L.J. Mahoney

New MBE buffer used to eliminate backgating in GaAs MESFETs

The buffer is grown by molecular beam epitaxy (MBE) at low substrate temperatures (150-300 degrees C) using Ga and As/sub 4/ beam fluxes. It is highly resistive, optically inactive, and crystalline, and high-quality GaAs active layers can be grown on top of the buffer. MESFETs fabricated in active layers grown on top of this new buffer show improved output resistance and breakdown voltages; the DC and RF characteristics are otherwise comparable to MESFETs fabricated by alternative means and with other buffer layers.<<ETX>>

GaN avalanche photodiodes operating in linear-gain mode and Geiger mode

For solar-blind ultraviolet detection, AlGaN avalanche photodiodes (APDs) that operate in Geiger mode can outperform conventional AlGaN photodiodes in sensitivity and should compare favorably to photomultiplier tubes. Toward this goal, we report GaN APDs that operate in the linear-gain mode and in the Geiger mode. The APDs were fabricated from high-quality GaN epitaxial layers grown by hydride vapor phase epitaxy. The GaN layer structure consisted of a Zn-doped /spl pi/ layer, an unintentionally doped n layer, and a Si-doped n+ layer-all on top of a thick GaN unintentionally doped n layer on a sapphire substrate. Capacitance-voltage (C-V) measurements on photodiodes fabricated from some of these layers show that field strengths between 3 and 4 MV/cm are sustainable in the depletion region at voltages slightly below the observed breakdown of /spl sim/80 V. Both mesa-etched and planar devices exhibited avalanche gains of 10 in linear-gain mode and /spl sim/10/sup 6/ in Geiger mode when top illuminated with a 325 nm HeCd laser. Raster measurements of the photoresponse show highly uniform response in gain mode that becomes slightly more inhomogeneous in Geiger mode.

High-power-density GaAs MISFETs with a low-temperature-grown epitaxial layer as the insulator

A GaAs layer grown by molecular beam epitaxy at 200 degrees C is used as the gate insulator for GaAs MISFETs. The gate reverse breakdown and forward turn-on voltages, are improved substantially by using the high-resistivity GaAs layer between the gate metal and the conducting channel. It is shown that a reverse bias of 42 V or forward bias of 9,3 V is needed to reach a gate current of 1 mA/mm of gate width. A MISFET having a gate of 1.5*600 mu m delivers an output power of 940 mW (1.57-W/mm power density) with 4.4-dB gain and 27.3% power added efficiency at 1.1 GHz. This is the highest power density reported for GaAs-based FETs.<<ETX>>

InGaAsP/InP avalanche photodiodes for photon counting at 1.06 μm

Geiger-mode (photon-counting) operation at 1.06 μm has been demonstrated with InGaAsP/InP avalanche photodiodes operated at room temperature. A photon detection efficiency of 33% was measured on uncoated detectors, representing an internal avalanche probability of 60%. Under identical bias conditions a dark count rate as low as 1.7 MHz was measured at 290 K, consistent with a primary dark current of <0.3 pA. Dark count rates drop by approximately 50–200× by cooling the detectors to 210 K (−63 °C).

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.

Afterpulsing in Geiger-mode avalanche photodiodes for 1.06μm wavelength

We consider the phenomenon of afterpulsing in avalanche photodiodes (APDs) operating in gated and free-running Geiger mode. An operational model of afterpulsing and other noise characteristics of APDs predicts the noise behavior observed in the free-running mode. We also use gated-mode data to investigate possible sources of afterpulsing in these devices. For 30-μm-diam, 1.06-μm-wavelength InGaAsP∕InP APDs operated at 290K and 4V overbias, we obtained a dominant trap lifetime of τd=0.32μs, a trap energy of 0.11eV, and a baseline dark count rate 245kHz.

Three-dimensional metallodielectric photonic crystals exhibiting resonant infrared stop bands

Using standard microelectronic techniques, we have fabricated arrays of infrared metallodielectric photonic crystals (IR MDPCs) on silicon substrates. The metallic “atoms” are located on a three-dimensional (100)-oriented face-centered-cubic lattice. Resonant stop-band characteristics have been measured with rejection levels of up to 20 dB and widths of up to 83% of the center frequency. We demonstrate structures with stop bands across the midinfrared wavelength range from 2 to 12 μm. Angular studies of the photonic stop bands show an insensitivity to incident angle for some of the structures. The IR MDPC results are compared with measurements made on microwave-scale MDPC structures to help in understanding the infrared results.

High-breakdown-voltage MESFET with a low-temperature-grown GaAs passivation layer and overlapping gate structure

GaAs MESFETs were fabricated using a low-temperature-grown (LTG) high-resistivity GaAs layer to passivate the doped channel between the gate and source and between the gate and the drain. The gate was fabricated such that the source and drain edges of the metal gate overlapped the LTG GaAs passivation layer. The electric fields at the edges of the gate were reduced by this special combination of LTG GaAs passivation and gate geometry, resulting in a gate-drain breakdown voltage of 42 V. This value is over 60% higher than that of similar MESFETs fabricated without the gate overlap.<<ETX>>

Ultraviolet photon counting with GaN avalanche photodiodes

Photon counting, utilizing Geiger-mode avalanche response, has been demonstrated at 300 K in avalanche photodiodes fabricated in GaN grown by hydride vapor-phase epitaxy. Measurements have been made using both passive-quench and time-gated modes of operation. The two important figures of merit for photon-counting applications, photon detection efficiency (PDE) and dark count rate, were measured. A maximum PDE of 13% was measured at 325 nm with a dark count rate of 400 kHz. Typical mesa-etched devices exhibit a parasitic shunt leakage current of less than 20 nA at 90% of breakdown voltage.

Reduction of sidegating in GaAs analog and digital circuits using a new buffer layer

Sidegating effects relevant to GaAs digital, analog, and monolithic microwave integrated circuits have been significantly reduced or eliminated by using a low-temperature buffer layer grown by molecular-beam epitaxy. At radio frequencies the low-temperature buffer layer reduced the signal coupling between devices, which is an important consideration in microwave integrated circuits. For digital circuit applications, the low-temperature buffer layer eliminated the dependence of the voltage level of an inverter on the logic state of adjacent devices and on the duty cycle of a pulse train encountered in the circuit. The highly resistive nature of the low-temperature buffer allows experimental identification of the role that a buffer layer plays in sidegating. >