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.
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.
Bipolar Transistor Construction
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 Bipolar Transistor basic construction consists of two PN‐junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B ) and the Collector ( C ) respectively.
Bipolar Transistors are current regulating
devices that control the amount of current flowing through them in proportion
to the amount of biasing voltage applied to their base terminal acting like a
current‐controlled switch. The principle of operation of the two transistor
types PNP and NPN, is exactly the same the only difference being in their
biasing and the polarity of the power supply for each type.
Details of internal structure
- 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).
- There are two junctions in this transistor – junction J‐1 and junction J‐2.
- 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 diode.
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.
- 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.
Reverse Biasing
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.
Operation of PNP transistors
Working of a PNP transistor is exactly the same as that of NPN transistors discussed earlier, if the role played by the electrons in NPN transistors is interchanged with holes as given below;
In a PNP transistor,
‐ the majority current carriers are holes instead of electrons
‐ the
minority current Ico is due to electrons in the N type base material
instead of holes all the polarities for a PNP transistor are reversed which
means that it “sinks” current into its Base as opposed to the . which “sources”
current through its Base. The main difference between the two types of
transistors is that holes are the more important carriers for PNP transistors,
whereas electrons are the important carriers for NPN transistors. Then, PNP
transistors use a small base current and a negative base voltage to control a
much larger emitter‐collector current. In other words for a PNP transistor, the
Emitter is more positive with respect to the Base and also with 274 respect to
the Collector. The construction of a “PNP transistor” consists of two P‐type
semiconductor materials either side of an N‐type material as shown below.
A PNP Transistor
Configuration
The construction and terminal voltages for an NPN transistor are shown above. The PNP Transistor has very similar characteristics to their NPN bipolar cousins
PNP Transistor Connection
he voltage between the Base and Emitter ( VBE ), is now negative at the Base and positive at the Emitter because for a PNP transistor, the Base terminal is always biased negative with respect to the Emitter.
Also the Emitter supply voltage is positive with respect to the Collector ( VCE ). So for a PNP transistor to conduct the Emitter is always more positive with respect to both the Base and the Collector.
The voltage sources are connected to a PNP transistor are as shown. This time the Emitter is connected to the supply voltage VCC with the load resistor, RL which limits the maximum current flowing through the device connected to the Collector terminal. The Base voltage VB which is biased negative with respect to the Emitter and is connected to the Base resistor RB, which again is used to limit the maximum Base current.
To cause the Base current to flow in a PNP transistor the Base
needs to be more negative than the Emitter (current must leave the base) by
approx. 0.7 volts for a silicon device or 0.3 volts for a germanium device with
the formulas used to calculate the Base resistor, Base current or Collector
current are the same as those used for an equivalent NPN transistor .
