# how NPN transistor control collector using base current

Joined Feb 4, 2022
145
In NPN Current flow into the base/emitter junction. I am trying to understand how NPN transistor control collector current using a base current ?

#### Papabravo

Joined Feb 24, 2006
20,369
In the relm of semiconductor physics it is helpful to think about "potential barriers". Inside the transistor with no voltage present on the terminals there is no current flow because the charge carriers, called "electrons" and "holes" cannot flow in large numbers across these "potential barriers in any significant quantities. There may be some quantum tunneling through those barriers, but the currents are very small -- like femto or ato amperes.

When we introduce a positive power supply across the collector and emitter, we now have a source of charge carriers (electrons). As we supply small amounts of current to the base, the voltage across the BE-junction rises, and at the same time one of the potential barriers is lowered. Then just like a waterfall the charges have enough energy to get over the potential barrier and roll downhill like a thundering avalanche.

That's the heuristic description of what is going on. You seem like you don't want to get deep in the weeds with quantum mechanics and mathematics that will make your head spin.

#### crutschow

Joined Mar 14, 2008
32,846
This will likely explain it better than I could.

Joined Feb 4, 2022
145
In the relm of semiconductor physics it is helpful to think about "potential barriers". Inside the transistor with no voltage present on the terminals there is no current flow because the charge carriers, called "electrons" and "holes" cannot flow in large numbers across these "potential barriers in any significant quantities. There may be some quantum tunneling through those barriers, but the currents are very small -- like femto or ato amperes.

When we introduce a positive power supply across the collector and emitter, we now have a source of charge carriers (electrons). As we supply small amounts of current to the base, the voltage across the BE-junction rises, and at the same time one of the potential barriers is lowered. Then just like a waterfall the charges have enough energy to get over the potential barrier and roll downhill like a thundering avalanche.

That's the heuristic description of what is going on. You seem like you don't want to get deep in the weeds with quantum mechanics and mathematics that will make your head spin.
Correct me if I am misunderstanding concept

When we don't supply power across the base and emitter, there will be no base current flowing

When we supply power across the base and emitter, there will be base current flowing through base to emitter

When we don't supply power across the collector and emitter, there will be no collector current flowing through collector to emitter

When we supply power across the collector and emitter, there will be collector current flowing through collector to emitter

#### crutschow

Joined Mar 14, 2008
32,846
When we supply power across the collector and emitter, there will be collector current flowing through collector to emitter
Only if there is also some base-emitter current.

Joined Feb 4, 2022
145
Only if there is also some base-emitter current.
Understood, Collector current will only flow when base current will flow

#### k1ng 1337

Joined Sep 11, 2020
701
https://www.falstad.com/circuit/ is an easy to use circuit simulator that may help. There is an example in Circuits - Transistors - NPN Transistor that shows voltage, current, power and provides a visual of current flow. Edit some values to see the response.

#### ci139

Joined Jul 11, 2016
1,874