A W-band unamplified direct detection radiometer module is described that provides a wideband response and is
scalable to large arrays. The radiometer design is intended to provide sufficient sensitivity for millimeter wave imaging
applications with a goal of 2K noise equivalent temperature difference (NETD) at a 30 Hz frame rate.
This effort leverages previously reported device scaling to increase sensitivity. We present a radiometer module
designed for 60 GHz RF bandwidth that utilizes HRL's antimonide-based backward tunnel diode. An impedance
matching circuit with on- and off-chip elements, as well as ridged waveguide, provides a wideband match to the
detectors. Modules were designed with two different microwave substrates: 125 micron thick quartz and 100 micron
thick alumina. flip-chip bonding of the detectors is amenable to automated pick-and-place for high volume
manufacturing. The modular nature of the array approach allows large arrays to be manufactured in a straightforward
manner. We present the design approach along with both electromagnetic simulations and measured performance of the
modules. This work was supported by phase II of DARPA's MIATA program.
We are developing a passive W-band millimeter wave imaging array that operates without the use of RF low noise
amplifiers. The work is supported by the DARPA MIATA program. Previously reported Phase I results were a noise
equivalent temperature difference (NETD) of 4.8°K. The goal of Phase II, currently underway, is to decrease this to 2°K
or less. There are two improvements that must be made to achieve the goal. The square law diode detector sensitivity
and the RF bandwidth reaching the detector must be increased significantly. This paper mainly deals with the first issue,
the effort to increase the sensitivity by decreasing the diode area and capacitance, using electron beam lithography.
Brief mention will be made of the redesign of the antenna-to-diode transition that simulations indicate will provide a
doubling of bandwidth from 30 to 60 GHz.
Sb-heterostructure diodes have become the detector of choice for W-band millimeter wave imaging cameras now
being commercialized or in prototype development. Here we optimize the diode impedance to yield a minimum noise equivalent
power (NEP). The goal is to decrease the gain required of the front-end LNA. Measured W-band sensitivities for two diodes are
3500 and 5500V/W. Their zero bias differential resistance values imply Johnson noise limited NEP's of 0.98 and 0.83pW/Hz1/2,
respectively, much less than obtained from conventional biased Schottky diodes. A MMIC version of the diode detector has been
simulated with an integrated bandwidth of ~ 30 GHz at W-band. The simulated temperature sensitivity (NEΔT) with an HRL W-band
LNA on the front end is <1°K.
We have demonstrated a high efficiency package for zero bias Sb-based backward tunnel diodes developed for passive millimeter wave imaging. Flip-chip mounting of detector MMICs onto quartz substrates permit placement of the detector directly within the WR-10 waveguide feeds for diagonal horn antennas. This arrangement minimizes the losses between the detectors and antennas while providing an impedance match over a majority of W band. A 2x2 array of radiometers was fabricated, assembled, and measured using coherent measurement techniques. The resulting noise equivalent temperature difference, calculated assuming a 30 Hz frame rate is 10 degrees K.
The figure of merit for RF detectors, noise-equivalent power (NEP), is determined by the noise divided by the sensitivity. Thus, the challenge is to design a diode structure that has low junction resistance while maintaining a large nonlinearity. This work presents sensitivity and noise measurements for Sb-heterostructure backward diodes with varying barrier thicknesses and cross-sectional areas. Nominal diode areas are 2x2mm2 and 3x4mm2 with 15Å and 20Å barriers. The best NEP measured to date is 1.19 pW/rtHz at 36.5 GHz.
High-resolution passive millimeter wave imaging cameras require per pixel detector circuitry that is simple, has high sensitivity, low noise, and low power. Detector diodes that do not require bias or local oscillator input, and have high cutoff frequencies are strongly preferred. In addition, they must be manufacturable in large quantities with reasonable uniformity and reproducibility. Such diodes have not been obtainable for W-band and above. We are developing zero-bias square-law detector diodes based on InAs/Alsb/GaAlSb heterostructures which for the first time offer a cost-effective solution for large array formats. The diodes have a high frequency response and are relatively insensitive to growth and process variables. The large zero- bias non-linearity in current floor necessary for detection arises from interband tunneling between the InAs and the GaAlSb layers. Video resistance can be controlled by varying an Alsb tunnel barrier layer thickness. Our analysis shows that capacitance can be further decreased and sensitivity increased by shrinking the diode area, as the diode can have very high current density. DC and RF characterization of these devices and an estimate of their ultimate frequency performance in comparison with commercially available diodes are presented.
We have developed a new kind of millimeter wave diode for zero-bias detection up to and beyond 100GHz. The diode is based on the InAs/GaSb/AlSb heterostructure, which has a staggered Type II band gap alignment in which the conduction band of the InAs is lower in energy than the GaSb valence band edge. This produces a built-in asymmetry which produces a high zero bias nonlinearity in the current-voltage characteristic. The heterostructure is a relatively simple one that is reliably and reproducibly grown using standard molecular beam epitaxy techniques. The diodes we have fabricated demonstrate that this is a new and superior solution as compared with its predecessors, the Ge backward diode and the planar doped barrier diode, for detection and mixing of small input power-level signals without the added complexity of DC bias or local oscillator.
The influence of double-barrier quantum well asymmetry on the switching time of RTDs is studied. For circuit simulation purposes the RTD is modeled as a resistance in series with a parallel combination of a non-linear dependent current source and a non-linear capacitor. The current source embodies an analytic expression for the I(V) characteristics derived from basic principles. The capacitor embodies a C(V) equation derived from a self-consistent numerical two-band model simulation of the structure. It is found that the switching time is most sensitive to asymmetry on the emitter-side barrier, and that the smallest emitter barrier width structure exhibits the smallest switching time.