Johan Das

On-chip manipulation and magnetization assessment of magnetic bead ensembles by integrated spin-valve sensors

Manipulation and detection of magnetic beads on a semiconductor chip opens up new perspectives for analysis of magnetically labeled specimens in biomechanical micro-electromechanical systems for biological applications. Sensitive spin-valve sensors were integrated with magnetic field generating conductors to assess the behavior of ensembles of superparamagnetic nanoparticles 300 nm in diameter that contain 75%–80% magnetite. The spin-valve multilayer including a nanooxide layer achieves 8% magnetoresistance (MR) for an integrated device of 2×16 μm2. Motion of the magnetic particles towards and across the sensor is achieved by two tapered magnetic field generating current conductors. The spin-valve sensor detects the stray magnetic field that emanates from the ensemble of magnetic particles. We study the transients in the magnetic signal on the order of 1% MR. These results lead to a model that describes magnetization configurations of the cluster of beads.

Technology assessment for the implementation of magnetoresistive elements with semiconductor components in magnetic random access memory (MRAM) architectures

We describe the DRAM-like approach towards a non-volatile magnetoresistive memory integrating magnetic and semiconductor devices into one cell. The speed at which the magnetic memory signal can be read depends on many factors. An important factor is the magnetic element itself, the size, magnetic characteristics and absolute resistance. Secondly, the design of the read-out electronics is a key issue. A third determining factor is the technology in which the electronics are fabricated. Some features are indicated that are essential in optimizing MRAM in future.

A comprehensive reliability investigation of the voltage-, temperature- and device geometry-dependence of the gate degradation on state-of-the-art GaN-on-Si HEMTs

In this work, the gate degradation of GaN-based HEMTs is analyzed. We find that the gate degradation does not occur only beyond a critical voltage, but it has a strong voltage accelerated kinetics and a weak temperature dependence. By means of a statistical study we show that the time-to-failure can be fitted best with a Weibull distribution. By using the distribution parameters and a power law model it is possible to perform lifetime extrapolation based on the gate degradation at a defined failure level and temperature for the first time. From this elaboration, the lifetime of a given device geometry can also be extracted. Eventually, the strong bias dependence of the gate degradation reported here implies that this phenomenon should be assessed by means of a voltage-based accelerated investigation as described in this work.

Experimental and simulation study of breakdown voltage enhancement of AlGaN/GaN heterostructures by Si substrate removal

The breakdown mechanism in GaN-based heterostructures (HFETs) grown on silicon substrate is investigated in detail by TCAD simulations and silicon substrate removal technique. High-voltage electrical measurements show that the breakdown voltage saturates for larger gate-drain distances. This failure mechanism is dominated by the avalanche breakdown in the Si substrate. High-voltage TCAD simulations of AlGaN/GaN/Si substrate structures show higher impact ionization factor and electron density at the Si interface indicating a leakage current path where avalanche breakdown occurs. Experimentally, by etching off the Si substrate the breakdown voltage no longer saturates and linearly increases for all gate-drain gaps. We propose the silicon removal technique as a viable way to enhance the breakdown voltage of AlGaN/GaN devices grown on Si substrate.

A 96% Efficient High-Frequency DC–DC Converter Using E-Mode GaN DHFETs on Si

III-Nitride materials are very promising to be used in next-generation high-frequency power switching applications. In this letter, we demonstrate the performance of normally off AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs) using a boost-converter circuit. The figures of merit of our large (57.6-mm gate width) GaN transistor are presented: RON * QG of 2.5 Ω·nC is obtained at VDS = 140 V. The switching performance of the GaN DHFET is studied in a dedicated high-frequency boost converter: both the switching times and power losses are characterized. We show converter efficiency values up to 96.1% at 500 kHz and 93.9% at 850 kHz at output power of 100 W.

Low leakage high breakdown e-mode GaN DHFET on Si by selective removal of in-situ grown Si 3 N 4

We describe the fabrication and characteristics of high voltage enhancement mode SiN/AlGaN/GaN/AlGaN double heterostructure FET devices. The Si3N4 not only acts as a passivation layer but is crucial in the device concept as it acts as an electron donating layer (1). By selective removal under the gate of the in-situ SiN, we realize e-mode operation with a very narrow threshold voltage distribution with an average value of +475 mV and a standard deviation of only 15 mV. Compared to the reference depletion mode devices, we see no impact of the e-mode architecture on the breakdown behaviour. The devices maintain very low leakage currents even at drain biases up to 80% of the breakdown voltage.

Si Trench Around Drain (STAD) technology of GaN-DHFETs on Si substrate for boosting power performance

We report on the first measurement results to obtain over 2 kV breakdown voltage (VBD) of GaN-DHFETs on Si substrates by etching a Si Trench Around Drain contacts (STAD). Similar devices without trenches show VBD of only 650 V. DHFETs fabricated with STAD technology show excellent thermal performance confirmed by electrical measurements and finite element thermal simulations. We observe lower buffer leakage at high temperature (100°C) after STAD compared to devices with Si substrate, enabling high temperature device operation.

InAs/(Al,Ga)Sb quantum well structures for magnetic sensors

This paper reports on the fabrication and characterization of Hall and magnetoresistive sensors with high sensitivity and good temperature stability, InAs/AlGaSb quantum well structures grown by molecular beam epitaxy (MBE) on semiinsulating GaAs substrates were used as active layers for magnetic field sensing. The excellent transport properties (electron mobilities up to 30,000 cm/sup 2/Ns at room temperature) resulted in high sensitivity for room temperature operation of magnetoresistors (relative sensitivity of 0.46%/mT) and Hall elements (magnetic sensitivity of 5.5 V/T),.

Limitations of Field Plate Effect Due to the Silicon Substrate in AlGaN/GaN/AlGaN DHFETs

We investigated the limitations of the field plate (FP) effect on breakdown voltage <i>V</i>_BD that is due to the silicon substrate in AlGaN/GaN/AlGaN double heterostructures field-effect transistors. In our previous work, we showed that in devices with large gate-drain distance (L_GD > 8 μm), the breakdown voltage does not linearly increase with <i>L</i>_GD because of a double leakage path between the silicon substrate and the metal contacts, which makes the device break at the silicon interface. In this paper, we showed that the effect of the FP for such large <i>L</i>_GD is not significant because the breakdown is still dominated by the silicon substrate. The increase in <i>V</i>_BD due to the FP is significant only for devices with small gate-drain distances (<i>L</i>_GD <; 8 μm). Indeed we show that for such small <i>L</i>_GD the increase in the breakdown voltage is more than double, whereas for larger <i>L</i>_GD, this is only about 10%. Simulations of AlGaN/GaN/AlGaN devices for small <i>L</i>_GD are carried out with different FP lengths and passivation thickness in order to study the electric field distribution.

Degradation and time dependent breakdown of stressed ferromagnetic tunnel junctions

Ferromagnetic tunnel junctions are very sensitive to degradation and breakdown, due to the ultrathin (∼1 nm) tunnel barrier. When the junction is stressed with a constant current or voltage, a conductance change of the tunnel junction is observed. Sufficiently high stress will lead to breakdown of the junction. As in SiO2 gate oxide reliability studies, the Weibull distribution plot can be obtained from the time to breakdown data. The dependence of the Weibull function on the area and the stress conditions is studied for the Al2O3 barrier of the tunnel junctions. This is the first step of a systematic study of reliability, which is an important issue for the use of tunnel junctions in, e.g., magnetic random access memory applications.