collector current

[Ankit Gupta]

How do collector current is controlled by base current in a CE configuration transistor ?

[Externet]

As a simple analogy, if you are familiar with the workings of relays and potentiometers; imagine a relay that pulls its pivoting contact with the coil magnetism.

 

Now replace the pivoting contact with the wiper of a potentiometer.

 

Weaker and stronger magnetic fields created by the solenoid will pull the wiper terminal and cause the resistance of the potentiometer to vary accordingly. That is, a weak current fed to the solenoid will decrease the resistance less than a stronger one.

 

A change in the current of the coil transfers to a change in the resistance of the potentiometer, or transferred to resistor = transistor effect.

If the coil terminals are called base and base2, and the potentiometer terminals are called collector and emitter; joining base 2 and emitter together (CE) still behaves the same. Current from base to emitter now changes resistance collector to emitter.

[Danijel Gorupec]

What Externet described may be considered incorrect in all but most general explanations.

 

As you are asking your question for bipolar-junction transistors (BJT), you probably know that the base current controls the collector current (precisely as you asked) and not the collector-emiter resistance (as Externet explained).

 

You asked your question in the physics forum, so I suppose you want some in-depth picture about what happens inside BJT (or?). As this is somewhat complex mater, it could be better if you systematically read about it somewhere else (internet or books) and then come here and discuss more specific questions... Or tell us what you know already so that people here can start answering from somewhere.

[Ankit Gupta]

What Externet described may be considered incorrect in all but most general explanations.

 

As you are asking your question for bipolar-junction transistors (BJT), you probably know that the base current controls the collector current (precisely as you asked) and not the collector-emiter resistance (as Externet explained).

 

You asked your question in the physics forum, so I suppose you want some in-depth picture about what happens inside BJT (or?). As this is somewhat complex mater, it could be better if you systematically read about it somewhere else (internet or books) and then come here and discuss more specific questions... Or tell us what you know already so that people here can start answering from somewhere.

 

yes u r right and I read it in many books, internet ,videos but didn't get any thing in most of the books only the circuit diagram is explained but not why and how it happens , I have spent a whole month on semiconductor but till now I didn't understand much of it so I asked it here , I asked to another students also but they also knew only the book stuff

[Sensei]

It's very simple if you realize what really happens to electrons.

In NPN transistor electrons that are on Base, must escape it, to start flowing electrons between Emitter and Collector.
That's why you usually connect NPN base to + in battery (through resistor).

In PNP it's reversed.

Arrow on circuits is always showing opposite direction to flow of electrons (they flow from - to + in battery).

ps. you can even make human touch detector with plain NPN transistor. Connect Collector to LED diode with resistor, and Base to some wire. When you will touch wire diode will start shining.

ps2. Buy tens of NPN/PNP (f.e. C945), universal circuit board, resistors and LEDs, and start experimenting. Always check in 3+ different specifications whether you have right E/C/B on legs (I have seen specifications that were mutually exclusive for the same model of transistor on the Internet). Connecting Base to + or - without any resistor means usually instant damage of transistor (in such case it's usually permanently conducting). So better use 1k+ ohms resistor in test stage, while checking what are E/C/B on legs. Once you know that f.e. middle leg is Base, you can use smaller/right resistor.

 

 

[studiot]

http://www.allaboutcircuits.com/vol_3/chpt_2/8.html

 

If this was too far advanced for you back up to the beginning of the chapter.and watch the video lectures.

 

http://www.allaboutcircuits.com/vol_3/chpt_2/1.html

[Enthalpy]

In planar epitaxial transistors, the base current is essentially injected by the base into the emitter. The collector current is injected by the emitter in the base but sucked by the collector and doesn't exit at the base. Because these two currents depend on the base-emitter voltage roughly in the same way, they are more or less proportional to an other.

 

So the base current sets a base-emitter voltage which defines a collector current.

 

In bipolars of allied germanium technology, recombination in the base was the main contribution to base current. This isn't the case with planar epitaxial transistors. Many books are outdated.

 

Many bipolar have an emitter/base heterojunction now, which reduces the injection in the emitter. I'm not sure this injection is still the main contribution for their base current.

 

For planar epitaxial transistors, the gradient of doping in the base determines the speed of the carriers injected there, much more than thermal diffusion does. With a heterojunction, or if a thin oxide separates the polysilicon emitter from the base, the carriers are shot in (if possible through) the base with a high initial speed.

 

Things may have changed further with gallium nitride.