leaps and bounds over the intervening decades, none can simultaneously deliver an electrical signal and a laser output. The invention of such device had to wait until 2004, when I, Milton Feng, in partnership with co-worker Holonyak, produced the world’s first transistor laser.
This revolutionary semiconductor device that offers three- port operation – an electrical input, an electrical output and an optical laser output (see Figure 1) – has the potential to a make important contributions to integrated photonic and electronic integrated circuits, the computer industry and new information technology. Amongst these many promises, it is capable of redefining the approach made to the transfer of digital data. Today, PCs operate solely in the electrical domain, and hooking up to the internet requires an infrastructure involving transmitters and receivers that can provide an interface with the optical domain. The transistor laser, however, is capable of performing all these functions itself. One of its roles could be to act as an optical interconnect that could allow incredibly fast data flow to and from memory chips, graphics processors and microprocessors.
Our transistor laser, which emits infrared light, is a modified, high-speed HBT with a quantum well in its base region. In conventional high-speed HBTs, which inevitably operate at a high current density, the base provides the pathway for electrons to travel from the emitter to the collector. In our device, the quantum well in the heavily-doped base traps some of these electrons, which diminishes the transistors gain, but allows this device to realize radiative recombination between holes and electrons. Thanks to the geometry of our device – the chip has cleaved facets that act as mirrors – the light that is emitted is bounded by a cavity, enabling stimulated emission, one hallmarks of a laser.
Our first transistor laser needed to be cooled with liquid nitrogen. But a year later it could be run at room temperatures, thanks to improvements in MOCVD growth
Fig. 1 Three-port operation of the transistor laser provides an electrical input (port 1), an electrical output (port 2) and an optical laser output (port 3).
and the design of the quantum well. Since then we have focused on improving the quality of light output from our transistor laser and understanding its electrical behavior.
Revolutionary modulation speeds One of the really encouraging attributes of our transistor laser is its incredibly fast radiative recombination lifetime: it is below 30 ps. This can spur the direct-modulation bandwidth in an LED to 10 GHz, and to 100 GHz in a laser. The far higher bandwidth will accelerate the deployment of LEDs and lasers in optoelectronic interconnects and open the door to a new generation of high-performance, electronic-photonic integrated circuits.
Traditional laser diodes suffer from a resonance peak in the frequency response. To combat this, resonance compensation circuits are included in transistor laser driver circuits. Our transistor lasers, however, do not have to contend with this thanks to a shift in the carrier-photon damping ratio and elimination of the resoance peak. Thanks to these attributes, our laser transistors could be used to build an ultra-low power transmitter and array for 100 Gbit/s Ethernet and optical interconnect applications. In addition to the high speeds, there is also the possibility to tap into our device’s non-linear characteristics, and exploit flexible signal mixing and processing.
An additional weakness of the conventional laser is a pulsation or “ripple” in its output. This phenomenon is well understood. It was observed in 1959 in masers under certain pump conditions, and has been studied in detail by the researchers Statz and deMars. They explained its occurrence in the 1960s by studying the transient solution of a pair of coupled carrier-photon rate equations
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