J. N. Schulman

Resonant tunneling diodes: models and properties

The resonant tunneling diode (RTD) has been widely studied because of its importance in the field of nanoelectronic science and technology and its potential applications in very high speed/functionality devices and circuits. Even though much progress has been made in this regard, additional work is needed to realize the full potential of RTDs. As research on RTDs continues, we will try in this tutorial review to provide the reader with an overall and succinct picture of where we stand in this exciting field or research and to address the following questions: What makes RTDs so attractive? To what extent can RTDs be modeled for design purposes? What are the required and achievable device properties in terms of digital logic applications? To address these issues, we review the device operational principles, various modeling approaches, and major device properties. Comparisons among the various RTD physical models and major features of RTDs, resonant interband tunneling diodes, and Esaki tunnel diodes are presented. The tutorial and analysis provided in this paper may help the reader in becoming familiar with current research efforts, as well as to examine the important aspects in further RTD developments and their circuit applications.

Physics-based RTD current-voltage equation

An analytic expression for the current-voltage characteristics of resonant tunneling diodes is derived from basic principles. The form is ideal for insertion into circuit simulation models. It is demonstrated for a conventional InGaAs-AlAs RTD and for an InAs-AlSb-GaSb RIT diode. The expression is based on the quantum tunneling formalism and contains parameters that originate from physical quantities, but which can also be treated as empirical. Empirical fitting is straightforward and results in an excellent match to the data. Additional levels of physical realism can be incorporated in a natural way.

Effect of size nonuniformity on the absorption spectrum of a semiconductor quantum dot system

The interband optical absorption of a nonuniform semiconductor quantum dot system is calculated. The effect of dot size variation on the resolvability of the absorption peaks is estimated. The dots are assumed to be cubic, with a size distribution described by a Gaussian function. It is shown that the total absorption spectrum of such a dot system depends strongly on the dot size distribution described by the parameter ξ, the ratio of the standard deviation of the dot size to the average dot size of the system. The range of ξ values for which the absorption peaks are resolvable is given.

Passive Millimeter-Wave Imaging Module With Preamplified Zero-Bias Detection

An analytical model and supporting measured data are presented for a preamplified W-band radiometer with a zero-bias detector appropriate for commercial millimeter-wave imaging cameras. Basic radiometer parameters, including RF bandwidth, are computed directly from simple low-frequency measurements and compare well with those obtained from RF measurements. A detailed analytical model shows how radiometer performance depends on internal component parameters, such as low-noise amplifier gain, noise factor, reflection coefficient, detector responsivity, etc. The measurements suggest that performance is sufficient for operation without a Dicke switch or mechanical chopping. A measured noise equivalent temperature difference of 0.45 K was obtained, assuming a single sensor is scanned across a focal plane, forming 32 pixels with 3.125-ms integration time per pixel. This sensitivity is considered sufficient by commercial manufacturers to obtain quality images in low-contrast (e.g., indoor) environments.

Sb-heterostructure interband backward diodes

Backward diodes are a version of Esaki tunnel diodes that are useful for mixing and detection. Ge backward diodes in particular have been used as temperature insensitive, zero bias square law detectors, capable of translating low level RF power into DC voltage or current with extreme linearity and low noise. However, Ge diodes are difficult to reproducibly manufacture and are physically fragile. Here we demonstrate specially designed Sb-heterostructure-based backward diodes grown by molecular beam epitaxy. These diodes have superior figures of merit compared to Ge diodes, especially the current density and junction resistance, and are reproducible and physically rugged. In addition, the flexibility of MBE growth allows easy tailoring of the layer structure to maximize the desired figure of merit for a given application.

