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### Course: Modern Physics (Essentials) - Class 12th>Unit 5

Lesson 4: How current flows in transistors

# Transistor working

Amplifying an electric signal. Let's explore how a transistor (more specifically a Bipolar junction transistor) works as an amplifier.  Created by Mahesh Shenoy.

## Want to join the conversation?

• At , why exactly is electron recombination required for electrons to be pulled out into the base??
• It's not that an electron needs to be "blessed" by being recombined, it's that only the electrons that occupy low enough energy band can participate in orderly movement and thus create the base current.
• why you grounded emitter?it always confuses me that where should we apply ground?But why we apply ground?should there not be some -2 or -3 or anyone else?
• From the author:That's a very good question.
The answer is, you can ground any of the three parts. When you ground emitter, the circuit is called common emitter connection (Simply because in a real circuit, the ground would be negative terminal of a battery, and so the emitter would be a common connection for both base and collector). In this entire course, we will only be dealing with common emitter circuit and so the emitter will always be grounded.

If we ground collector or base, we will call it common collector circuit and * common base circuit* respectively. These connections are beyond the scope of our course syllabus and so we will not be discussing them.

Finally, all voltage levels are relative. For example, if we have a 10 V cell, then we could say, it's positive terminal is at 10V and the negative terminal is at 0. But we could also say it's positive terminal is at +5V and the negative terminal is at -5V. Remember what we really care about is the voltage difference. So it's just convenient to call the common voltage as 0 and hence we ground it.
• During transistor action, is there any time lag when electrons are injected from emitter to base and collected by the collector due to forward and reverse bias respectively?
• Where can I find a introduction to transistors? (I'm trying to understand CMOS and TTL)
• But shouldn't the current be flowing from the mike towards the npn transistor when we speak into the microphone?
• The current is flowing from the mic (I/P) to the transistor at the base as the electron from the base moves towards the input wire. Flow of current is opposite to the flow of current
(1 vote)
• How can you distinguish those electrons coming from base to be as minority charge carriers and those which are in collector as majority charge carriers, as after the base electrons reach the collector region they are all same ...and what is the ideology behind those electrons only could by +5 v?
• In n type semiconductor, the dopant used is pentavalent which provide excess of electron thus for n type electrons are the majority charge carriers and holes minority charge carrier while in p type, the dopant used is trivalent which accepts electron and has more number of hole( absence of electrons) this holes are the majority charge carrier in n-type semiconductors and electrons minority charge carriers.
• How come the 5V supplied isn't enough for diffusion current in the forward-biased PN junction to flow (barrier voltage .6 V)? I understand that the upper PN junction is in reverse bias, but wouldn't the lower one be open?
• There probably is, but with so few vacant holes on the valence band in P very few charges can participate in diffusion.
(1 vote)
• At he emphasizes that "recombination is necessary for the electrons to get pulled out", but then he proceeds to show that they get "pulled out" without undergoing recombination. What gives?
• At we established that the voltage in the P region is smaller than that of the voltage in the N region.
What will happen if the voltage in the N region was smaller than that of P region? Say P region has +2V and N region has +0.5V.