G. Verzellesi

Surface-related drain current dispersion effects in AlGaN-GaN HEMTs

Drain current dispersion effects are investigated in AlGaN-GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects characterized by time constants in the order of 10/sup -5/-10/sup -4/ s cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge.

Efficiency droop in InGaN/GaN blue light-emitting diodes: Physical mechanisms and remedies

Physical mechanisms causing the efficiency droop in InGaN/GaN blue light-emitting diodes and remedies proposed for droop mitigation are classified and reviewed. Droop mechanisms taken into consideration are Auger recombination, reduced active volume effects, carrier delocalization, and carrier leakage. The latter can in turn be promoted by polarization charges, inefficient hole injection, asymmetry between electron and hole densities and transport properties, lateral current crowding, quantum-well overfly by ballistic electrons, defect-related tunneling, and saturation of radiative recombination. Reviewed droop remedies include increasing the thickness or number of the quantum wells, improving the lateral current uniformity, engineering the quantum barriers (including multi-layer and graded quantum barriers), using insertion or injection layers, engineering the electron-blocking layer (EBL) (including InAlN, graded, polarization-doped, and superlattice EBL), exploiting reversed polarization (by either inv...

Current Collapse and High-Electric-Field Reliability of Unpassivated GaN/AlGaN/GaN HEMTs

Long-term ON-state and OFF-state high-electric-field stress results are presented for unpassivated GaN/AlGaN/GaN high-electron-mobility transistors on SiC substrates. Because of the thin GaN cap layer, devices show minimal current-collapse effects prior to high-electric-field stress, despite the fact that they are not passivated. This comes at the price of a relatively high gate-leakage current. Under the assumption that donor-like electron traps are present within the GaN cap, two-dimensional numerical device simulations provide an explanation for the influence of the GaN cap layer on current collapse and for the correlation between the latter and the gate-leakage current. Both ON-state and OFF-state stresses produce simultaneous current-collapse increase and gate-leakage-current decrease, which can be interpreted to be the result of gate-drain surface degradation and reduced gate electron injection. This study shows that although the thin GaN cap layer is effective in suppressing surface-related dispersion effects in virgin devices, it does not, per se, protect the device from high-electric-field degradation, and it should, to this aim, be adopted in conjunction with other technological solutions like surface passivation, prepassivation surface treatments, and/or field-plate gate

Investigation of High-Electric-Field Degradation Effects in AlGaN/GaN HEMTs

High-electric-field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate-source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters.

Impact of programming charge distribution on threshold voltage and subthreshold slope of NROM memory cells

The aim of this paper is to achieve a correct description of the programming charge distribution in NROM memory devices. This is essential to prove device functionality and to extrapolate scaling limits of devices. For this purpose we employ an inverse modeling based methodology using measurements easily performed, such as subthreshold characteristics and threshold voltage measurements. We show a simple model of programming charge distribution that can be easily implemented in two-dimensional (2-D) TCAD simulations. Results show good agreement between measured and simulated currents under different bias conditions and for different programming levels.

Mechanisms of RF Current Collapse in AlGaN–GaN High Electron Mobility Transistors

The physical mechanisms underlying RF current- collapse effects in AlGaN-GaN high-electron-mobility transistors are investigated by means of measurements and numerical device simulations. This paper suggests the following conditions: 1) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; 2) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; 3) for surface-trap densities lower than 9 x 1012 cm-2, surface-potential barriers in the 1-2-eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants.

Influence of Buffer Carbon Doping on Pulse and AC Behavior of Insulated-Gate Field-Plated Power AlGaN/GaN HEMTs

Pulse behavior of insulated-gate double-field-plate power AlGaN/GaN HEMTs with C-doped buffers showing small current-collapse effects and dynamic RDS,on increase can accurately be reproduced by numerical device simulations that assume the CN-CGa autocompensation model as carbon doping mechanism. Current-collapse effects much larger than experimentally observed are instead predicted by simulations if C doping is accounted by dominant acceptor states. This suggests that buffer growth conditions favoring CN-CGa autocompensation can allow for the fabrication of power AlGaN/GaN HEMTs with reduced current-collapse effects. The drain-source capacitance of these devices is found to be a sensitive function of the C doping model, suggesting that its monitoring can be adopted as a fast technique to assess buffer compensation properties.

A review of failure modes and mechanisms of GaN-based HEMTs

Failure modes and mechanisms of AlGaN/GaN HEMTs, observed during accelerated tests at various bias conditions are reviewed.

Study of breakdown effects in silicon multi-guard structures

Many applications of silicon p/sup +/-n junctions as detectors require high voltage operation. In particular the LHC experiments, because of the radiation damage level, need very high bias working voltage to fully collect the ionising generated charge multi-guard structures can be used in order to improve the breakdown voltage of microstrip detectors, limiting the occurrence of critical fields in the proximities of a reverse biased p/sup +/-n junction. In this work we present results for different designs of multi-guard structures, before and after irradiation with ionising and non-ionising radiation sources (p,n,/spl gamma/), and for different doses. Various experimental techniques have been used, like DC and AC electrical characterizations, and light emission microscopy. Moreover, a simulation work is presented. Its purpose is to improve the design on the basis of the experimental results.

Extension of impact-ionization multiplication coefficient measurements to high electric fields in advanced Si BJT's

Measurements of the impact-ionization multiplication coefficient M-1 in advanced Si BJTs up to values in excess of 10 (corresponding to a peak electric field at the base-collector junction of about 9*10/sup 5/ V/cm) are presented. The intrinsic limitations affecting M-1 measurements at high electric fields are discussed. In particular, the fundamental role played by the negative base current and the parasitic base resistance in determining instabilities during M-1 measurements is pointed out. An accurate theoretical prediction of the M-1 coefficient on collector-base voltages close to BV/sub CBO/ requires that the contribution of holes to impact ionization be properly accounted for.<<ETX>>