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In this schematic, what do the Schottky diode like "barbs" mean on some of the BJTs where the base is connected? I haven't encountered this symbol before.

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3 Answers 3

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You need to understand three things in order to understand a Schottky transistor.

  1. In an ordinary BJT, the saturation state collector-emitter voltage falls below or equal to 0.2 V.
  2. In an ordinary BJT, the base-emitter voltage must be equal to or above 0.8 V.
  3. The saturation state voltage drop of a Schottky diode is around 0.3 V.

Now, if we consider all this information together; a Schottky transistor cannot enter into saturation state, because, voltage drop on the diode (0.3 V) added to the voltage drop on the transistor (0.2 V) doesn't reach the required saturation base-emitter voltage of the transistor.

Why do we need a transistor like this?
One of the biggest disadvantages of BJT is that it gets out of the saturation state very slowly. This causes a low switching speed. But, if it doesn't enter into the saturation state, it can be turned off much faster. That's the reason why the Schottky transistor is used.

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Very nice explanation. I have always stood in a bit of confusion when talking about turn on/turn off times of transitors. I know these times are consequence of internal junction parasitic capacitance, but I never fully understood why such things exist. Does the schotkey transistor, in essence, keep excess charge from entering the base and charging these parasitic capacitances? –  sherrellbc Jun 26 at 14:13
    
I don't remember the solid-state explanation, but I don't think that it is related with "base charge". Because there is not much base charge in BJTs like in MOSFETs. Maybe someone else could give an explanation of this. –  hkBattousai Jun 26 at 14:21
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In the figure "Schotkey" should be "Schottky". –  Peter Mortensen Jun 27 at 14:27

It's called a Schottky transistor. Basically, it's a transistor that incorporates a Schottky diode junction that keeps the transistor from saturating, speeding it up in switching (logic) applications.

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And the nice thing it's that when you are integrating a BJT, making that diode is free: the base contact is made a little wider so that it contacts the collector diffusion too. –  Vladimir Cravero Jun 26 at 14:18

It's a stylized "S", for Schottky.

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Interesting I never noticed that, is the Zener symbol a stylized "Z" for Zener? –  jreimers Jul 2 at 14:12
    
I think so. My favorite one, though, is the varicap symbol. –  EM Fields Jul 2 at 14:22

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