Olivier Latry

Physical analysis of Schottky contact on power AlGaN/GaN HEMT after pulsed-RF life test.

This paper deals with the physical study of the Schottky contact after pulsed-RF saturated life test under enhanced drain bias voltage on power HEMTs. Electrical measurements showed a pinch-off voltage (VP) shift, a decrease of output power and average drain current while Photon Emission Microscopy (PEM) was used to identify the degradation distribution along the 80 fingers die. Finally, Transmission Electron Microscopy (TEM) is performed to point out the different Schottky degradation between a central finger and an outer one.

Compared deep class-AB and class-B ageing on AlGaN/GaN HEMT in S-Band pulsed-RF operating life

AlGaN/GaN HEMTs are on the way to lead the RF-power amplification field according to their outstanding performances. However, due to its relative youth, reliability studies in several types of operating conditions allow to understand mechanisms peculiar to this technology and responsible for the wearing out of devices. This paper reports the reliability study on two power amplifiers using AlGaN/GaN HEMT. Based on results of a previous study of 1280 h in standard operating conditions wherein no evolution of electrical parameters have been observed, two ageing tests in deep class-AB (432 h) and class-B (795 h) are performed under pulsed-RF operating life at high drain bias voltages and saturated operation. This study shows a drift in RF performances which is linked with the evolution of electrical parameters (RDSON, gm and VP). Similar kinetics and amplitude of degradations are observed revealing quasi-similar contribution of thermal effects in both cases. Degradations are supposed to be related to trapped charges phenomena induced by high voltage operating conditions. Although, several results attest to this hypothesis, a part of the evolutions seems to be linked with structural changes.

A 5000 h RF life test on 330 W RF-LDMOS transistors for radars applications

A reliability test bench dedicated to RF power devices is used to improve 330 W LDMOS in a radar conditions. The monitoring of RF power, drain, gate voltages and currents under various pulses and temperatures conditions are investigated. Numerous duty cycles are applied in order to stress LDMOS. It shows with tracking all this parameters that only few hot carrier injection phenomenon appear with no incidence on RF figures of merit (Pout or PAE). Robustness and ruggedness are shown for LDMOS with this bench for radar applications in L-band.

Achieving of an optical very high frequency modulated wave source using heterodyne technique

We describe a method for generating an amplitude modulated optical wave in the range of the telecommunication frequencies. The originality is to guaranty a tunability of the source up to 275 GHz with about 45 MHz spectral width. The principle is based on the optical heterodyne of two DFB laser sources emitting around 1550 nm. After describing various existing methods for optical wave modulation and for millimetric signal generation, we concentrate our interest on the heterodyne technique. We carry out a study concerning the spectral width influence on the spectral purity of the generated signal. Then, the experimental measurement set-up is described, as well as characterization of the spectral width of the lasers. Finally, we present the results obtained in reception of the beat of two identical lasers on a 25 GHz bandwidth photodiode.

Characterization and modeling of hot carrier injection in LDMOS for L-band radar application

Abstract This paper reports a methodology to correlate Hot Carrier Injection (HCI) degradation mechanism and electrical figures of merit on Lateral-Diffused Metal–Oxide-Semiconductor (LDMOS) transistor. This method is based on RF life test in radar operating conditions coupled to a high drain voltage in order to make visible HCI degradation. We propose drain current modeling vs. time based on a simple extraction procedure. The electron density trapped in the oxide is extracted from hot carrier induced series resistance enhancement model (HISREM – i.e. Δ R d model). From this methodology, the degradation of RF-LDMOS due to HCI is quantified and could be simulated with EDA.

Fiber-based Mach-Zehnder interferometric structures: principles and required characteristics for efficient modulation format conversion

Since new modulation schemes have become a great center of interest, Mach-Zehnder fiber fused interferometers (MZI) became necessary to ensure the direct detection (DD) from phase modulated signals, like DPSK (Differential Phase Shift Keying) or DQPSK. As these modulation schemes are very interesting in optical transmissions due to their capabilities, MZIs cannot be overlooked in the future optical links. In this paper, we review the base principles for using MZI as DPSK demodulators, and the major characteristics associated to these devices. In order to evaluate the minimal characteristics required for efficient modulation format conversion, we propose some numerical simulation results. Based on these results, we depicted a new fabrication process for the realization of MZIs by adiabatic stretching of one of the interferometers arm. This is carried out by a CO2 laser, and the fabrication major points that must be respected are discussed. After review the MZI thermal dependency mechanism, the last part presents the experimental results obtained for the quantification of MZI temperature control accuracy needed for its utilization as an 43 Gbit/s PSBT (Phase Shaped Binary Transmission) encoder.

CO2-laser-beam-based technique for producing optical fiber components

A simple and handy technique to produce optical fiber components such as Bragg gratings has been used. A CO2 laser beam, focused on a single mode fiber, can change its optical properties locally, allowing therefore the creation of a Bragg grating inside the fiber itself. In the present study we investigate the effect of this laser beam on the fiber and show how optical fiber components can be produced. When exposing a single-mode optical fiber to a high-power CO2 laser beam a small device is developed insdie this fiber. Inside this micro-structure, the optical power is exchanged between the core and the cladding modes. This device may exhibit either a very selective rejection behavior similar to Bragg grating or an oscillating behavior like tapered fiber. The coupled mode theory combined with a mathmatical algorithm can be used to study the propagation of the modes involved in the fiber.

Characterization of ESD stress effects on SiC MOSFETs using photon emission spectral signatures

This article presents a robustness study of SiC power MOSFETs to Electrostatic Discharge (ESD), by photon emission (PE) and spectral PE techniques (SPE). Investigations in different polarization modes are performed in photoemission PE, and a linear dependence in gate voltage has been identified. A decrease in the intensity of the ESD degraded device emissions has been noticed. A SPE system has been developed, and PE spectrum has been extracted for both fresh and degraded devices. SPE analyses are reported and correlated with electrical analyses to localize and identify the failure. The ESD degradation is seems to be related to a pn junction degradation. C-V analyses are conducted to prove this hypothesis.

Leakage current effects on N-MOSFETs after thermal ageing in pulsed life tests

This paper presents a synthesis of leakage current effects on N-MOSFET performances, after thermal ageing in pulsed life tests. A 3000h pulsed RF life test has been conducted on a dedicated RF S-band bench in operating modes. It is interesting to understand the degradation mechanism effects caused by the increase leakage current and in turn on drifts of critical parameters. It shows with tracking of a set of RF parameters (Pout, Gain and Drain Efficiency: DE) that only Hot Carrier Injection (HCI) phenomenon appears with incidence on RF. It is the main cause for device degradation leading to the interface state generation (traps), which results in a build up of negative charge at Si/SiO2 interface. The physical processes responsible for the observed degradation at different stress conditions are studied by means of 2D ATLAS-SILVACO simulations to locate and confirm these phenomena.

Generation of an amplitude modulated optical wave up to 275 GHz by laser wave mixing for optical component bandwidth measurement

This paper presents a method to achieve an amplitude-modulated optical wave in the range of telecommunication frequencies for optical components bandwidth tests. The principle is based on the optical heterodyning of two free distributed feedback lasers. The frequency tunabil- ity of the modulated wave reaches 275 GHz with high stability. These characteristics constitute the originality of our work. Our laboratory has developed an experimental fusion setup that enables the realization of numerous optical fiber components. Component bandwidth measurement enables their validation for high-rate data transmissions. We present in this paper results obtained for a 57-GHz beat signal in a fast photodiode and we describe bandwidth optical components tests in the 0 to 40 GHz range.