Trade-offs between gate oxide protection and performance in SiC MOSFETs

The reliability of gate oxides in SiC MOSFETs has come under increased scrutiny due to reduced performance under time dependent dielectric breakdown and increased threshold voltage instability. This paper investigates how 10% gate voltage (V GS ) derating in SiC MOSFETs can be implemented with minimal impact on loss performance. Using experimental measurements and electrothermal simulations of power converters, the trade-off between reduced V GS and conversion loss is investigated. It is shown that 10% V GS de-rating increases the ON-state resistance by 10% and the turn-ON switching energy by 7% average while the turn-OFF switching energy is unaffected. The low temperature sensitivity of the ON-state losses in SiC MOSFETs can be exploited since the rise in junction temperature due to V GS derating is marginal, unlike Si devices where ON-state resistance rises significantly with temperature. The load current and switching frequency influences the effectiveness of V GS derating. It is also shown that reducing the gate drive output impedance can compensate for V GS derating at high switching frequencies, with reduced total loss penalization. This may be important for protecting the gate oxide and enhancing its reliability

Polar domain walls trigger magnetoelectric coupling

Interface physics in oxide heterostructures is pivotal in material's science. Domain walls (DWs) in ferroic systems are examples of naturally occurring interfaces, where order parameter of neighboring domains is modified and emerging properties may develop. Here we show that electric tuning of ferroelastic domain walls in SrTiO3 leads to dramatic changes of the magnetic domain structure of a neighboring magnetic layer (La1/2Sr1/2MnO3) epitaxially clamped on a SrTiO3 substrate. We show that by exploiting the resposiveness of DWs nanoregions to external stimuli, even in absence of any domain contribution, prominent and adjustable macroscopic reactions of neighboring layers can be obtained. We conclude that polar DWs, known to exist in other materials, can be used to trigger tunable responses and may lead to new ways for manipulation of interfacial emerging properties

Sigma-Delta control of charge trapping in heterogeneous devices

Dielectric charging represents a major reliability issue in a variety of semiconductor devices. The accumulation of charge in dielectric layers of a device often alters its performance, affecting its circuital features and even reducing its effective lifetime. Although several contributions have been made in order to mitigate the undesired effects of charge trapping on circuit performance, dielectric charge trapping still remains an open reliability issue in several applications. The research work underlying this Thesis mainly focuses on the design, analysis and experimental validation of control strategies to compensate dielectric charging in heterogeneous devices. These control methods are based on the application of specifically designed voltage waveforms that produce complementary effects on the charge dynamics. Using sigma-delta loops, these controls allow to set and maintain, within some limits, the net trapped charge in the dielectric to desired levels that can be changed with time. This allows mitigating long-term reliability issues such as capacitance-voltage (C-V) shifts in MOS and MIM capacitors. Additionally, the bit streams generated by the control loops provide real-time information on the evolution of the trapped charge. The proposed controls also allow compensating the effects of the charge trapping due to external disturbances such as radiation. This has been demonstrated experimentally with MOS capacitors subjected to various types of ionizing radiation (X-rays and gamma rays) while a charge control is being applied. This approach opens up the possibility of establishing techniques for active compensation of radiation-induced charge in MOS structures as well as a new strategy for radiation sensing. A modeling strategy to characterize the dynamics of the dielectric charge in MOS capacitors is also presented. The diffusive nature of the charge trapping phenomena allows their behavioral characterization using Diffusive Representation tools. The experiments carried out demonstrate a very good matching between the predictions of the model and the experimental results obtained. The time variations in the charge dynamics due to changes in the volatges applied and/or due to external disturbances have been also investigated and modeled. Moreover, the charge dynamics of MOS capacitors under sigma-delta control is analyzed using the tools of Sliding Mode Controllers for an infinite sampling frequency approximation. A phenomenological analytical model is obtained which allows to predict and analyze the sequence of control signals. This model has been successfully validated with experimental data. Finally, the above control strategies are extended to other devices such as eMIM capacitors and perovskite solar cells. Preliminary results including open loop and closed loop control experiments are presented. These results demonstrate that the application of the controls allows to set and stabilize both the C-V characteristic of an eMIM capacitor and the current-voltage characteristic (J-V) of a perovskite solar cell.La carga atrapada en dieléctricos suele implicar un problema importante de fiabilidad en muchos dispositivos semiconductores. La acumulación de dicha carga, normalmente provocada por las tensiones aplicadas durante el uso del dispositivo, suele alterar el rendimiento de éste con el tiempo, afectar sus prestaciones a nivel de circuital e, incluso, reducir su vida útil. Aunque durante años se han realizado muchos trabajos para mitigar sus efectos no deseados, sobre todo a nivel circuital, la carga atrapada en dieléctricos sigue siendo un problema abierto que frena la aplicabilidad práctica de algunos dispositivos. El trabajo de investigación realizado en esta Tesis se centra principalmente en el diseño, análisis y validación experimental de estrategias de control para compensar la carga atrapada en dieléctricos de diversos tipos de dispositivos, incluyendo condensadores MOS, condensadores MIM fabricados con nanotecnología y dispositivos basados en perovskitas. Los controles propuestos se basan en utilizar formas de onda de tensión, específicamente diseñadas, que producen efectos complementarios en la dinámica de la carga. Mediante el uso de lazos sigma-delta, estos controles permiten establecer y mantener, dentro de unos límites, la carga neta atrapada en el dieléctrico a valores prefijados, que pueden cambiarse con el tiempo. Esto permite mitigar problemas de fiabilidad a largo plazo como por ejemplo las derivas de la curva capacidad-tensión (C-V) en condensadores MOS y MIM. Adicionalmente, las tramas de bits generadas por los lazos de control proporcionan información en tiempo real sobre la evolución de la carga. Los controles propuestos permiten también compensar los efectos de la carga atrapada en dieléctricos debida a perturbaciones externas como la radiación. Esto se ha demostrado experimentalmente con condesadores MOS sometidos a diversos tipos de radiación ionizante (rayos X y gamma) mientras se les aplicaba un control de carga. Este resultado abre la posibilidad tanto de establecer técnicas de compensación activa de carga inducida por radiación en estructuras MOS, como una nueva estrategia de sensado de radiación. Se presenta también una estrategia de modelado para caracterizar la dinámica de la carga dieléctrica en condensadores MOS. La naturaleza difusiva de los fenómenos de captura y eliminación de carga en dieléctricos permite caracterizar dichos fenómenos empleando herramientas de Representación Difusiva. Los experimentos realizados demuestran una muy buena correspondencia entre las predicciones del modelo y los resultados experimentales obtenidos. Se muestra también como las variaciones temporales de los modelos son debidas a cambios en las formas de onda de actuación del dispositivo y/o a perturbaciones externas. Además, la dinámica de carga en condensadores MOS bajo control sigma-delta se analiza utilizando herramientas de control en modo deslizante (SMC), considerando la aproximación de frecuencia de muestreo infinita. Con ello se obtiene un modelo analítico simplificado que permite predecir y analizar con éxito la secuencia de señales de control. Este modelo se ha validado satisfactoriamente con datos experimentales. Finalmente, las estrategias de control anteriores se han extendido a otros dispositivos susceptibles de sufrir efectos de carga atrapada que pueden afectar su fiabilidad. Así, se han llevado a cabo experimentos preliminares cuyos resultados demuestran que la aplicación de controles de carga permite controlar y estabilizar la característica C-V de un condensador eMIM y la característica corriente-tensión (J-V) de una célula solar basada en perovskitas.Postprint (published version