Micro/Nano Scale

Researchers at the University of Illinois have improved the performance of a Quantum Well Transistor and Laser. Their invention helps improve the performance of optical communications systems and can also be used to determine the noise figure of semiconductor optical amplifiers.

Overview of the Transistor Laser (from the website of Professor Feng)

Described in the Novermeber 15 issue of the journal Applied Physics Letters in 2004, Milton Feng, Nick Holonyak, postdoctoral research associate Gabriel Walter, and graduate research assistant Richard Chan demonstrated operation of the first heterojunction bipolar transistor laser by incorporating quantum well in the active region of light emitting transistor. Just as light emitting transistor, transistor laser was made of indium gallium phosphide, indium gallium arsenide, and gallium arsenide, but emitted coherent beam by stimulated emission, which differed from their previous device that only emitted incoherent photons. Despite their success, the device was not useful for practical purposes since it only operated at low temperatures about minus 75 in Celsius degrees. Within a year, though, the researchers finally fabricated a transistor laser operating at room temperature by using metal organic chemical vapor deposition (MOCVD), as reported in the September 26 issue of the same journal. At this time, the transistor laser had 14-layer structure including aluminium gallium arsenide optical confining layers and indium gallium arsenide quantum wells. The emitting cavity was 2,200nm wide, 0.85mm long, had continuous modes at 1,000nm. Plus, it had threshold current of 40mA and direct modulation of the laser at 3 GHz. 

This invention is the world's first light emitting transistor. The HBLET extends the capabilities of light-emitting diodes and could make this transistor the fundamental element in electronics and optoelectronics.

Details

In a bipolar device there are two kinds of injected carriers, negatively charged electrons and positively charged holes. Some of these carriers, for example, a transistor, recombine rapidly, supported by a base current essential for normal transistor function. In the past, this base current has been regarded as a waste current that generates unwanted heat. However, this technology shows the base current creates light that can be modulated at transistor speed. This recombination process is the same (but enhanced by carrier transport) as the one used in LEDs to produce visible, rather than infared, light. The transistor laser produces infared radiation in phase with its base current, so it can be modulated at a switching speed impossible to attain with an LED. The switching speed obtained is fast enought to operate in fiber optic networks, as well as other applications.

These transistors are made with indium gallium phosphide and gallium arsenide, unlike traditional transistors which have been built from silicon and germanium. The recombination process in indium gallium phosphide and gallium arsenide materials create infared photons, the "light" in the light emitting transistor. This contributes to the device's ability to operate as both a laser and a transistor. The existence of a third port (the photon output) can interconnect optical and electric signals for display or communication purposes. Additionally, electrical and optical qualities are increased by the incorporation of quantum wells into the active region of the transistors. The transistor laser has a unique capability in signal processing and electronic-photonic integrated circuits.

Applications

• Electronics
• Optoelectronics
• Displays

Benefits

• Speed: These transistors produce infared light in phase with their base current, allowing it to attain a switching speed superior to light emitting diodes (LEDs).
• Controlled light emission: Light intensity can be controlled by varying the base current.
• Simultaneous control of optical and electrical outputs: The addition of a third port allows greater control.
• Utilizes base current: Once considered wasteful, the base current is used to create light. 

Semiconductor light emitting diodes (LEDs) and lasers using direct gap III-V materials and electron-hole injection and recombination, have over the years led to numerous applications in display and light-wave communications. While semiconductor lasers typically dominate long-distance communications links, fast spontaneous light-wave can be an attractive solution for short range optical data communications and optical interconnections as their threshold-less operation, high fabrication yield and reduced driver and feedback control complexity significantly reduce the overall cost, form factor and power consumption of transmitters. However, the fastest spontaneous light source shown to date (a light emitting diode) employs p-doping as high as 7X1019 cm-3 to achieve a bandwidth of 1.7 GHz, at the cost of reduced internal quantum efficiency to 10% or less. In practice, higher efficiency spontaneous devices such as LEDs or RCLEDs operate with bandwidths that are less than 1 GHz, restricting actual commercial application of spontaneous light transmitters (LEDs and RCLEDs) to less than 1 Gbits/s. With this technology an improved 4.3 GHz optical microwave performance of a three-port light emitting transistor is achieved.

Details

The hetero-junction bipolar light emitting transistor (HBLET), which uses a high-speed hetero-junction bipolar transistor (HBT) structure, could potentially function as a light source with speeds exceeding tens of GHz. The room temperature, continuous wave operation of a transistor laser further demonstrates that a practical radiative recombination center (i.e. undoped quantum well) can be incorporated in the heavily doped base region of a HBLET. In practice, despite the high intrinsic speed of the HBT, the microwave performance of an HBLET is severely limited by parasitic capacitances, partly owing to the need to include light extraction features (such as oxide apertures) not present in traditional high speed HBT devices. With this technology an improved 4.3 GHz optical microwave performance of a three-port light emitting transistor is achieved which significantly reduces the overall parasitic capacitances. Also, the three-terminal nature of the light emitting transistor offers two input-output configurations for electrical-to-optical output conversion, e.g., via the common-collector BC- and EC-input ports, each with its own unique advantages.

Applications

Together with the advantages of higher yield, reduced complexity and three-terminal high-speed modulation capabilities as both a transistor (amplifier and switch) and electrical-to-optical convertor, the HBLET could be an attractive solution for short range optical data communications, and has significant implications for the development of high-speed semiconductor lasers and integrated optoelectronics.

Benefits

This hetero-junction bipolar light emitting transistor (HBLET) technology indicates that spontaneous recombination can be fast and higher modulation speeds are possible by further reducing the undesirable parasitic. In addition, due to the absence of the relaxation oscillations typically observed in laser devices and the lesser attenuation slope, an HBLET could potentially be deployed at data rates much higher than 4.3 Gb/s. Together with the advantages of higher yield, reduced complexity and three-terminal high-speed modulation capabilities as both a transistor (amplifier and switch) and electrical-to-optical convertor, the HBLET could be an attractive solution for short range optical data communications, and has significant implications for the development of high-speed semiconductor lasers and integrated optoelectronics.

This invention provides processing steps, methods and materials strategies for making patterns of structures for integrated electronic devices and systems. Processing methods of the present invention are capable of making micro- and nano-scale structures, such as Dual Damascene profiles, recessed features and interconnect structures, having non-uniform cross-sectional geometries useful for establishing electrical contact between device components of an electronic device.

The invention provides device fabrication methods and processing strategies using sub pixel-voting lithographic patterning of a single layer of photoresist useful for fabricating and integrating multilevel interconnect structures for high performance electronic or opto-electronic devices, particularly useful for Very Large Scale Integrated (VLSI) and Ultra large Scale Integrated (ULSI) devices. Processing methods of the present invention are complementary to conventional microfabrication and nanofabrication methods for making integrated electronics, and can be effectively integrated into existing photolithographic, etching, and thin film deposition patterning systems, processes and infrastructure.