Thermionic emission | EPFL Graph Search

Thermionic emission (also known as thermal electron emission or the Edison effect) is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission. The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum in a vacuum tube. The hot cathode can be a metal filament, a coated me

Transient Current Technique characterization of bonded interfaces for monolithic silicon radiation detectors

Jacopo Bronuzzi

This thesis aims at demonstrating a novel technique for the characterization of interfaces obtained by a CMOS-compatible Surface Activated Bonding (SAB) process between silicon wafers. This enables the optimization of the two main components of monolithic silicon detectors, the CMOS circuitry for the read-out and the sensing layer, by fabricating them in different substrates and then by bonding them together. Therefore, to be collected by the read-out circuitry, charges generated by radiation in the bulk have to traverse the bonding interface, whose electrical properties need to be characterized. The first part of this thesis is focused on the evaluation of Transient Current Technique (TCT) for this purpose. TCT is largely used for the study of radiation damage in silicon detectors, and consists in the injection of a localized cloud of electrons inside a detector based on a reverse-biased diode, that is drifted by the electric field. A transient current signal is generated, whose shape is related to the electric field profile that may be affected by lattice defects generated by radiation. In this context, the bonding process is expected to generate a thin amorphous silicon interface between the two bonded substrates. This layer can be seen as full of defects and therefore it is expected to influence the electric field, and the TCT current signal. This is demonstrated by means of Sentaurus TCAD numerical simulations and an analytical model, using a diode with the bonding interface in the middle of the bulk as test structure. The second part of the thesis describes the characterization of the interface, generated by bonding high resistivity wafers at CEA-Leti in Grenoble. For this purpose, Schottky diodes are fabricated on these stacks at EPFL, and then characterized with CV, IV and TCT techniques. The results obtained are compared to simulation data, to show that the electric field did not extend to the bulk, preventing charges to be collected. This is an issue for the fabrication of radiation detectors, since there would not be collection of charges generated at the sensing bulk. Following these conclusions, two solutions are proposed. First, the optimization of the bonding process to reduce the number of traps. Second, a modification of the detector design in such a way that the bonding interface is located at the PN junction, since the electric field is maximum at this position, and therefore the influence of traps is less important. The last part of this thesis is devoted to the description of a new charge injection technique for TCT measurements. Instead of using a laser, charges are injected by means of nanosecond voltage pulses, applied to dedicated wells fabricated on the PN junction contact. Injection occurs by thermionic emission, while the charges drift, as in standard TCT measurements. This novel method of charge injection is called electrical injection TCT (el-TCT). It could allow to perform on-line TCT measurements during experiments.

EPFL2018

III-V Nanowire Hetero-junction Tunnel FETs integrated on Si

Davide Cutaia

In the last decade the power consumption of electronic devices has increased for both static and active components. Following the Dennard's scaling rule, as long as the transistor sizes are reduced then the supply voltage (VDD) can also be scaled in order to increase the area density and maintain or improve the performances. However, scaling VDD and thus shifting the threshold voltage (Vth) of 60mV in a metal-oxide semiconductor field-effect transistor (MOSFET) increases the OFF state current (Ioff ) by a factor of 10 in the ideal case. The reason for this is that the inverse subthreshold slope (SS) in an ideal MOSFET is limited by the thermionic emission of electrons over a potential barrier, which has an intrinsic physical limit of 60mV/decade. To overcome this limit, novel device concepts have been proposed and the tunnel field-effect transistor (TFET) is one of the most promising because it resembles a MOSFET and should enable to achieve sub-60mV/dec SS, low Ioff and small VDD. The aim of this thesis is to investigate the potential of TFETs by the fabrication and characterization of InAs/Si as p-channel and InAs/GaSb as n-channel device for a complementary TFET technology. III-V nanowires are grown via metal-organic chemical vapor deposition (MOCVD) and are integrated on Si(100) substrates using a novel technique called template-assisted selective epitaxy (TASE), which enables the fabrication of vertical and lateral III-V hetero-structure TFETs. Sub-40nm nanowire cross-section InAs/Si p-TFETs and InAs/GaSb n-TFETs in-plane on Si are demonstrated for the first time. The InAs/Si p-TFETs exhibit state-of-the art performances with an ON state current (Ion) of 4uA/um at VGS=VDS=-0.5V, ON/OFF ratio of 1x10E5 with SSavg of 70-80mV/dec at room temperature. The InAs/GaSb n-TFETs have an order of magnitude larger Ion but the SS is limited by the non-optimized gate-stack on InAs channel and by the depletion of the undoped GaSb source. Different operation regimes of TFETs are investigated by temperature-dependent measurements,Wentzel-Kramers-Brillouin (WKB) modeling and TCAD simulations, indicating that the switching region for InAs/Si is dominated by the presence of traps at the hetero-junction. The abruptness of the band-edges in InAs/GaSb is studied by the extraction of the conductance slope in fabricated p-n and p-i-n tunnel diodes.

EPFL2016

Thiazole Boron Difluoride Dyes with Large Stokes Shift, Solid State Emission and Room-Temperature Phosphorescence

Shuo Tong, Wei Wang, Jieping Zhu

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Delta lambda, 77-101 nm) with quantum yields (phi(FL)) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (phi(PL)) and lifetime (tau(p) up to 251 mu s). The presence of intramolecular H-bonds and negligible pi-pi stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, pi-pi and C-H center dot center dot center dot pi interactions could be responsible for the observed RTP of iodine containing phosphors.