David W. Woodard

Surface and interface depletion corrections to free carrier-density determinations by hall measurements

Errors in the determination of (ND-NA) for semiconductor epitaxial layers by the Hall method can result if corrections for carrier depletion are omitted in the calculations. Simple practical procedures are discussed to correct for carrier depletion that occurs in epitaxial layers at their free surfaces, and their interfaces with semi-insulating substrates. Theoretical estimates of carrier depletion in GaAs indicate that depletion regions can extend several microns into high purity epitaxial layers, and can cause (ND-NA) to be considerably underestimated. Experimental evidence is presented in support of the theory.

High-frequency, high-efficiency MSM photodetectors

Metal-semiconductor-metal (MSM) photodiodes with submicron spaced interdigitated Schottky barrier fingers have been developed for applications in monolithic integrated optical receiver circuits capable of detecting a millimeter-wave modulation signal. Each photodetector layer, is designed for optimal absorption about a narrow linewidth centered on a specific wavelength between 700 and 800 nm. The MBE grown layers consist of an Al/sub x/Ga/sub 1-x/As cap layer, to prevent any surface recombination of carriers and to minimize top surface reflections; a thin GaAs absorption layer (375 nm), to achieve a high-frequency response (>39 GHz) by minimizing the collection times of optically generated carriers; and a buried Bragg reflector stack which reflects unabsorbed light back into the GaAs absorption layer. Using this layer design, we are able to fabricate detectors that have millimeter-wave bandwidths without sacrificing quantum efficiency. The measured internal quantum efficiency of an MSM photodiode, fabricated on such a layer structure, was 82% at 5 V and close to 94% at 10 V. >

GaAs integrated circuits by selected‐area molecular beam epitaxy

Working GaAs integratted logic elements have been fabricated using selected‐area molecular beam epitaxy. Thermally grown native oxides ∼250 A thick have been used to define regions of single‐crystalline device‐quality GaAs in a matrix of semi‐insulating polycrystalline GaAs. The planarity of this process and the inherent resistivity of the polycrystalline GaAs ( ρ≳105 Ω cm) have been used in the fabrication of working NAND and NOR logic gates (Schottky diode/field‐effect transistor logic).

The effect of a high energy injection on the performance of mm wave Gunn oscillators

Abstract The “dead zone” existing in conventional n + nn + GaAs Gunn diodes was eliminated by the addition of a cathode structure which launches electrons into the active region at nearly the energy required for intervally transfer. The experimental efficiency and output power from devices made with the launcher were up to three times higher at 80–100 GHz than from conventional n + nn + devices designed for these frequencies. The launcher was realized by the energy discontinuity at the interface of a n -AlGaAs/n-GaAs heterojunction, where the transport is perpendicular to the interface.

New coherent detector for terahertz radiation based on excitonic electroabsorption

We demonstrate a new technique for the coherent measurement of free‐space THz electrical transients, based on the parallel‐field excitonic electroabsorption effect in GaAs quantum wells. A THz transient generated from a photoconductive dipole antenna is measured with a rise time of 290 fs and a full width at half maximum of 360 fs. The initial rise of the THz wave form is abrupt, and does not display the exponential leading edge apparent in waveforms measured with photoconductive techniques. The detector sensitivity is sub‐100 mV/cm.

High speed optical detectors for monolithic millimeter wave integrated circuits

Metal-semiconductor-metal photodiodes with interdigitated Schottky barrier fingers are being developed for applications in monolithic optical receiver circuits with the purpose of detecting millimeter-wave modulation signals being transmitted via an optical carrier. The devices are planar and incorporate submicron finger spacings and a thin absorption region for speed with a buried stack of tuned Bragg reflectors for enhanced sensitivity at the carrier wavelength. These devices are being integrated with short-gate MODFET (modulation-doped field-effect transistor) amplifiers to form the complete monolithic integrated optical receiver circuit. Device measurements indicate a top surface reflectance of 0.5% with an operation bandwidth in excess of 40 GHz and a dynamic range of 33 dB.<<ETX>>

High-speed MSM photodetectors for millimeter waves

This presentation deals with MSM photodetectors and post-detection amplifiers capable of responding at millimeter-wave frequencies. Monolithic integration, incorporating pseudomorphic MODFETs (SMODFETs) as the amplifiers, is used with coplanar waveguide resonant impedance transformers. Interdigitated Schottky barrier metal fingers, with submicron spacing and unique Bragg reflector layers to maximize light absorption, are used for the photodetector. For a 0.5-micron gap detector, a measurement shows a flat frequency response at least up to 40 GHz. Quantum efficiency is expected to be more than 60 percent for a 0.75 micron gap and 0.25 micron finger detector with 0.375 micron absorption layer.

The effect of subband quantization in the 2-D electron gas on thermionic current and heterojunction capacitance in an n-AlxGa1−xAs/n-GaAs heterojunction

Abstract The partition of the applied voltage between both sides of an n -AlGaAs/ n -GaAs heterojunction is calculated, considering energy subband quantization in the very narrow triangular-like well in the GaAs and is used to calculate the thermionic current and the heterojunction capacitance as functions of the applied voltage. Comparison with classical calculations shows a difference of a few orders of magnitude in the current, especially in reverse bias, and a small difference in capacitance. By applying the model to self capacitance measured on heterojunction devices bounded with ohmic contacts on both sides, it is also shown how to obtain such parameters as conduction band discontinuity, doping in the AlGaAs, average distance of 2-D electron gas from the heterojunction, and sheet concentration. Experimental results using this method are within 10% of other recent determination of ΔE c .

Space Charge Effects On Heterojunction Cathode (al-Ga)as Gunn Oscillators

A new transport formulation for large signal time-dependent hot electron transport in graded and abrupt heterostructures has been developed and implemented in a self-consistent ensemble Monte Carlo code. It has been used to explore the microscopic physics of Gunn diodes with a heterojunction launcher cathode. It is shown that previous frequency limits for Gunn diodes are too conservative and that a significant reduction of the dead zone at the cathode is possible with heterojunction designs.

High-speed optical detectors for monolithic millimeter wave integrated circuits

Metal-semiconductor-metal photo-diodes with interdigitated Schottky barrier fingers have been developed for applications in monolithic optical receiver circuits with the purpose of detecting millimeter wave modulation signals being transmitted via an optical carrier. The devices are planar and incorporated submicron finger spacings and a thin absorption region for speed with a buried stack of tuned Bragg reflectors for enhanced sensitivity at the carrier wavelength. These devices have been integrated with short-gate MODFET amplifiers to form the complete monolithic integrated optical receiver circuit. The circuits comprise of the active devices and coplanar waveguide matching/filter networks for optimal MMW performance. DC bias networks have also been integrated with the circuits.