R. K. Mains

Detection and discrimination of radar targets

A new method for detection and discrimination of radar targets is described. The basis for this method is that the gross structure of a radar target can be identified from scattered fields of the target at harmonic radar frequencies located just in the low resonance region. This is in sharp contrast to many target signature schemes that operate at much higher frequencies and observe many of the details of the target in lieu of its gross features. Multiple frequency radar scattering data and the complex natural resonant frequencies of radar targets are integrated into a predictor-correlator processor. The method is illustrated using as target models both classical shapes and thin-wire configurations of simple geometry. Integral equation programs are utilized to calculate multiple frequency backscatter data for the wire geometries and to deduce the complex natural resonant frequencies of the wire structures. Discrete multiple frequency radar scattering data spanning a particular spectral range are shown to be desirable for optimum capability but discrimination and detection can be achieved using just two near-conventional radars, even if the radars are located at different sites and hence view the target from different aspects.

Simulation of GaAs IMPATT diodes including energy and velocity transport equations

Simulations have been performed of GaAs hybrid double-drift IMPATT diodes at 60 and 94 GHz using a transport model which includes equations for the average per-carrier velocity and energy. These equations are obtained from the second and third velocity moments of the Boltzmann transport equations, respectively. The relaxation-time formulation is used to characterize the collision terms. Simulations were also carried out for the same structures using the standard drift-diffusion transport model. It was found that inclusion of the energy-velocity equations significantly modifies the predicted carrier transport behavior and results in somewhat better RF performance under large-signal conditions than that predicted by the drift-diffusion simulation.

Observation of intrinsic bistability in resonant tunneling diode modeling

Intrinsic bistability has been observed experimentally and attributed to the effect on the potential profile from stored charge in the quantum well through Poisson’s equation. This effect leads to two possible current states corresponding to a single voltage within the negative resistance region. In this letter a simulation method is presented which clearly shows bistability in the current‐voltage curve of a resonant tunneling diode. This method self‐consistently combines a Thomas–Fermi equilibrium model for the electron concentrations outside the double‐barrier structure with a quantum calculation for the concentration inside the structure.

Large-signal numerical and analytical HBT models

Several large-signal heterojunction bipolar transistor (HBT) models are investigated to determine their usefulness at millimeter-wave frequencies. The most detailed model involves numerically solving moments of the Boltzmann transport equation. A description of the numerical model is given along with several simulated results. The numerical model is then used to evaluate two analytical HBT models, the conventional Gummel-Poon model and a modified Ebers-Moll model. It is found that the commonly used Gummel-Poon model exhibits poor agreement with numerical and experimental data at millimeter-wave frequencies due to neglect of transit-time delays. Improved agreement between measured and modeled data results b including transit-time effects in an Ebers-Moll model. The simple model has direct application to millimeter-wave power amplifier and oscillator design. Several measured results are presented to help verify the simple model. >

C-V and I-V characteristics of quantum well varactors

A theoretical model for quantum well varactors is presented. The model is used to calculate the device C‐V and I‐V characteristics and very good agreement has been found between the calculated and measured results. Based on the model, a triple barrier double well varactor has been designed and fabricated. A very high capacitance ratio within a very small bias range is achieved, as designed. Details of the design calculations and experimental results are presented.

A proposed narrow-band-gap base transistor structure

Abstract A transistor structure is proposed which alleviates the problem of high base resistance in the narrow quantum-well base region of the Resonant-Tunneling Transistor (RTT). This idea may also be used to improve the performance of the Induced Base Transistor (IBT) and other related structures. Bound states are created in the quantum well by using a base material with lower band gap than the contact layers. Electrons in these bound states form a low-resistance base region for application of bias to the device. Current flow is due to resonant tunneling via the second energy level in the well. Calculated I–V curves and switching transients for the RTT are presented. The issue of undesired tunneling current from base to collector is addressed, and a modified RTT structure is proposed which significantly reduces this problem.

High-power generation in IMPATT devices in the 100-200-GHz range

A silicon double-drift IMPATT diode with high uniform doping levels was simulated. Simulation results show that it is possible for silicon IMPATT diodes to generate extremely high pulsed output power for frequencies above 100 GHz under high current-density operation. The highest output power matched to a 1- Omega load resistance obtained at 150 GHz is 37.7 W with a DC current density of 200 kA/cm/sup 2/, although the calculated power conversion efficiency is low. It is also shown that the low-power conversion efficiency limits the diodes continuous wave power operation. >

Theoretical studies of the applications of resonant tunneling diodes as intersubband laser and interband excitonic modulators

We present a theoretical analysis of the optical applications of resonant tunneling diodes. The electronic properties are calculated with a self‐consistent traveling‐wave model that includes effective‐mass mismatches. The interband optical properties are calculated from a 4×4 k⋅p band structure in the dipole approximation. We find that it is possible to operate a conventional device as an intersubband laser if the transition energy is large (∼0.5 eV) and the linewidth in minimal (∼5 meV). A bound‐state device can produce a modulation ratio of 5:1 at the excitonic peak with an absorption length of ∼ 40 μm in a waveguide geometry.

NOVEL USE OF RESONANT TUNNELING STRUCTURES FOR OPTICAL AND IR MODULATORS

The basic concepts and some preliminary calculations are presented showing the feasibility of high speed optical and infrared modulators baaed on resonant tumreling structures. A modulator based on a conventional resonant tunneling structure can be operated in the infrared region by using the intersubband transitions and in the optical region by using the valence band to conduction band transitions in the quantum well of the double barrier structure. A novel resonant tunneling structure is proposed as a modulator in which the quantum well conduction band at zero bias is below the collector layer conduction band. This deep well resonant tunneling structure can allow intersubband transitions with higher energy than its conventional counterpart and with a careful design may even result in population inversion and gain. The band-to-band transitions are also possible in the deep well resonant tunneling modulator. Due to the inherent negative resistance of the device which persists to ultrahigh frequencies, it is possible to operate the device as a self-oscillating modulator.

The bound-state resonant tunneling transistor (BSRTT): Fabrication, D.C. I-V characteristics and high-frequency properties

Abstract The output characteristics of resonant tunneling transistors with a charge filled bound state quantum well base obtained by a self-consistent solution of Poissons and Schrodingers equations show the effect of coupling between the input and output ports of the device and the effect on the current-voltage characteristics. Using a self-aligned process transistors were fabricated which showed a current gain of 3 and transconductances of 30 mS. The output characteristics do not saturate and this is in qualitative agreement with theoretical predictions. The charge and potential distributions obtained from the self-consistent calculations are used in a quasi-static analysis of the small signal parameters for a hybrid-π model, and the high-frequency performance of the transistor is analyzed.