How NPN transistor works? Tutorials of forward & reverse biasing of an NPN transistor
This is the discussion series of bipolar transistors. This is Part-1 of this series.
Bipolar Transistor – in this type of transistor, holes as well as electrons, are responsible to the flow of electric current. These transistors are used in amplifier circuits. They are of two types –
1) NPN Transistor – when a p-layer is sandwiched between two n-layers, as shown in the following figure, the NPN transistor is formed. There are following important points about this transistor –
a) Area of collector layer is largest.
b) Area of base layer is smallest. It is very thin layer.
c) Area of emitter layer is medium.
d) Collector layer is moderately doped. It has medium number of charges (electrons).
e) Base layer is lightly doped. It has a very few number of charges (holes).
f) Emitter layer is heavily doped. It has largest number of charges (electrons).
g) There are two junctions in this transistor – junction J1 and junction J2.
h) The junction between collector layer and base layer is called as collector-base* junction
or c-b junction.
i) The junction between base layer and emitter layer is called as base-emitter* junction
or b-e junction.
j) The two junctions have the same potential barrier or depletion layer, as it is in diode.
k) When n-p-n transistor is forward biased, its base-emitter junction is forward biased and its collector-base junction remains always reverse biased. Now current flows through the transistor from collector to emitter*.
l) When n-p-n transistor is reverse biased, its both junctions remain reverse biased. Now the current through the transistor is practically zero.
2) PNP Transistor – when an n-layer is sandwiched between two p-layers, as shown in the following figure, the PNP transistor is formed. There are following important points about this transistor –
a) Area of collector layer is largest.
b) Area of base layer is smallest. It is very thin layer.
c) Area of emitter layer is medium.
d) Collector layer is moderately doped. It has medium number of charges (holes).
e) Base layer is lightly doped. It has a very few number of charges (electrons).
f) Emitter layer is heavily doped. It has largest number of charges (holes).
g) There are two junctions in this transistor – junction J1 and junction J2.
h) The junction between emitter layer and base layer is called as emitter-base* junction
or e-b junction.
i) The junction between base layer and collector layer is called as base-collector* junction
or b-c junction.
j) The two junctions have the same potential barrier or depletion layer, as it is in diode.
k) When p-n-p transistor is forward biased, its emitter-base junction is forward biased and its
base-collector junction remains always reverse biased. Now current flows through the transistor from emitter to collector*.
l) When p-n-p transistor is reverse biased, its both junctions remain reverse biased. Now the current through the transistor is practically zero.
m) The symbol of both transistors is given above. The arrow is always shown at emitter region of transistor and its direction indicates the direction of conventional current through the transistor.
Working of NPN Transistor –
1) Forward Biasing – to forward bias an n-p-n transistor it is connected as shown in the following figure. Note the important points of its forward biasing –
a) The collector is connected to high positive voltage with respect to base i.e. Vcb is very high.
b) So c-b junction is reverse biased.
c) The base is connected to low positive voltage with respect to emitter i.e. Vbe is low.
d) Also Vcb is always greater than Vbe.
e) When Vbe is greater than or equal to potential barrier voltage of b-e junction, the transistor is forward biased.
f) Now large number of electrons in emitter layer is repelled by negative terminal of Vbe and they flow towards b-e junction.
g) They cross the junction and enter into small base layer. Here some electrons combine with holes, some of them are attracted by positive terminal of Vbe and remaining large number of electrons flow into collector layer, crossing c-b junction.
h) The resident electrons of collector are repelled by these (guest) electrons and thus, all the electrons are attracted by positive terminal of Vcb.
i) Thus, all these electrons complete their journey back into emitter layer and thus, produce currents in the transistor as shown in the above circuit.
j) Thus, as per KCL, we can write à Ic + Ib = Ie
k) Now as Vbe increases, more electrons from base are attracted by positive terminal of Vbe. So base-collector junction is more and more forward biased and base current (Ib) increases.
l) Hence, we conclude that collector current (Ic) is the function of base current (Ib) i.e. Ic µ Ib.
