George I. Haddad

Digital circuit applications of resonant tunneling devices

Many semiconductor quantum devices utilize a novel tunneling transport mechanism that allows picosecond device switching speeds. The negative differential resistance characteristic of these devices, achieved due to resonant tunneling, is also ideally suited for the design of highly compact, self-latching logic circuits. As a result, quantum device technology is a promising emerging alternative for high-performance very-large-scale-integration design. The bistable nature of the basic logic gates implemented using resonant tunneling devices has been utilized in the development of a gate-level pipelining technique, called nanopipelining, that significantly improves the throughput and speed of pipelined systems. The advent of multiple-peak resonant tunneling diodes provides a viable means for efficient design of multiple-valued circuits with decreased interconnect complexity and reduced device count as compared to multiple-valued circuits in conventional technologies. This paper details various circuit design accomplishments in the area of binary and multiple-valued logic using resonant tunneling diodes (RTDs) in conjunction with high-performance III-V devices such as heterojunction bipolar transistors (HBTs) and modulation doped field-effect transistors (MODFETs). New bistable logic families using RTD+HBT and RTD+MODFET gates are described that provide a single-gate, self-latching majority function in addition to basic NAND, NOR, and inverter gates.

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.

Finite-element simulation of GaAs MESFET's with lateral doping profiles and submicron gates

Results of a two-dimensional finite-element simulation of a GaAs MESFET are presented. The simulation is used to determine the drain current and transconductance as well as the two-dimensional voltage, electron density, and electric-field distributions. It is shown that placement of a compensated doping region in the high electric-field region between gate and drain increases the drain current and transconductance by reducing the velocity-saturation effect. The transconductance and drain conductance of the MESFET in the saturation region of devices having different channel heights are compared with previous analysis.

Nonlinear properties of IMPATT devices

The basic principles of IMPATT diodes as microwave devices are reviewed and the current status of these devices concerning power output and efficiency is given. The main purpose of this paper, however, is to discuss the nonlinear properties of these diodes which are useful in the design of amplifiers, oscillators, and other microwave devices. The main results of this paper are obtained from a digital computer analysis where an approximate, but realistic, diode model is employed. A detailed comparison of complementary silicon diodes as well as GaAs diodes concerning power output and efficiency is given. The effects of doping profile, current density, temperature, and material parameters on the performance of these devices have been investigated and are summarized. Saturation effects which limit the efficiency and power output of these devices are described and optimum efficiencies which can be achieved for various doping profiles are given. A comparison between single-sided and double-drift diodes in both silicon and GaAs is also presented.

Frequency-Dependent Characteristics of MicrostripTransmission Lines

A method for determining the frequency-dependent characteristics of both single and coupled lines in shielded microstrip is presented. Numerical results are given for a variety of dielectric configurations and the effects of geometry on the dispersion characteristics are examined in detail. Of particular interest are the characteristics of coupled lines on compensated dielectric structures, i.e., structures that are capable of achieving equal even- and odd-mode phase velocities, and the effects of dispersion on the directivity characteristics of such lines are discussed. In addition, the variation of impedance as a function of frequency, where the impedance is defined as the ratio of the power to the square of the longitudinal current, is presented for representative cases of single and coupled lines.

High-efficiency class-A power amplifiers with a dual-bias-control scheme

A new scheme for power amplifiers is proposed, which can provide both high efficiency and linearity. The proposed amplifier operates in a virtual class-A mode under dual-bias control to maximize the power-added efficiency along with its inherent class-A linearity. The dynamic dual-bias control involves controlling both bias current and voltage of the amplifier with a varying envelope of input RF signals. The efficiency of the proposed amplifier is theoretically evaluated and compared with that of other conventional amplifier schemes. Based on theoretical analyses, several promising schemes for dual analog and digital bias control are proposed and discussed.

Basic Principles and Properties of Avalanche Transit-Time Devices

The basic principles and characteristics of the various modes of operation of avalanche transit-time devices are presented. Theoretical and experimental results are also presented in order to indicate the present state of development and the kind of performance which has been achieved in these devices.

High-Frequency Limitations of IMPATT, MITATT, and TUNNETT Mode Devices

High-frequency limitations of IMPATT and other mode devices are explored by concentrating on the details of the Iarge-signal injected current pulse formation. Simple waveform models are given for injected current pulses of large widths, and various scaling relations are also included. The large-signal injected current pulse is calculated by use of a modified Read equation where attention is given to the effect of the intrinsic response time and the tunneling current. The poor high-frequency performance of GaAs devices is explained by postulating that the intrinsic response time is larger than expected. Tunneling current is shown to increase the high-frequency performance of GaAs diodes. Device efficiencies are calculated for specific diode structures by using a computer simulation which includes mixed avalanche-tunnel breakdown. The results for GaAs and Si devices are given, and the results are discussed and compared.

Semiconductor Device Simulation

Two of the numerical methods most widely used in solving the set of partial differential transport equations for holes, electrons, and electric field in semiconductor devices and the various numerical instability phenomena which can be encountered are described in detail. Also presented are approaches, using these methods, to calculate dc static solutions and small-signal solutions, and to simulate devices in voltage-driven, current-driven, and circuit-loaded operation. Sample results are given for each mode of operation for the case of Si avalanche-diode oscillators. The numerical methods and approaches are those developed at our laboratory and sufficient detail is presented to permit the development of similar Fortran codes by others.

Large-Signal Equivalent Circuits of Avalanche Transit-Time Devices

A large-signal analysis for IMPATT diodes is derived, which allows carrier multiplication by impact ionization to occur at every point in the diode. Therefore, the operating characteristics of IMPATT diodes with a wide range of realistic doping profiles can be investigated. For a given operating frequency, RF voltage, dc bias current, and doping profile, the admittance, power output, efficiency, bias voltage of a diode can be obtained. An equivalent circuit the diode package, microwave circuit mount and diode, is obtained experimentally. Using this circuit, the admittance of the diode is measured by a reflection-type circuit and an oscillator circuit as a function of the RF voltage, dc bias current, and frequency.