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Chip Innovators Eye Airport Surveillance Equipment

European researchers are developing high-frequency transistors based on a variety of radical technologies to power security-camera systems, probe distant galaxies and boost computer performance. Richard Stevenson investigates.

The events of 9/11 have changed our lives. Plane travel requires longer check-ins through increased security, while unidentified liquids have the capacity to ground aircraft. Unsurprisingly, there has also been a surge in the development of high-performance millimeter-wave screening systems that are able to expose dangerous and hidden substances (see figure 1), and consequently the transistor chips which are needed in the detectors.

Screening systems can distinguish between various objects by comparing their emissive and reflective properties at frequencies of tens to hundreds of gigahertz. But the radiation emitted by objects at these frequencies is weak, so the signals have to be amplified before detection. Transistor-based amplifiers produce the necessary signal gain and both GaAs – and InP-based technologies can be designed to operate at 94 GHz, 140 GHz and 220 GHz, the atmospheric windows where absorption is relatively low. These transistors also need to have low noise characteristics so that the amplifiers can produce sufficiently high contrast images.

At the Fraunhofer Institute for Applied Solid State Physics (IAF) in Freiburg, Germany, a team is developing HEMTs to meet these needs. The devices are less noisy than HBTs and are the foundation for many different components like amplifiers, mixers and detectors, which can all be formed on the same chip.
The IAF team s metamorphic InAlAs/InGaAs HEMTs have a 100 nm gate-length, an fT of 220 GHz, an fmax of 300 GHz and an estimated mean time to failure of 30 million h. These devices amplify signals at 94 GHz, 140 GHz and 220 GHz (see figure 2), and are grown on 4 inch GaAs substrates. Michael Schlechtweg, the Fraunhofer IAF s head of RF circuit development, says that this platform offers benefits over InP in terms of cost, robustness and greater design freedom in the InGaAs layer composition.

With these components it is possible to make active and passive imaging systems that are analogous to cameras taking pictures with and without a flash. The primary contrast mechanism for these images is the temperature difference between objects. In the outside environment variations in temperature are significant and so passive systems can be employed, but for indoor screening active illumination at the detection frequency is required.

Active systems will be needed for airport security applications, while passive systems can serve a wide variety of other applications. This includes observations from space of the Earth s climate for weather forecasting and studies of climate change, and measurements of the cosmic background radiation that should be beneficial to cosmologists.

IAF is predominantly directing efforts at 94 GHz MMICs based on HEMTs, which are designed for both active illumination and detection elements. Systems incorporating these circuits will probably be available within the next few years. However, IAF transistors have also been used in the first complete MMIC-based 220 GHz passive imaging system, which was built late last year in partnership with the Research Institute for High Frequency Physics and Radar Techniques in Wachtberg, Germany (see figure 3). Higher frequencies improve the spatial resolution of the images or enable reduced antenna size.

The IAF team has scaled gate lengths to 50 nm, boosting gain, reducing noise and increasing fT and fmax to 400 GHz and 420 GHz, respectively.

Frequency triplers

Faster, more powerful chips based on InP are on the agenda at the Microwave and Terahertz Technology Lab, Chalmers University, Sweden. Instead of working with transistor technology Jan Stake s team is developing heterostructure barrier varactor (HBV) diodes that were invented there in 1989 (see box "HBV design"). These devices do not generate high-frequency emission themselves, but can triple or quintuple the frequency produced by another source efficiently.

The Chalmers group is targeting the frequency range from 200 GHz to 2 THz, which includes the so-called "terahertz gap", a range of frequencies that are not accessible with conventional transistors or quantum cascade lasers. Work at the lower end of this frequency range is being conducted under a Swedish defense project that is developing active imaging systems at 210 GHz for security applications. HBVs could boost the output power of these active systems, thereby improving image contrast. At higher frequencies HBVs could also be used in space-based radio astronomy instruments that can identify cosmic gases by measuring their absorption spectra.

