Unipolar transistors or Field Effect Transistors (FETs), as the name suggests use only one type of charge carrier, either electrons or holes depending on the type of semiconductor material used. If an N-type semiconductor is used the charge carriers are electrons while if a P-type semiconductor is used the charge carriers are holes. In the case of a BJT, both electrons and holes could be used as charge carriers.
A Bipolar Junction Transistor (BJT or BJT Transistor) is a three-terminal semiconductor device composed of two P-N junctions that can amplify or magnify a signal. The three terminals of the BJT are the base, the collector, and the emitter. The primary function of a BJT is to amplify current, which allows BJTs to be used as amplifiers. Another important application of BJT is switching circuits. Hence they are widely used as amplifiers and switches in electronic equipment such as mobile phones, industrial control, television, and radio transmitters.
A bipolar junction transistor is created by combining two doped semiconductor materials that are back-to-back. In other words, a BJT is formed by a "sandwich" of extrinsic semiconductor materials placed back to back. Two PN junction diodes are sandwiched together to form a BJT. The PN junction that connects the base and emitter region is called the BE-junction while the junction connecting the base and collector is called the BC-junction. The Base region is slightly doped and is thinner as compared to other regions. The Emitter region is highly doped while the Collector has balanced doping. The Emitter terminal emits charge carriers, which are electrons in the case of NPN type and holes in the case of PNP type BJT.
Charge flow in a BJT is due to the diffusion of charge carriers across a junction between two regions of different charge carrier concentrations. In a typical operation, the BE-junction is forward biased, which means that the P-type layer is at a more positive potential than the N-type layer, and the BC-junction is reverse-biased. When forward biased the electrons (in case of NPN) and holes (in case of PNP) get thermally excited causing them to break the depletion layer and enter into the Base layer and recombine with holes or electrons depending upon the type of transistor. The Base layer is thinner and lightly doped so as to reduce the number of recombinations in the base layer, ensuring electrons/holes reach the CB junction. The CB junction is reverse-biased prevening movement of majority carriers from collector to base, but the carriers from the emitter break the CB depletion region to reach the collector layer because of the applied electric field.
A BJT has four distinct regions of operation depending on the biasing polarity and magnitude they are discussed below:
Forward-active mode: This is the most commonly used mode of operation where the BE junction is forward biased and the CB junction is reverse biased. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations. This mode offers the greatest amount of current gain.
Reverse-active mode: By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles.
Saturation mode: When both the junctions are forward biased and the BJT acts as a closed switch facilitating the flow of current.
Cut-off mode: In the cut-off mode both the junctions are reverse biased and the BJT does not allow current to flow through it, hence it acts like an open switch.
The Forward-active mode of operation is used when the BJT is to be used as an amplifier and the Cut-off and Saturation mode are together used when the BJT is to be used as a switch.
Based on the nature of doping of the three semiconductor layers, BJTs are classified as PNP or NPN. A PNP transistor is made up of two P-type semiconductor junctions that share a thin N-doped region, whereas an NPN transistor is made up of two N-type semiconductor junctions that share a thin P-doped region. N-type means doped with impurities that provide mobile electrons (such as phosphorus or arsenic), whereas P-type means doped with impurities that provide holes that readily accept electrons (such as boron). A PNP BJT will function like two diodes that share an N-type cathode region, and the NPN like two diodes sharing a P-type anode region.
Both types of BJT work by giving amplified output from the collector with a small current input to the base. As a result, the BJT functions well as a switch controlled by its base input. The BJT is also an excellent amplifier because it can boost a weak input signal to about 100 times its original strength. BJT networks are used to create powerful amplifiers with a wide range of applications.
Both NPN and PNP transistors have differences based on their construction, operation, and applications. One of the striking differences is in the NPN transistor, the current flows from the collector to the emitter when a positive supply is applied to the base, whereas in the PNP transistor, the charge carrier flows from the emitter to the collector when a negative supply is applied to the base. The following table shows the comparisons between the two types of BJT based on different parameters.
|Parameter||NPN type||PNP type|
|Structure||Two N-type layers separated by one P-type layer||Two P-type layers separated by one N-type layer|
|Direction of current||Collector to Emitter||Emitter to Collector|
|Turn-on instance||When electrons enter into the base||When holes enter the base|
|Majority carriers||Electrons are the majority carriers||Holes are the majority carriers|
|Minority carriers||Holes are the minority carriers||Electrons are the minority carriers|
|Switching time||Lesser switching time hence faster switching||Higher switching time hence slower switching|
|Inside current||Develop because of the varying position of electrons||Originate because of the varying position of holes|
|Turn-off instance||Switches off by applying a low base voltage||Switches off by applying a positive base voltage|
A transistor is generally used for two types of applications, switching type operations and amplification of signals. A solid-state switch can be created using a transistor. The transistor functions as a closed switch when operated in the saturation zone and as an open switch when operated in the cutoff area. Both PNP and NPN transistors can be utilized as switches. A transistor conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When the base voltage is present, the switch is on.
To operate the transistor in the cut-off region (open-switch) both the PN junctions are reversed biased causing input and output current to be zero and voltage across the transistor to be maximum. Hence operating as an open switch. To operate the transistor in the saturation region (open-switch) both the PN junctions are forward biased causing input and output current to be maximum and voltage across the transistor to be minimum. Hence operating as a closed switch.
To operate the NPN transistor as a switch the voltage applied at the base terminal is varied. When a sufficient voltage (generally greater than 0.7V) is applied between the base and emitter terminals, collector to emitter voltage is approximately equal to 0 which implies that the transistor acts as a short circuit or closed switch. Similarly, when no voltage or zero voltage is applied at the input, the transistor operates in the cutoff region and acts as an open circuit.
In a PNP-type transistor the base terminal is always negatively biased with respect to the emitter terminal and hence the current always flows from the base. When the voltage is applied at the base the transistor acts as a closed switch and when the voltage applied at the base terminal is zero the transistor acts as an open switch.
A large number of electronics that we use in our daily lives use both NPN and PNP types of transistors in different switching and amplification applications. As amplifiers, they are used in various oscillators, modulators, and detectors. Some of the applications are mentioned below:
Bipolar Junction Transistors(BJT) amplify current from the emitter to the collector when a small amount of current is passed through the base. This property is used in communication circuits to amplify weak signals over long-distance communications.
They are used in sound modulators and amplifier circuits.
They are used as switches to control the switching of appliances like LEDs and motors.
Darlington pair is two back-to-back BJTs used to obtain high current gain for high power applications.
Phototransistors are bipolar devices that conduct when light falls on the base. This property is used to detect the presence of light in various automation applications.
They are in multivibrator circuits which are used to implement a variety of simple two-state devices such as relaxation oscillators, timers, and flip-flops.
Bipolar junction transistors find a variety of applications in different electronic equipment. They are solid state devices that are used in switching and amplification operations. The article would help readers understand and appreciate the engineering marvel which forms the base of almost every complex electronic device that exists today. Some of the key takeaways from the article are:
BJT is a three-terminal semiconductor device formed consisting of three semiconductor layers.
Based on the type and placement of the semiconductor layer they are divided into NPN and PNP types.
PNP type has two P-type layers with an N-type layer between them. While the NPN type has two N-type layers with a P-type layer between them.
The main differences between these two types of transistors were also understood.
Working of BJTs as switches were also studied. Both NPN and PNP types can be used as switches but the NPN type is preferred as the majority carriers are electrons which are more mobile.
The applications of BJT were also discussed towards the end.