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Metal-oxide-semiconductor structure[ edit ] The traditional metal-oxide-semiconductor MOS structure is obtained by growing a layer of silicon dioxide SiO 2 on top of a silicon substrate and depositing a layer of metal or polycrystalline silicon the latter is commonly used.
As the silicon dioxide is a dielectric material, its structure is equivalent to a planar capacitorwith one of the electrodes replaced by a semiconductor. When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor.
Conventionally, the gate voltage at which the volume density of electrons in the inversion layer is the same as the volume density of holes in the body is called the threshold voltage. When the voltage between transistor gate and source VGS exceeds the threshold voltage Vththe difference is known as overdrive voltage.
This structure with p-type body is the basis of the n-type MOSFET, which requires the addition of n-type source and drain regions. Let's consider a MOS capacitor where the silicon base is of p-type.
If a positive voltage is applied at the gate, holes which are at the surface of the p-type substrate will be repelled by the electric field generated by the voltage applied.
At first, the holes will simply be repelled and what will remain on the surface will be immobile neutral atoms of the donor type, which creates a depletion region on the surface.
Remember that a hole is simply an acceptor atom, i. Boron, which has one less electron than Silicon and therefore contributes an overall positive charge.
What actually happens is that because Boron has one less electron in its outer shell than Silicon, it will in fact attract one electron from a nearby Silicon atom, leaving that Silicon atom with a positive charge, and that is actually how the hole generation happens in this case. When the voltage is applied then, these holes will be repelled by the electric field, leaving immobile and neutral donor atoms on its places, i.
One might ask how can holes be repelled if they are actually non entities in themselves? The answer is that what really happens is not that a hole is repelled, but electrons are attracted by the positive field, and fill these holes, neutralizing the atom and creating a depletion region, where no charge carriers exist.
As the voltage at the gate increases, there will be a point when the depletion region disappears, and the surface will be converted from p-type into n-type, as electrons from the bulk area will start to get attracted by the larger electric field.
This converts the surface from p-type into n-type, and this is known as inversion. The voltage at which this conversion happens is known as the threshold voltage and is one of the key and most important parameters in a MOSFET. In the case of a p-type bulk, Inversion happens when the intrinsic energy level at the surface becomes smaller than the Fermi level at the surface.
One can see this from a band diagram.
Remember that the Fermi level defines the type of semiconductor in discussion. If the Fermi level is equal to the Intrinsic level, the semiconductor is of intrinsic, or pure type.
If the Fermi level lies closer to the conduction band valence band then the semiconductor type will be of n-type p-type.The perfect solution for a non-DAB car radio, the Dension DAB+A gives you an exceptional user experience with a simple and intuitive display of the DAB station’s information and menu on your RDS capable FM-radio.
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As a follow-up to Tuesday’s post about the majority-minority public schools in Oslo, the following brief account reports the latest statistics on the cultural enrichment of schools in Austria. Vienna is the most fully enriched location, and seems to be in roughly the same situation as Oslo. Many thanks to Hermes for the translation from rutadeltambor.com Nano, the Tata’s brainchild has not been able to taste as big a success as the product was capable of.
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In fact, for the last two months it has been having a dismal show at the sales charts despite Tata pushing it tooth and nail. A few days back we brought to you exclusive spyshots of Tata Nano.
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