lunes, 28 de junio de 2010

Breakdown voltage

The breakdown voltage of an Insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.
The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse. Some devices (such as TRIACs) also have a forward breakdown voltage.

High voltage dielectric breakdown within a block of plexiglas

In Detail

Breakdown voltage is a characteristic of an insulator that defines the maximum voltage difference that can be applied across the material before the insulator collapses and conducts. In solid insulating materials, this usually creates a weakened path within the material by creating permanent molecular or physical changes by the sudden current. Within rarefied gases found in certain types of lamps, breakdown voltage is also sometimes called the "striking voltage".[2]
The breakdown voltage of a material is not a definite value because it is a form of failure and there is a statistical probability whether the material will fail at a given voltage. When a value is given it is usually the mean breakdown voltage of a large sample. Another term is also 'withstand voltage' where the probability of failure at a given voltage is so low it is considered, when designing insulation, that the material will not fail at this voltage.[3]
Two different breakdown voltage measurements of a material are the AC and impulse breakdown voltages. The AC voltage is the line frequency of the mains (either 50 or 60 Hz depending on where you live). The impulse breakdown voltage is simulating lightning strikes, and usually uses a 1.2 microsecond rise for the wave to reach 90% amplitude then drops back down to 50% amplitude after 50 microseconds.[4]
Two technical standards governing performing these tests are ASTM D1816 and ASTM D3300 published by ASTM.

Breakdown in vacuum
In standard conditions at atmospheric pressure, gas serves as an excellent insulator, requiring the application of a significant voltage before breaking down (e.g. lightning). In partial vacuum, this breakdown potential may decrease to an extent that two uninsulated surfaces with different potentials might induce the electrical breakdown of the surrounding gas. This has some useful applications in industry (e.g. the production of microprocessors) but in other situations may damage an apparatus, as breakdown is analogous to a short circuit.

The breakdown voltage in a partial vacuum is represented as

where Vb is the breakdown potential in volts DC, A and B are constants that depend on the surrounding gas, p represents the pressure of the surrounding gas, d represents the distance in centimetres between the electrodes, and γse represents the Secondary Electron Emission Coefficient.

Breakdown voltage is a parameter of a diode that defines the largest reverse voltage that can be applied without causing an exponential increase in the current in the diode. As long as the current is limited, exceeding the breakdown voltage of a diode does no harm to the diode. In fact, Zener diodes are essentially just heavily doped normal diodes that exploit the breakdown voltage of a diode to provide regulation of voltage levels.

iode I-V diagram
Avalanche Breakdown
Avalanche Breakdown in Germanium
It is shown that all germanium junctions studied break down as the result of the same avalanche process found in silicon. An empirical expression for the multiplication inherent in this breakdown process is given for step junctions. Ionization rates for holes and electrons in Ge are derived with the use of this expression. The ionization rate for holes is larger than that for electrons by about a factor of two. The agreement between these ionization rates as a function of field and the theory of Wolff is excellent. It is determined that the threshold for electron-hole pair production is about 1.50 ev and the mean free path for electron (or hole)-phonon collisions is about 130 A.

Low-voltage organic transistors with an amorphous molecular gate dielectric
Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays1, 2, 3, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20 V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides2, 3, 4, 5, 6, 7, 8, or insulating polymers9, and are often thicker than 100 nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2 V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation.

A new drain-current injection technique for the measurement of off-state breakdown voltage in FET's
We present a new simple three-terminal technique to measure the off-state breakdown voltage of FET's. With the source grounded, current is injected into the drain of the on-state device. The gate is then ramped down to shut the device off. In this process, the drain-source voltage rises to a peak and then drops. This peak represents an unambiguous definition of three-terminal breakdown voltage. In the same scan, we additionally obtain a measurement of the two-terminal gate-drain breakdown voltage. The proposed method offers potential for use in a manufacturing environment, as it is fully automatable. It also enables easy measurement of breakdown voltage in unstable and fragile devices

Lenny Z Perez M

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