Zero-bias InAs/AlGaSb detectors and a host of other III-V components are at the heart of a camera that is expected to dramatically improve mm-wave imaging systems. Tim Whitaker reports.

HRL Laboratories has developed InAs/AlGaSb detectors operating at zero bias that could enable an order-of-magnitude improvement in the sensitivity of mm-wave imaging systems. The research organization, which is based in Malibu, CA, has already produced and delivered several hundred thousand InAs/AlGaSb diodes to Trex Enterprises, which is developing an advanced version of its passive mm-wave camera. The HRL diode is expected to help enable the practical implementation of mm-wave imaging for a range of future military and commercial applications. The mm-wave spectrum allows detectors to "see through" obstructions much more effectively than other imaging bands such as the infrared. Prototype mm-wave cameras are already being developed to detect concealed weapons or to act as a navigational aid when visibility is poor. The Sb-heterostructure devices developed by Joel Schulman, David Chow and colleagues at HRL are superior to existing device types in terms of zero-bias and high-frequency operation. At present, the diodes operate up to the W-band (75-100 GHz), but have the potential to extend well into the sub-mm (>300 GHz) wavelength range. The diodes are grown by MBE, which allows them to be produced in large quantities and with the flexibility to tailor the layer structure to optimize the desired figure of merit for a given application. Interband tunnelingHRL s diode operates on the principle of interband tunneling between the valence and conduction bands of adjacent semiconductor regions. The principle is the same as that in Esaki tunnel diodes, in which the required energy offset is achieved with a very heavily doped p-n junction. In the Sb-heterostructure devices, also pioneered by Leo Esaki at IBM in the late 1970s, tunneling takes place between the conduction band of an n-type InAs layer and the valence band of a p-type AlGaSb layer, which are separated by a thin AlSb barrier (figure 1a). Related heterostructures were explored in the early 1990s for their negative resistance properties, although not for the zero-bias behavior exploited here. The Type II bandgap line-up between InAs and AlGaSb results in the most attractive feature of such devices, namely a high curvature in the current-voltage characteristics at zero bias (figure 1b). This allows the device to function, at zero bias, as a square-law detector, in which the power (voltage squared) of the incoming signal is converted into a proportional DC offset. In comparison, high-frequency (above 10 GHz) Schottky diodes must be biased in order to lower their junction resistance, or require input signals significantly higher than the sub-mW level. Other device types have other problems; germanium-based Esaki diodes are limited to operating below 40 GHz and cannot be made in large quantities, while planar doped barrier devices work well up to W-band but are very difficult to produce reliably and reproducibly. As a consequence of enabling zero-bias direct detection, the Sb-heterostructure device eliminates the need for local oscillators or bias currents that add complexity, power consumption and noise. Also, since 1/f noise is proportional to bias current, a significant improvement in signal-to-noise ratio can be expected using Sb-heterostructure devices in place of Schottky diodes in practical system applications. In addition, the tunneling nature of the current transport virtually eliminates the influence of temperature on detection sensitivity; this is a major issue with Schottky diodes, where the current has an exponential dependence on inverse temperature. HRL says that its Sb-based devices lead to an order-of-magnitude improvement in the noise-equivalent power, the key figure-of-merit for detector sensitivity. Measurements made by the National Institute of Standards and Technology in Boulder, CO, have confirmed these results. Device performanceThe layers for the Sb heterostructures are grown by MBE on semi-insulating GaAs substrates, and consist of an n+ InAs contact layer, a p+ GaSb layer, an undoped Al0.12Ga0.88Sb layer, a thin undoped AlSb barrier, a low-doped n-InAs layer and an n+ InAs contact layer (Meyers et al. 2004). The devices made for Trex Enterprises have 2 µm x 2 µm active areas defined using I-line stepper lithography. In test circuits, record voltage responsivities of up to 8000 mV/mW have been recorded from 75 to 93 GHz, with input powers from -50 to -30 dBm (0.01-1 µW). Tests were also carried out using an actual camera frequency-processor board with 128 tuned channels; 72% of detectors had responsivities at or above 6000 mV/mW, while 3% of channels were above 10,000 mV/mW (Schulman et al. 2004). As the Sb-heterostructure diodes are simple to grow and fabricate, these results indicate the potential for large-scale arrays operating at W-band and above and providing low-noise, zero-bias, square-law detection. Phased-array imagersThe test results described above were performed by HRL in collaboration with Trex Enterprises, a San Diego, CA, company that has been involved in the development of passive mm-wave cameras for more than 10 years. Millimeter-wave imaging detects radiation that is emitted or reflected passively to some degree by all objects. The wavelength is such that the radiation passes through clouds, fog, clothing and some building materials. Systems are being developed that can provide video-rate imagery of scenes such as airports, enabling pilots to taxi and land in conditions of severely restricted visibility. The systems can also be used to detect concealed weapons, drugs and other contraband - all of which have mm-wave signatures that contrast with the human body (figure 2). The HRL diodes are being incorporated by Trex into a real-time, phased-array imaging system with a wide 20º x 30º field of view. At the heart of the Trex system is a 192 x 128 array, with a detector diode at every pixel, meaning that the imager contains almost 25,000 detectors in total. Prior to detection, the signals must be amplified, so the system also contains more than 1000 low-noise amplifiers (LNAs). The LNAs used by Trex are InP-based devices also manufactured by HRL on a 0.1 µm process and operating at 75-94 GHz. The systems also contain AlGaAs pin diode switches, supplied by M/A-COM, which toggle the input signal to the LNAs between the scene being viewed and a matched resistive load that acts as a reference. The sensitivity of the HRL diodes is a key factor in improving system performance, according to John Lovberg, CTO of Trex. "The costliest components in the system are the amplifiers, so the more sensitive the detector diodes, the less money we need to spend on the LNAs," he said. The HRL diodes also enable direct detection in the W-band, rather than requiring downconversion to X-band as in previous iterations of the phased-array imaging system. "The first system was very large and weighed around 2000 lb, but the latest imager is much smaller and weighs around 150 lb," said Lovberg. "The HRL diodes enabled us to build the phased-array imager in a compact package that can be flown on a helicopter." The sensor is shown in figure 3 (Martin and Kolinko 2004). Packaging the amplifiers and assembling the detector circuit boards, passive delay lines and other elements is currently all done manually, making the system cost very high. "We ve built two systems so far, each costing around $1 million," said Lovberg. "With automated assembly and a reduction in component costs, particularly for the amplifiers, we expect this can be reduced to around $200,000." The phased-array systems, providing video-rate imagery at 30 frames/s, are likely to be used in applications such as aircraft landing. Trex has also built mechanically scanned systems, which do not provide real-time images but are much smaller and less expensive, partly because they contain only 128 detectors and three amplifiers. "These systems could sell for less than $10,000, for portable security applications such as weapons detection," said Lovberg. Advances in semiconductor technology have enabled the new architectural design of the phased-array imaging sensor. "As this continues to happen we re also looking at higher frequencies, such as 140 and 220 GHz," said Lovberg. "Hopefully some day we ll push into the terahertz region, which is becoming a hot area for the detection of explosives." Further readingC Martin and V Kolinko 2004. SPIE Proc. 5409.
R G Meyers et al. 2004 IEEE Elec. Dev. Lett. 25 (1) 4.
J N Schulman et al. 2004 IEEE Microwave & Wireless Components Lett. 14(7) (in press).