Transistor laser could change communications

A team headed by professors Milton Feng and Nick Holonyak, Jr., has developed a transistor that emits light.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 7/1/2010 12:00:00 AM

Since their inception in 1947 by John Bardeen and Walter Brattain, transistors have always been three-terminal devices with electrical inputs and electrical outputs. Researchers at the University of Illinois at Urbana-Champaign have changed that. A team headed by professors Milton Feng and Nick Holonyak, Jr., has developed a transistor that emits light—enough to send data over fiber-optic links and across silicon wafers.

The three-port transistor laser emits photons from its base that are strong enough for use as data-communications signals.

Typical transistors emit a small number of photons, but not enough to be useful. The transistor laser differs from other transistors through the addition of a quantum well in the transistor's base structure, the P region of an NPN transistor. Tests have shown that the quantum well boosts optical signal intensity by as much as 40 times.

The researchers fabricated several hundred transistor lasers and characterized them. From the test results, they developed circuit models of the device's performance. Tests involved using a photodetector to receive the optical signal, from which the researchers measured three-port S-parameters (s₁₁, s₁₂, s₂₁, s₂₂, s₃₁, and s₃₂) with an Agilent Technologies N5230 parametric network analyzer (s₁₃, s₂₃, and s₃₃ represent the parameters of the optical port that were not used in this work). The DC and S-parameter measurements let the researchers obtain values for the device's I-V and L-I characteristics as well as resistance, capacitance, and inductance of the device's junctions (Ref. 1).They also measured eye diagrams at data rates up to 13 Gbps with an Agilent 86100A sampling oscilloscope using a 2⁷ PRBS (pseudorandom bit sequence) data stream. From the models, the researchers have demonstrated the performance of the transistor lasers at data rates up to 40 Gbps.

At first glance, the transistor laser appears to defy Kirchhoff's current law, which states that the charge that enters a node must exit it (charge conservation). In a typical transistor, I_EMITTER = I_BASE + I_COLLECTOR, but with the transistor laser, the equation needs an additional term for energy conservation, I_STIM, which represents an additional base-charging current for photon continuity and energy conservation.

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Reference
1. Then, H.W., Milton Feng, and Nick Holonyak, Jr., "Microwave circuit model of the three-port transistor laser," Journal of Applied Physics, May 10, 2010. American Institute of Physics, jap.aip.org.