MOS-FET ”depletion mode”

[sesebe]

Intrebare:

 

1) Un MOS-FET de genul celui din titlu se poate folosi si se comporta similar cu un J-FET?

2) Daca la prima intrebare raspunsul este "YES" atunci se pot reproecta schemele care sint initial concepute cu J-FET-uri in asa fel incit sa functioneze cu MOS-FET-uri ”depletion mode” de ambele polaritati?

[sesebe]

Deci nu stie nimeni nimic?

 

Ps: rog un moderator sa corecteze titlul. Aseara titlul era corect dar peste noapte "cineva" l-a stricat............

[Vizitator]

Pai inginerii de pe forum ii numeri pe degete. Plus ca ar trebui sa ai si pregatire de specialitate.

Majoritatea suntem depanatori si amatori care folosim la greu google.

 

JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward biased, excess- carrier injunction occurs and the gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but the device is never operated with gate forward biased because gate current is undesirable.

MOSFETs have input impedance much higher than that of JFETs. This is due to negligibly small leakage current.

JFETs have characteristic curves more flat than those of MOSFETs indicating a higher drain resistance.

When JFET is operated with a reverse bias on the junction, the gate current IG is larger than it would be in a comparable MOSFET. The current caused by minority carrier extraction across a reverse-biased junction is greater, per unit area, than the leakage current that is supported by the oxide layer in a MOSFET. Thus MOSFET devices are more useful in electrometer applications than are the JFETs.

For the above reasons, and also because MOSFETs are somewhat easier to manufacture, they are more widely used than are the JFETs.

Are depletion mode devices being made obsolete by the development of the technology of enhancement MOSFETs?

No, I wouldn't say so. Depletion mode devices, like most discrete devices, are being made obsolete by the development of integrated circuits. As time goes on, ICs are developed for progressively more specialized purposes, reducing the need for discretes of all sorts. This is particularly true for signal processing circuits.

Are there still applications where the use of a depletion device gives distinct advantages, especially as discrete components?

Yes. Depletion devices are awesome for startup circuits, to provide some small current during the circuit's power-up sequence and to be shut off when it is finished. Their "default-on" characteristic is what makes them useful here. You can find a number of high voltage depletion devices built for this purpose on the market.

It is also quite simple to make a "2-terminal" current source using a depletion-type device: it simply requires the FET and a resistor. Making a similar current source using enhancement MOSFETs is much clumsier.

On a related note, I'd like also to know whether JFETs are still actively used in new designs, since it seems that they could be replaced by depletion MOSFETs.

JFETs tend to have less flicker noise than depletion, and especially enhancement, MOSFETs, due to the fact that their conductive channel is buried deep within the silicon, far away from crystal defects at the surface. This makes them useful for low-noise amplifiers where the base current of a BJT would be unacceptable.

.JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward biased, excess- carrier injunction occurs and the gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but the device is never operated with gate forward biased because gate current is undesirable.

2.MOSFETs have input impedance much higher than that of JFETs. This is due to negligibly small leakage current.

3.JFETs have characteristic curves more flat than those of MOSFETs indicating a higher drain resistance.

4.When JFET is operated with a reverse bias on the junction, the gate current IG is larger than it would be in a comparable MOSFET. The current caused by minority carrier extraction across a reverse-biased junction is greater, per unit area, than the leakage current that is supported by the oxide layer in a MOSFET. Thus MOSFET devices are more useful in electrometer applications than are the JFETs.

Editare numai pt corectare.

Editat Aprilie 12, 2017 de Vizitator

[nico_2010]

Un raspuns il gasesti aici: http://www.aldinc.com/pdf/IntroDepletionModeMOSFET.pdf