High performance AlGaN/GaN high electron mobility transistors (HEMTs) with 0.12 /spl mu/m gate-length have been fabricated on an insulating SiC substrate. The devices exhibited an extrinsic transconductance of 217 mS/mm and current drive capability as high as 1.19 A/mm. The threshold voltage of the devices was -5.5 V. For AlGaN/GaN HEMTs with the same gate-length, a record high unity current gain cut-off frequency (f/sub T/) of 101 GHz and a maximum oscillation frequency (f/sub max/) of 155 GHz were measured at V/sub ds/=16.5 V and V/sub gs/=5.0 V. The microwave noise performances of the devices were characterized from 2 to 18 GHz at different drain biases and drain currents. At a drain bias of 10 V and a gate bias of -4.8 V, the devices exhibited a minimum noise figure (NF/sub min/) of 0.53 dB and an associated gain (G/sub a/) of 12.1 dB at 8 GHz. Also, at a fixed drain bias of 10 V with the drain current swept, the lowest NF/sub min/ of 0.72 dB at 12 GHz was obtained at I/sub ds/=100 mA/mm and a peak G, of 10.59 dB at 12 GHz was obtained at I/sub ds/=150 mA/mm. With the drain current held at 114 mA/mm and drain bias swept, the lowest NF/sub min/ of 0.42 dB and 0.77 dB were obtained at V/sub ds/=8 V at 8 GHz and 12 GHz, respectively. To our knowledge, these are the best microwave noise performances of any GaN-based FETs ever reported.

AlGaN/GaN HEMTs on SiC with over 100 GHz f/sub T/ and low microwave noise

We propose an original design for a neuron-inspired photonic computational primitive for a large-scale, ultrafast cognitive computing platform. The laser exhibits excitability and behaves analogously to a leaky integrate-and-fire (LIF) neuron. This model is both fast and scalable, operating up to a billion times faster than a biological equivalent and is realizable in a compact, vertical-cavity surface-emitting laser (VCSEL). We show that-under a certain set of conditions-the rate equations governing a laser with an embedded saturable absorber reduces to the behavior of LIF neurons. We simulate the laser using realistic rate equations governing a VCSEL cavity, and show behavior representative of cortical spiking algorithms simulated in small circuits of excitable lasers. Pairing this technology with ultrafast, neural learning algorithms would open up a new domain of processing.

A Leaky Integrate-and-Fire Laser Neuron for Ultrafast Cognitive Computing

We have analyzed the effect of the photon energy and temperature dependence of both the gain and the total losses inside the cavity to understand the polarization behavior of vertical-cavity surface-emitting lasers. The assumption that the losses are dominated by free-carrier absorption in the p-doped mirror is made. Developing a new theoretical approach, we are able to predict different polarization switching regimes in which switching occurs from the high- to the low-frequency mode, from the low- to the high-frequency mode, or both consecutively. All these predictions have been experimentally verified by our measurements on GaAs/AlGaAs proton-implanted vertical-cavity surface-emitting lasers.

Effect of photon-energy-dependent loss and gain mechanisms on polarization switching in vertical-cavity surface-emitting lasers

The effect of mode-profile specific etching of the top layer in selectively oxidized vertical-cavity surface-emitting laser (VCSEL) structures at 850-nm emission wavelength is examined. For high reproducibility, a self-aligned etching technique is demonstrated which aligns surface etch and oxide aperture by only one additional photoresist step. By optimizing layer structure and etch spot size, completely single-mode devices with aperture diameters up to 16 /spl mu/m are obtained. Maximum single-fundamental-mode output power of 3.4 mW at room temperature and over 1 mW at 0/spl deg/C is obtained with a maximum far-field angle of 5.5/spl deg/. Using parameters for etch spot height and diameter, Gaussian beam spot size and phase curvature, the measured diffracted far-field distribution is fitted well over a 20-dB intensity range. The chosen fit parameters therefore enable one to estimate the amount of phase curvature within the VCSEL for different operation currents, which cannot be obtained with available measurement methods.

Large-area single-mode VCSELs and the self-aligned surface relief

A vertical cavity apparatus includes first and second mirrors, a substrate, first and second active regions and a plurality of individual active regions. Each of an individual active region is positioned between the first and second active regions. A first oxide layer is positioned between the first mirror and the second mirror. A plurality of tunnel junctions are positioned between the first and second mirrors.

Vertical cavity apparatus with tunnel junction

Single-mode operation from an index-guided vertical-cavity surface-emitting laser (VCSEL) has been achieved by using a fiber Bragg grating (FBG) as an external selective wavelength reflector. The spectral characteristics of the external cavity controlled VCSEL under static and dynamic conditions have been studied. The FBG provided stable single mode operation under pseudorandom modulation with a sidemode suppression ratio of /spl sim/30 dB from 500 Mb/s up to 4 Gb/s at room temperature.

Single-mode operation from an external cavity controlled vertical-cavity surface-emitting laser

Differential carrier lifetime measurements were performed on index-guided oxide-confined vertical cavity surface emitting lasers operating at 980 nm. Lifetimes were extracted from laser impedance measurements at subthreshold currents, with device size as a parameter, using a simple small-signal model. The carrier lifetimes ranged from 21 ns at 9 μA, to about 1 ns at a bias close to threshold. For a 6×6 μm2 oxide aperture device the threshold carrier density was nth∼2×1018 cm−3. The effect of carrier diffusion was also considered. An ambipolar diffusion coefficient of D∼11 cm2 s−1 was obtained.

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Differential carrier lifetime in oxide-confined vertical cavity lasers obtained from electrical impedance measurements

Measurements of differential carrier lifetimes on gain-guided proton-implanted vertical-cavity surface-emitting lasers with device size as a parameter are reported. The lifetimes were obtained from laser impedance measurements and from small-signal modulation optical response at subthreshold currents. A simple small-signal equivalent circuit was used to correct the optical data and to extract the carrier lifetimes from the impedance data. Carrier lifetimes ranged from 4.2 ns at 0.04 mA, to about 0.6 ns at a bias close to threshold. The measured carrier lifetimes were used to calculate the corresponding threshold carrier density (n/sub th//spl sim/6/spl times/10/sup 18/ cm/sup -3/) and recombination parameters.

Measurement of differential carrier lifetime in vertical-cavity surface-emitting lasers

The authors report low-frequency noise measurements of doped-channel AlGaN-GaN heterostructure field-effect transistors grown on insulating silicon carbide substrates. In the frequency range 1 Hz-100 kHz the observed noise is of the 1/f type, and the estimated value of the Hooge constant /spl alpha//sub H/ is /spl sim/10/sup -3/.

Low-frequency noise in AlGaN-GaN doped-channel heterostructure field effect transistors grown on insulating SiC substrates

Small-signal impedance characteristics of quantum-well laser structures are analyzed. A simple analytical expression for the frequency dependence of below-threshold small-signal impedance is derived and verified experimentally. It is shown that the differential carrier lifetime and quantum-well transport and capture times can be extracted from electrical impedance measurements.

G.E. Giudice

Papers

Single-mode operation from an external cavity controlled vertical-cavity surface-emitting laser

Differential carrier lifetime in oxide-confined vertical cavity lasers obtained from electrical impedance measurements

Measurement of differential carrier lifetime in vertical-cavity surface-emitting lasers

Low-frequency noise in AlGaN-GaN doped-channel heterostructure field effect transistors grown on insulating SiC substrates

Small-signal impedance characteristics of quantum-well laser structures