Transistor is a semiconductor device having three layers, three terminals and two junctions. Since we have only two types of semiconductors i.e. the p-type and n-type, there are two types of transistors: they are NPN transistor and PNP transistor.
It is named as transistor which is an acronym of two terms: “transfer-of-resistor.” It means that the internal resistance of transistor transfers from one value to another values depending on the biasing voltage applied to the transistor. Thus it is called TRANSfer resISTOR: i.e. TRANSISTOR. You can watch our video lectures of XI Bifocal Electronics to understand all the basic topics of semiconductor physics.
A bipolar transistor is a semiconductor device in which electric current flows due to electrons and holes BOTH, simultaneously. Thus both types of charges take part in the conduction of current through it. Hence it is called bipolar transistor. There are two types of bipolar transistors, NPN transistor and PNP transistor.
Internal structure of NPN transistor
NPN transistor: It uses three semiconductor layers: two n-type layers and one p-type layer. The p-layer is sandwiched between two n-layers, as shown below.
The direction of arrow shown in the symbol of transistor indicates the direction of electric current through it. Thus in NPN transistor, current comes out from emitter of the transistor. But in PNP, the current goes into the emitter.
Details of Internal Structure
- It contains three semiconductor layers: one p-layer and two n-layers.
- The area of collector layer is largest. So it can dissipate heat quickly.
- Area of base layer is smallest and it is very thin layer.
- Area of emitter layer is medium.
- Collector layer is moderately doped. So it has medium number of charges (electrons).
- Base layer is lightly doped. So it has a very few number of charges (holes).
- Emitter layer is heavily doped. So it has largest number of charges (electrons).
- THE P-LAYER IS SANDWICHED BETWEEN TWO N-LAYERS.
- So two junctions are formed: C-B junction and B-E junction.
- The junction between collector layer and base layer is called as collector-base junction
or c-b junction.
- The junction between base layer and emitter layer is called as base-emitter junction
or b-e junction.
- The two junctions have almost same potential barrier voltage of 0.6V to 0.7V, just like in a general purpose rectifier diode.
How does it work?
The NPN transistor can be used in two different modes: forward biased mode and the reverse biased mode. In forward biased mode, the electric current can easily flow through it. So it acts like a CLOSED SWITCH. However, in reverse biased mode, the current through it is practically zero and thus, it acts like an OPEN SWITCH.
Forward biasing: To forward bias an NPN transistor it is connected as shown in the above circuit. Read following points to understand the process easily –
- The collector is connected to high positive voltage with respect to base i.e. Vcb is very high. So c-b junction is reverse biased. Vcb >> Vbe.
- The base is connected to low positive voltage with respect to emitter i.e. Vbe is low.
- When we increase Vbe ≥ 0.7V (the value 0.7V is a typical value of potential barrier voltage) the transistor is forward biased.
- Now large number of electrons in emitter layer is repelled by negative terminal of Vbe and they flow towards b-e junction.
- They cross the junction and enter into small base layer. Here some electrons combine with holes. Also some of them are attracted by positive terminal of Vbe and remaining maximum number of electrons flow into collector layer, crossing the second junction i.e. c-b junction.
- The resident electrons of collector are repelled by these (guest) electrons and thus, then all the electrons are present in collector layer are attracted by positive terminal of Vcb.
- Thus, all these electrons complete their journey back into emitter layer and produce conventional currents in the transistor as shown in the above circuit.
- Thus, as per Kirchhoff Current Law, we can write, Ic + Ib = Ie.
- Now when Vbe is still increased, more electrons are repelled by negative terminal of Vbe. So base-emitter junction is more and more forward biased. Thus the base current (Ib) increases, which in turn increases Ic.
- Hence, we can say that collector current (Ic) is the function of base current (Ib).
- But there is a typical value of Vbe for each transistor, at which the collector current Ic no longer remains the function of base current Ib.
- Also collector current is directly proportional to the base current.
- In all this process, maximum number of electrons from emitter layer flow into collector layer. So collector current is ALMOST EQUAL to emitter current. Hence we say that, collector current is proportional to emitter current.
The beauty of transistor structure (DON’T SKIP THIS)
The beauty of transistor structure lies in its different areas of each of the layers. Also, the base layer is kept between collector and emitter. We can say that the base acts just like a ‘door’ for the transfer of electrons from emitter to collector through it. When base voltage increases, this ‘door’ opens wider and more number of electrons are allowed to flow through it.
The area of base layer is deliberately kept small because of two reasons. First, it creates strong repulsive forces among the crowded electrons in base layer. When electrons from emitter layer come into base layer, they get trapped, as they cannot go back, due to repulsive force of negative ions of base layer near B-E junction. In base layer they find very small number of holes to recombine. The base voltage is also small. So very small number of electrons are attracted towards positive terminal of Vbe battery.
Eventually, all these electrons crowded in base layer have but one option, to surmount the ‘gap’ of C-B junction and enter into collector layer. But this junction is already reverse biased (note the polarity connections of Vcb battery). Now how they will jump over this ‘large gap’ of C-B junction? They will jump this gap because, the strong positive electric field of collector layer is ‘inviting’ them or attracting them. So with two forces: one is mutual repulsive force among them in base layer and one is the strong attractive force from collector layer, they succeed to surmount this large gap of C-B junction and enter into collector layer.
The discussion in expert notes ignores the dynamic conditions for the time being. So the student will get the idea about basic working of the transistor.
Watch this video to understand more clearly
Click this link for more videos on Semiconductor Physics by Prof. Dattaraj Vidyasagar.
In this method both the junctions are reverse biased as the batteries are connected in opposite direction as shown in the adjacent diagram. Due to Vcb battery, the collector-base junction is reverse biased. Similarly, due to Veb battery, the base-emitter junction is also reverse biased. So charges cannot flow and current in the transistor is practically zero. This method is not useful as the transistor is in “cut-off” state since current is zero.
What do you mean by transistor biasing?
When external voltage is applied to the junction of transistor in such a direction that it cancels out the potential barrier, so that electric current flows through it, is called as transistor biasing. Now to obtain easy current flow through the transistor it must be biased by connecting external batteries. So there must be two batteries to apply proper bias across the two junctions of the transistor. For example, the NPN transistor can be biased using three different methods –
- FF biasing: In this method both the junctions are forward biased. For this, two external batteries are connected across two junctions such that collector is negative w.r.t. base and base is positive w.r.t. emitter. This method is not useful as the transistor is in “saturation” and the current cannot be controlled easily.
- RR biasing: In this method both the junctions are reverse biased. For this, two external batteries are connected across two junctions such that collector is negative w.r.t. base & base is negative w.r.t. emitter. This method is also not useful as the transistor is in “cut-off” state since current is zero.
- FR biasing: This is the most common and popular method used in transistor biasing. In this method, the base-emitter junction is forward biased and collector-base junction is reverse biased. For this, two external batteries are connected across two junctions such that collector is positive w.r.t. base and base is positive w.r.t. emitter. So by adjusting base voltage we can control total current in the transistor easily.
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