A complete quantum generalization of microwave mixer theory is constructed for nonlinear single-particle tunnel junctions. The result represents a unification of the concepts used to describe these "classical" resistive mixers with the language of photon detection. Tunneling devices are predicted to undergo a transition from energy detectors to photon counters when operated at frequencies where the photon energy becomes comparable to the voltage scale of the dc nonlinearity. The small-signal video current response is found to approach one electron for each photon absorbed at high frequencies. In a heterodyne receiver, sufficiently nonlinear tunnel junctions are predicted to be capable of achieving the fundamental quantum noise limit for sensitivity in the detection of electromagnetic radiation. The theory presented here thus provides a framework for systematically extending the techniques of quantum electronics to considerably lower frequencies than are currently being exploited. Recent measurements of heterodyne mixer performance using superconductive tunneling devices are already beginning to approach quantum limited results at microwave and millimeter wave frequencies. Eventual application of tunnel barriers as photon detectors in the submillimeter and infrared spectral regions also appears to be possible, and the fast response times of such devices could give them an advantage over photoconductors even at the higher frequencies. The development of suitable nonlinear tunnel junctions contains the potential to bridge the present gap in quantum detectors between the infrared photon devices and microwave masers.

Quantum limited detection in tunnel junction mixers

The traditional HF model of a bulk-type (nonepitaxial) Schottky-barrier diode is extended to include the influence of skin effect, carrier inertia, and displacement current. The parasitic cutoff frequency of the extended model is calculated for n-GaAs and n-Si and compared with that predicted by the traditional model. Below the plasma frequency, the two models are found to give similar results for n-Si. For n-GaAs, however, the extended model predicts a value of cutoff frequency only one-sixth that predicted by the traditional model. With both materials, operation near the plasma frequency is impractical since it would require unrealistically small contact dimensions. Above the plasma frequency, however, both materials display a broad frequency range where operation should again be feasible. For n-Si, the extended model predicts that operation above the plasma frequency can actually be achieved with Iarger contacts than is predicted on the basis of the traditional model.

Cutoff Frequency of Submillimeter Schottky-Barrier Diodes

A quantum generalization of classical microwave mixer theory is developed to project the performance of nonlinear tunneling barriers at high frequencies. The results demonstrate that such devices may be considered photon detectors when their conductance varies rapidly on a scale of voltage h/ω/e. Tunnel barrier mixers are thus shown to offer potential for the development of wide‐band quantum noise limited detectors in the wavelength region from 10 μm to a few millimeters.

Photon detection in nonlinear tunneling devices

An analysis of the noise of millimeter- and submillimeter-wavelength mixers with GaAs Schottky diodes is presented. This analysis accounts for the correlation of the downconverted components of the time-varying hot-electron noise in the series resistance, and is thus accurate even for cryogenically cooled mixers operated at submillimeter wavelengths. This paper shows that the terms of the series-resistance noise correlation matrix are functions of the Fourier coefficients of the squared diode current F(I/sup 2/) rather than the square of the Fourier coefficients of the diode current [F(I)]/sup 2/, as has been previously presented in the literature. The analysis is used to evaluate the optimization of cryogenic mixer diodes. It is shown that minimization of the diode's I-V slope parameter V/sub 0/ is more critical than reduction of the parasitic elements for millimeter-wavelength operation, while at frequencies above 600 GHz (Lambda<0.5 mm), the junction capacitance is the most crucial parameter. Experimental results from several research groups working with a variety of mixers are presented to substantiate these results.

Analysis and Optimization of Millimeter-And Submillimeter-Wavelength Mixer Diodes

Antennas and waveguides for the wavelength range 0.1-3 mm are considered. Emphasis is placed on those designs which lend themselves to integration with each other and with other components such as diodes. The general properties of FIR antennas are reviewed. A novel silicon waveguide antenna is discussed, and its design, simulation, fabrication, and performance at 119 μm are described. This antenna has a highly symmetrical, single-lobed beam with 3 dB beamwidths of 35 and 38° in the E - and H -planes, respectively. The gain (measured in microwave simulation) is 12.8 dB. This antenna is well suited for integration with Schottky diodes. The related subject of FIR waveguides is discussed. Experiments with metal transmission lines at 119 μm are described and dielectric guides related to the waveguide antenna are also considered. Using components such as these it may soon be possible to construct receiver front ends for this wavelength range in integrated-circuit form.

Antennas and waveguides for far-infrared integrated circuits

Schottky-barrier diode detection has been extended to 7.2 THz (42 mu m) using 0.5-mu m-diam diodes. The diodes were fabricated on bulk-doped n-type GaAs using electron lithographic techniques; diameters as small as 1000 /spl Aring/ have been achieved. A new approach in Schottky-barrier design, the contact array diode, is proposed. The diode is fabricated from readily available bulk doped material, and a performance is indicated that is competitive to the conventional epitaxial Schottky-barrier mixer well into the submillimeter wavelength region. A scanning electron microscope (SEM) photograph of diode array structures is shown.

D.T. Hoidges

Papers

Submillimeter-Wave Detection with Submicron-Size Schottky-Barrier Diodes