BJT problem

Discussion in 'General Electronics Chat' started by aina, Aug 22, 2011.

  1. aina

    Thread Starter New Member

    Aug 22, 2011
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    I am recently working on tcad simulation of Silicon Carbide Bjt . My main problem is the Characteristics Curve of common emitter junction. my curent curve starts from negative Y axis. i.e, at smaller voltage collecter current is negative and as voltage increase The current become positive. Can anybody explain me why this happened and its solution plzzzzzzzzzzzzzzzzzzz
     
  2. #12

    Expert

    Nov 30, 2010
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    This datasheet tells me that you are looking at a mistake because SiC transistors work the same as silicon transistors.
     
  3. aina

    Thread Starter New Member

    Aug 22, 2011
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    No, actually i am simulating a new device and while simulation wrong i-v curve is coming. i have applied 2.5volts at base and 5.5 volts at collector. emitter is grounded. now can you tell me that what is the problem
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you want people to be able to understand your problem, and so be able to comment, you need to supply more information. A schematic of your set-up would help, as would more details of your results.

    Perhaps you are plotting the common-emitter Ic vs. Vce characteristic with the collector voltage continued through the origin.
    In this case, if the base voltage is held at a constant positive level, negative collector current may flow due to the collector-base junction becoming forward-biased. There may also be significant "reverse transistor" action, where the roles of collector and emitter are swapped over. This, at any rate, would be expected with ordinary Silicon transistors: the behaviour of SiC may be different.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    We not only need the schematic, but also the complete .model and/or .subckt that you are using; otherwise we can only make wild guesses about what the problem might be.
     
  6. aina

    Thread Starter New Member

    Aug 22, 2011
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    Actually all you people are correct. I want to know that what is happening in the collecter-base junction that is causing the current to be negative? My main idea is that at collecter-base junction Drift Dominates instead of Diffusion . And i want to solve the problem . PLZZZZZZZZZZZZZZZZZZZZZ Help me
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    The OP has a similar thread in the physics forum. Multiple threads asking the same question is frowned on here. Since you are getting better response here this probably where you ought to stay.

    I could be wrong, but I will maintain that the mechanism for a BJT (bipolar junction transistor) is probably the same no matter what the material. At this time I'm only aware of two materials, silicon and germanium.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Bill,
    The use of Silicon Carbide aka SiC material in semiconductors is relatively new; it was developed originally by Cree in 2001; their application was essentially a high voltage version of a Schottky diode.

    I found this in an application note about different diode technologies:
    [​IMG]

    One advantage of typical Schottky diodes vs standard silicon diodes is the extremely fast reverse recovery time; generally less than 15nS. By comparison, even "ultra fast" recovery diodes take anywhere from 50nS to 75nS to recover. Standard diode recovery time is measured in the microseconds; far slower. Reducing recovery time is vital for things like high-frequency switching supplies; slow diode recovery means wasted power. The problem with Schottky diodes is that they are limited to around 200V maximum. SiC diodes can be made with voltage ratings exceeding 1kV; but the manufacturing process is currently rather difficult, so such diodes are rather expensive.

    I just recently realized that they existed myself.

    SiC transistors are in their infancy; they've only been available for a couple of years tops.
     
    Last edited: Aug 25, 2011
  9. Wendy

    Moderator

    Mar 24, 2008
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    OK, but the term BJT still defines the operation, wouldn't you say?

    Last I heard Silicon Carbide was used to create blue LEDs.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Sure, BJT defines the operation.

    Cree started making blue LEDs using SiC as a substrate in 1989, but they weren't very efficient. The SiC Schottky diodes was probably an offshoot of that program.
     
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