Stake thinks that the group s most impressive result is a HBV tripler that delivers 0.2 W at 114 GHz. "It s not a very high frequency, but it is still a good result," says Stake. According to him, this latest device will fulfill the need for a more powerful lower-frequency source with which to pump high-frequency HBVs.

Stake adds that the main challenge associated with making high-performance HBVs is maintaining high-quality epitaxy throughout the structures, which are relatively thick. In fact, he wants to develop thicker devices by widening the buffer layer from the 0.5 μm used today to several microns. This should cut series resistance, leading to a decline in conversion loss.

However, progress has been hampered by problems in maintaining the indium-to-gallium composition ratio. Outsourcing the growth process has also proved impossible. "It s still such a niche application that industry is not interested in developing a process to grow this structure, so we had to do it in-house".

Additional gates

At the University of Lille, France, researchers are trying to raise the operating frequencies of InP-based HEMTs by taking the unusual step of adding a second gate. Nicolas Wichmann, who is heading the project, says that they need to take this radical approach because conventional HEMT performances cannot be improved by more scaling of device dimensions without severe trade-offs. Without any redesign, the HEMTs are limited to an fT and an fmax of typically 500–600 GHz.

Wichmann and his co-workers have developed a process to make double-gate (DG) HEMTs, in which the gates can be either connected together or operated separately (see figure 4). When these gates are connected they reduce short channel effects, such as poor control of the electron concentration in the channel by the gate electrode, and boost transconductance.

The team has evaluated this new design by comparing the features of its DG HEMTs with conventional T-gate HEMTs that have an identical gate length of 100 nm. With its gates connected, the DG HEMT showed double the transconductance and a 30% rise in fmax to 288 GHz, but fT fell by 8% to 192 GHz.

Although encouraged by the results, Wichmann admits that the DG HEMT s need for gate alignment will make it harder to manufacture than its conventional cousin: "It s directly correlated to the precision of the e-beam system. If the gates are misaligned, charge control efficiency and charge confinement are not optimal." However, he points out that the DG process is only of comparable difficulty to that used for HBT production, which involves an etching undercut to define device dimensions.

Wichmann s team is now focused on another innovation – so-called velocity modulation transistors. These are DG HEMTs with two channels that have different charge transport properties. Modulating the electron velocity in these devices has a similar effect on drain current. This removes the intrinsic capacitance and should lead to very high cut-off frequencies.

Esoteric materials

The defense technology company QinetiQ is also developing high-speed transistors at its site in Malvern, UK. However, its approach involves using a more esoteric material, InSb, that provides the highest electron mobility and saturation velocity of any semiconductor. These properties are able to deliver transistors with very high speeds, low operating voltages and low power consumption.

Tim Ashley, who heads QinetiQ s InSb development, explained that the work began over a decade ago under funding from the Ministry of Defence (MoD). The aim was to investigate the potential of InSb and then advise the MoD of the implications. "At that time the only example of an InSb transistor was a thin-film device with a current on-off ratio of two", says Ashley, "and that s not a lot of good to anybody."

More recently the company teamed up with Intel. "We ve been focused on pushing the technology forward with a view to examining its potential for microprocessors," explained Ashley. The partnership has produced quantum well FETs with a 85 nm gate length that have been designed to assess the parameters for digital operation and have an fT of 340 GHz and a current on-off ratio of more than 1000.

QinetiQ s efforts are not the first attempt at developing III-Vs for logic applications. Supercomputer manufacturer Cray actually owned a 4 inch GaAs line before selling it to M/A COM in 1995. However, Ashley believes that the landscape has changed: "The complexity that s required for microprocessor circuits is so high that silicon has always kept ahead. What s changing now is that the silicon industry is looking to see what materials can be used to keep Moore s law running." The silicon industry is looking at a range of materials, says Ashley, and it s too early to claim that they ve focused on any one in particular.

Ashley s team is also developing analog FETs that could tackle the same types of applications as those targeted by the researchers at Freiberg, Chalmers and Lille, but with the added benefit of the low power consumption linked with InSb. This conversion will implement variants of structures already employed on other III–V FETs, according to Ashley, who hopes to present work on these developments later this year.

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