2) Reverse Biasing – to reverse bias an n-p-n transistor it is connected as shown in the following figure. Note the important points –
a) The collector is connected to high positive voltage with respect to base i.e. Vcb is very high.
b) So c-b junction is reverse biased.
c) The base is connected to low negative voltage with respect to emitter i.e. Veb is low.
d) Also Vcb is always greater than (–Vbe)*.
e) Since battery Veb is connected in opposite direction, the base-emitter junction is now reverse biased.
f) So the electrons in emitter layer are attracted by positive terminal of Veb and thus, the electrons cannot cross the junction.
g) So there are no electrons that can produce base current. So Ib = 0.
h) So collector current Ic = 0 and emitter current Ie = 0.
i) Thus, the KCL cannot be applied here, as the equation, Ic + Ib = Ie has no meaning.
j) Now if Veb is increas
ed further, nothing will happen as all the electrons in emitter layer are already attracted by positive terminal of Veb.
k) Hence, we conclude that transistor is reversed biased.
Album
Thank you for your blog post. Velupe and I are saving for a new e-book on this issue and your post has made us to save all of our money. Your thoughts really responded to all our queries. In fact, in excess of what we had recognized prior to when we came across your amazing blog. I no longer have doubts including a troubled mind because you have totally attended to our own needs in this article. Thanks
Nice blog
Very good and elaborated info about diffrent articles on this website.
This is a very useful website.
particularly for the students of electronics
needs a little more description
May I please know what sort of more description you need further?
Please reply, so that I can make necessary updates and enrich my website.
Thanks for visiting the site…
Points missing..How are the electrons penetrating through the reverse biased b-c junction(insulator)?
,what is the reason for small and lighted doped base(reduce time and recombination) and others too,
technically speaking transistor is dc to ac power converter.
Your queries can be explained in short as follows…
Also, I thank you for putting this query, as me too, was thinking of writing down a short note on these points in particular.
Thanks also for visiting the site.
Please don’t hesitate to point out the “points” missing…by sparing out of your valuable time….
really confusing for someone who has no background
from fwd bias-> f) Now large number of electrons in emitter layer is repelled by negative terminal of Vbe and they flow towards b-e junction.
why are the electrons in emitter layer is repelled when it’s connected to negative junction of the battery which is also full of electrons??
The electrons repel each other due to mutual repulsion…like charges, as they have…
THANKS ………
THANKS…….
Why base collector voltage >> base emiter voltage ?????
In the base junction negative terminal of Vcb and positive terminal of Vbe
is connected. Vcb grater then Vbe . So electrons should flow towards base .
So current should flow out of the base. ( opposite to diagram )
Then why current is flowing towards base ?
Please explain ………
I don’t understand this basic concept …..
The reason that the Vcb >> Vbe, is as follows….
My dear friend,
Please also read the explanation given on your next comment.
I like the way, you are asking the queries. It increases the wisdom of understanding this process in deep.
i have a doubt regarding forward bias and reverse bias connection for cc amplifier. can u plz help me out
Refer this article…
http://vsagar.com/2012/10/20/simple-details-common-collector-configuration-npn-transistor-2/
You may also refer other articles on configurations of transistor, to clear your doubts.
sir,
I have doubt when npn transistor is on or off i.e is it right if the voltage at the base is more than the emiter volage applied the transistor is on.
I would also like to know when does the transistor act as switch as well as CMOS working.
It is always the case, that when voltage across base-emitter junction of NPN transistor is greater than or equal to 0.7V, then it is forward biased.
When this voltage is quite large, say around 3V or more (upto certain limit), then total the number of free electrons in emitter layer start participating in conduction of electric current through the transistor.
This condition is called as saturation of transistor and in this condition we can use it just like a switch which is ON.
And when the base voltage is quickly reduced to zero, this process of conducting current suddenly stops and we can use the transistor just like a switch which is OFF…!
What type of CMOS info you need?
thank you for your immediate response.
sir I would like to know working of CMOSFET inverter.
Inverter is defined as the circuit that converts DC voltage source into AC voltage at required frequency.
Now CMOS-FET inverter is a device which is actually a FET (Field Effect Transistor) in CMOS technology.
CMOS-FET inverter is a simply a push-pull type FET circuit as shown below, which amplifies both halves of the AC cycle and give you reliable output of AC source.
is transistor simply means transfer of resistors? how can u say so?
Since the current through the transistor is controllable, it means that the internal resistance of the transistor is changing. The fact that when its base-emitter resistance changes it reflects the change in its collector-emitter resistance.
Hence it is termed as transfer of resistance-transistor.