R. Menozzi

Breakdown walkout in pseudomorphic HEMT's

In this work we show for the first time evidence of gate-drain breakdown walkout due to hot electrons in pseudomorphic AlGaAs-InGaAs-GaAs HEMTs (PHEMTs). Experiments performed on passivated commercial PHEMTs show that hot electron stress cycles induce a large and permanent increase of the gate-drain breakdown voltage. Three-terminal and two-terminal stress conditions are compared, the former producing a much larger walkout due to hot electrons flowing in the channel. Experimental results indicate that a build-up of negative charge in the region between gate and drain is responsible for the breakdown walkout, due to a local widening of the depletion region and a reduction of the peak electric field.

Alkali-templated surface nanopatterning of chalcogenide thin films: a novel approach toward solar cells with enhanced efficiency.

Concepts of localized contacts and junctions through surface passivation layers are already advantageously applied in Si wafer-based photovoltaic technologies. For Cu(In,Ga)Se2 thin film solar cells, such concepts are generally not applied, especially at the heterojunction, because of the lack of a simple method yielding features with the required size and distribution. Here, we show a novel, innovative surface nanopatterning approach to form homogeneously distributed nanostructures (<30 nm) on the faceted, rough surface of polycrystalline chalcogenide thin films. The method, based on selective dissolution of self-assembled and well-defined alkali condensates in water, opens up new research opportunities toward development of thin film solar cells with enhanced efficiency.

Hot electron degradation of the DC and RF characteristics of AlGaAs/InGaAs/GaAs PHEMT's

This paper reports on hot electron (HE) degradation of 0.25-/spl mu/m Al/sub 0.25/Ga/sub 0.75/As/In/sub 0.2/Ga/sub 0.8/As/GaAs PHEMTs by showing the effects of the hot electron stress on both the dc and rf characteristics. The changes of dc and rf behavior after stress turn out to be strongly correlated. Both can be attributed to a decrease of the threshold voltage yielding different effects on the device gain depending on the bias point chosen for device operation and on the bias circuit adopted: a fixed current bias scheme will minimize the changes induced by the stress. The work also presents a study of the dependence of device degradation on the stress bias condition.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Electric-field-related reliability of AlGaAs/GaAs power HFETs: bias dependence and correlation with breakdown

This work shows experimental and simulated data of hot electron degradation of power AlGaAs/GaAs HFETs with different gate lengths and recess widths, and uses them to infer general indications on the bias and geometry dependence of the device high-field degradation, the meaningfulness of the breakdown voltage figure of merit from a reliability standpoint, and the physical phenomena taking place in the devices during the stress and leading to performance degradation. Possible formulations of a voltage-acceleration law for lifetime extrapolation are also tested.

Small-signal modeling for microwave FET linear circuits based on a genetic algorithm

In this paper we present a new method to efficiently optimize small-signal equivalent circuits for microwave and millimeter-wave FET linear circuit design. The method couples the stochastic search of a Partially Elitistic Genetic Algorithm with a local search procedure. Up to 19 equivalent circuit elements have been included in the small-signal model for completeness and flexibility. Optimization examples are given for an ion-implanted MESFET up to 12 GHz, a pseudomorphic HEMT up to 50 GHz, and for synthetic data. The results show that the proposed algorithm is able to consistently provide an excellent fit between measured and calculated S-parameters without any need of a careful initial guess for the circuit element values. Also, once the device parasitics have been de-embedded, the algorithm is able to extract unique, physically meaningful values for the intrinsic device parameters, and it is numerically shown not to be affected by measurement uncertainties.

Three-dimensional finite-element thermal simulation of GaN-based HEMTs.

The aim of this paper is to show and discuss results of three-dimensional finite-element thermal simulation of GaN HEMT structures. HEMTs differing by geometry, substrate material, passivation, and cooling strategy are simulated and compared in order to give a picture of the complex interplay of factors that must be reckoned with for proper thermal modeling and management.

Stepping toward standard methods of small-signal parameter extraction for HBTs

An improved HBT small-signal parameter extraction procedure is presented in which all the equivalent circuit elements are extracted analytically without reference to numerical optimization. Approximations required for simplified formulae used in the extraction routine are revised, and it is shown that the present method has a wide range of applicability, which makes it appropriate for GaAs and InP-based single and double HBTs. Additionally, a new method is developed to extract the total delay time of HBTs at low frequencies, without the need to measure h/sub 21/ at very high frequencies and/or extrapolate it with -20 dB/dec roll-off. The existing methods of finding the forward transit time are also modified to improve the accuracy of this parameter and its components. The present technique of parameter extraction and delay time analysis is applied to an InGaP/GaAs DHBT and it is shown that: (1) variations of all the extracted parameters are physically justifiable; (2) the agreement between the measured and simulated S- and Z-parameters in the entire range of frequency is excellent; and (3) an optimization step following the analytical extraction procedure is not necessary. Therefore, we believe that the present technique can be used as a standard extraction routine applicable to various types of HBTs.

Enhancement and degradation of drain current in pseudomorphic AlGaAs/InGaAs HEMTs induced by hot-electrons

New failure mechanisms induced by hot-electrons in AlGaAs/InGaAs pseudomorphic HEMTs have been identified by means of accelerated testing of commercial devices from four different suppliers. Different degradation modes have been observed, depending on the device type, namely: (a) recoverable increase of I/sub D/ and |V/sub T/|, which has been attributed to recombination of electrons trapped under the gate with holes generated by impact ionization; (b) enhancement of the kink in the output characteristics, possibly due to the generation of deep levels with subsequent electron trapping/detrapping; (c) permanent increase of the breakdown voltage, due to creation of negatively charged traps in the gate-drain region, yielding a wider space-charge region, hence a reduced maximum electric field. The link between the observed degradation modes and the underlying physical mechanisms is investigated by means of different techniques, and the main functional effects of the degradation modes are addressed.

Off-state breakdown of GaAs PHEMTs: review and new data

This paper reviews the literature dealing with off-state gate-drain breakdown in MESFET and HEMT structures, with particular emphasis on GaAs PHEMTs, in terms of: 1) the physics of the breakdown phenomenon; 2) the breakdown walkout effect; 3) the impact of design and process choices on the breakdown behavior; and 4) the experimental techniques used for breakdown characterization. A thorough temperature-dependent breakdown characterization of commercial PHEMTs is also shown and discussed. It is found that different physical mechanisms may dominate the gate-drain leakage depending on the reverse bias and temperature range considered, and the particular PHEMT technology. The main results shown here tell us the following. 1) The breakdown voltages are decreasing functions of temperature between room temperature and 160/spl deg/C. 2) Between room temperature and 90-100/spl deg/C, thermionic-field emission seems be dominant, with low activation energies below 0.15 eV; as a consequence, the temperature dependence of the breakdown voltage is weak. 3) Between 110/spl deg/C and 160/spl deg/C, higher activation energy mechanisms (possibly trap-assisted tunneling and thermionic emission over a field-dependent barrier) tend to dominate, and the temperature dependence of the breakdown voltages is stronger.