As electronic devices continue to become more advanced and more compact, there is a constant need to improve their microelectronic and optoelectronic circuits.
This is to facilitate faster signal processing, higher speed operations, large-capacity seamless communications and enhanced performance characteristics.
One of the most promising developments in this regard has been the transistor laser invented by scientists at the University of Illinois at Urbana-Champaign (UIUC) just over half a decade ago. It has proven itself over and over again to be versatile in its capabilities. Unlike a light-emitting diode (LED), which emits broadband, incoherent light, the transistor laser emits a narrow, coherent beam, and when modulated at transistor speeds, the laser beam can be transmitted through an optical fibre as a high-speed signal.
Continuing their considerable work on the transistor laser, the UIUC researchers have now developed a microwave signal mixer made from a tunnel junction transistor laser. The novel mixing device accepts two electrical inputs and produces an optical signal output. Moreover, the tunnel junction provides a way for voltage-controlled modulation of the photon output of the transistor laser, in addition to the usual current-modulation capability.
This development could be quite significant as it brings us closer to higher speed electronics and higher performance electrical and optical integrated circuits. The innovative research work was spearheaded by Nick Holonyak Jr., professor at UIUC, and the inventor of the first practical LED and the first semiconductor laser to operate in the visible spectrum.
To develop this mixing device, Holonyak and his team first placed a quantum well inside the base region of the transistor laser. Then a tunnel junction was created within the collector region. The tunnelling process begins in the quantum well, where electrons and holes combine to generate photons, and within the transistor laser, the process occurs predominantly through photon-assisted absorption. The generated photons are reabsorbed to create new pairs of electrons and holes used for voltage modulation. The tunnel junction aids in annihilating an electron in the quantum well and tunnelling an electron out to the collector by the tunnel contact.
The researchers were able to successfully demonstrate microwave signal mixing with sinusoidal signals of frequencies of 2,0 GHz at the base using current modulation and 2,1 GHz at the collector using voltage modulation, resulting in an optical output with harmonics of up to 22,7 GHz, despite being limited by amplifier bandwidth. The fabrication and operation of the microwave mixer is described in detail in a paper published in a recent issue of the journal Applied Physics Letters.
This development is significant in that it has demonstrated a new type of transistor, or rather makes the transistor into a different device with additional properties by using the photon internally to modify its electrical operations. High-speed signal mixing is made possible by the nonlinear coupling of the internal optical fields to the base electron-hole recombination, minority carrier emitter-to-collector transport, and the base-to-collector electron tunnelling at the collector junction.
The flexibility offered by the sensitivity of the device to third-terminal voltage control, facilitates the design of new non-linear signal processing devices for improved optical power output. This device would find considerable applications in many signal mixing and signal processing applications, and the greater sensitivity offers new capabilities for innovative designs.
For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274, patrick.cairns@frost.com, www.frost.com
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