Infrared optical characterization of InAs/Ga1−xInxSb superlattices

InAs/Ga1−xInxSb superlattices have been examined by photoluminescence, photoconductivity, and infrared optical transmission. Samples display clear photoconductive thresholds at energies in agreement with band gaps derived from photoluminescence. Far‐infrared energy gaps (8–14 μm and beyond) are obtained for InAs/Ga0.75In0.25Sb superlattices with periods <75 A, in good agreement with gaps calculated from a simple two‐band model. An absorption coefficient of ∼2000 cm−1 at 10 μm is measured in a superlattice with an energy gap of 11.4 μm. The magnitude and shape of this absorption edge is comparable to that of bulk Hg1−xCdxTe, suggesting that infrared detectors based on InAs/Ga1−xInxSb superlattices may be competitive in the 8–14 μm range and beyond.

Sb-Heterostructure Millimeter-Wave Detectors With Reduced Capacitance and Noise Equivalent Power

InAs/AlSb/GaSb backward diodes are used for millimeter-wave square-law power detection. A new heterostructure design with low capacitance, low resistance, and high curvature coefficient as compared to previous designs is presented. Voltage sensitivity, which is directly proportional to curvature coefficient, is improved by 31% as compared to prior reports of devices with similar barrier thicknesses. The junction capacitance is also reduced by 24% to 13 . The improved sensitivity and decreased junction capacitance originate from the incorporation of a p-type-doping plane with sheet concentration of in the n-InAs cathode layer. The combination of low resistance (and thus Johnson noise) and high sensitivity results in an estimated noise equivalent power of 0.24 at 94 GHz for a conjugately matched source, whereas the reduced capacitance facilitates wideband matching and increases the detector cutoff frequency. These modified Sb-based detectors have promise for improving the performance of passive millimeter-wave and submillimeter-wave imaging systems.

High-performance antimonide-based heterostructure backward diodes for millimeter-wave detection

Small-area antimonide-based backward diodes for zero-bias millimeter-wave detection have been fabricated and tested. The devices were fabricated using high-resolution I-line stepper lithography, allowing accurate control of the small device active area required for operation at W-band. The devices exhibit excellent measured performance from 1-110 GHz, with responsivities when driven from a 50-/spl Omega/ source of 2540 V/W at 95 GHz. This translates to a projected responsivity of 11.5 /spl times/ 10/sup 3/ V/W at 95 GHz for a conjugately matched detector. The compression characteristics of the detectors have been measured, with 1.2 dB of responsivity compression for an input power of 8 /spl mu/W.

Bias and temperature dependence of Sb-based heterostructure millimeter-wave detectors with improved sensitivity

Nearly lattice-matched InAs/AlSb/GaSb-based heterostructure backward diodes for zero-bias millimeter wave detection were fabricated and measured. A record-high curvature, /spl gamma/=39.1 V/sup -1/, at zero bias was measured. On-wafer sensitivity measurements from 1 to 110 GHz gave a record-high average sensitivity of 3687 V/W for zero-bias operation. Further enhancement of detector sensitivity was observed with applied dc bias, with a sensitivity of 7996 V/W obtained for a 0.9 /spl mu/A bias. Extrapolating the conjugately-matched measured sensitivity suggests that 1000 V/W should be achievable at a record-high 541 GHz. The temperature dependence of detector sensitivity was evaluated from measured dc current-voltage characteristics and gave expected sensitivities ranging from 3910 V/W at 293 K to 7740 V/W at 4.2 K.

W-band direct detection circuit performance with Sb-heterostructure diodes

W-band direct detection circuits have been designed and fabricated for use in a passive millimeter wave camera. The circuits are based on the recently developed Sb-heterostructure diode. We measure record voltage responsivities in test circuits, up to 8,000 mV/mW from 75 to 93 GHz, with input power from -50 to -30 dBm. Performance was similar in an actual camera frequency processor board with 128 tuned channels. 72% of detectors showed responsivity at or above 6,000 mV/mW and 3% of channels were above 10,000 mV/mW. Since tens of thousands of Sb-heterostructure diodes can be reproducibly and inexpensively fabricated, this demonstrates for the first time the feasibility of large-scale detector arrays utilizing zero bias direct detection circuitry.