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### Course: Class 12 Physics (India)>Unit 14

Lesson 9: Transistor characteristics

# Input characteristics of NPN transistor

We will explore the behaviour of input current (base current) as the input voltage (base-emitter voltage) is changed.  Created by Mahesh Shenoy.

## Want to join the conversation?

• @ if there is very low recombination, why there would be very low electron flow from the base?
• Since electrons being pulled out into the Base voltage terminal depends on the recombination. The Base must maintain a certain number of holes in it since it's maintained at 0.7 V. Thus, when a recombination happens, the Base must then generate a new electron-'hole' pair and the 'hole' replaces that 'hole' that was recombined and the electron goes to the +0.7V terminal.
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• I am confused. You say at that the depletion region increases as a result of reverse bias which means more electrons flow to the collector. I thought we wanted the depletion region to decrease for output current to increase? Or are you saying that technically the output current would increase with more Vce....but maybe not as much as with more Ib?
• Yeah, I found that really weird as well. But after going back to my Reverse Bias notes, I think I have formed an answer to this question.

At first, it may seem counterintuitive to think that the current would increase when the depletion region widens. However, recall that the depletion region prevents the majority charge carriers of the top N node and the P node. At 0.7V, the electrons from the bottom N node are basically the "minority" charge carriers in the P node since the bottom depletion region has practically vanished.

Therefore, with stronger V_CE, the stronger is the pull for electrons to go to the Collector. But I admit it is still strange since when the depletion region increases, the negative charge on the Base's side must also increase effectively changing nothing, right? Well, I think about that, recall that one of the requirements for a transistor is a very thin Base. So, although the negative field have indeed increased, I think the positive field must've increased just enough to overpower the negative field of the depletion region of the Base-Collector.

Take this with a grain of salt though. I might be wrong but this is currently my best explanation for this.
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• My question is why so few questions are actually answered on this platform?
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• Can you tell me how to get voltage in the transistors
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• Hello Mahesh Sir I have a question...... why does the input characteristics graph does not have current plotted in the negative y-Axis ?? because here we can see that the direction of current is in the wire connecting the emitter and the base is from low potential i.e. from the emitter to the high potential i.e. to the base
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• Conventional current is in the opposite direction of electron current.
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