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#1
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FETs inserted the wrong way around
One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged, because a FET (JUGFET, MOSFET) is symmetrical in its construction? |
#2
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FETs inserted the wrong way around
On 18/09/2018 20:12, Gareth's Downstairs Computer wrote:
One thing puzzles me, why does it matter if a FET is put into a circuit the wrong way around, with source and drain interchanged, because a FET (JUGFET, MOSFET) is symmetrical in its construction? It depends. Some FETs are symmetric and should, in theory, work if 'reversed'. In JFETs it depends on the doping process. From memory, if a double diffusing process is used you're out of luck- but that is just from memory. Strictly speaking, symmetric JFETs are the only ones where the arrow should join the bar at mid-point in the symbol. The others it should be offset. Having said that, I've rarely seen that applied. -- Smile for the camera ;-) https://www.youtube.com/watch?v=HxyL2_38EsQ Remarkable Coincidences: The Stock Market Crashes of 1929 and 2008 happened on the same date in October. In Oct 1907, a run on the Knickerbocker Trust Company led to the Great Depression. |
#3
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FETs inserted the wrong way around
One thing puzzles me, why does it matter if a FET is put into
a circuit the wrong way around, with source and drain interchanged, because a FET (JUGFET, MOSFET) is symmetrical in its construction? Your "because" is too general a statement. Some types of FETs are, in fact, physically symmetrical in construction, and the source and drain are interchangeable. Most JFETs are like this, as are "lateral" MOSFETs (the usual sort in IC manufacturing, and some older audio power MOSFETs.) However, the commonly-used "vertical" MOSFET is different... it's very asymmetric in its construction. https://electronics.stackexchange.co...ted-with-arrow has a good set of explanations... including the detail that in a vertical MOSFET, the substrate is normally shorted to the source right at the semiconductor die, in order to suppress the "parasitic" BJT which lurks inside the silicon structure. |
#4
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FETs inserted the wrong way around
On 18/09/2018 20:17, Dave Platt wrote:
One thing puzzles me, why does it matter if a FET is put into a circuit the wrong way around, with source and drain interchanged, because a FET (JUGFET, MOSFET) is symmetrical in its construction? Your "because" is too general a statement. Some types of FETs are, in fact, physically symmetrical in construction, and the source and drain are interchangeable. Most JFETs are like this, as are "lateral" MOSFETs (the usual sort in IC manufacturing, and some older audio power MOSFETs.) Which is where I came in and what prompted my question. However, the commonly-used "vertical" MOSFET is different... it's very asymmetric in its construction. https://electronics.stackexchange.co...ted-with-arrow has a good set of explanations... including the detail that in a vertical MOSFET, the substrate is normally shorted to the source right at the semiconductor die, in order to suppress the "parasitic" BJT which lurks inside the silicon structure. Brilliant, Read, marked, learnt and inwardly digested, and recommended in turn to other enquirers. Thank-you very much. |
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