Hello Friends! This is a platform where we keep you updated with valuable information relating to engineering and technology, so you keep coming back every now and then. Today, I am going to unlock the details on the Introduction to NPN transistor. It is a bipolar junction transistor mainly used for amplification and switching purpose and composed of three semiconductor layers where one layer is P-doped semiconductor layer and other two layers are N-doped. The P-doped layer is sandwiched between two N-doped layers.
Main Function: Small current at one terminal is used to control large at the other terminals. This process is used for amplification purpose.
Major Charge Carriers: Electrons
BJTs (Bipolar Junction Transistor) are divided into two types i.e. NPN transistor and PNP transistor. Both transistors are different in terms of their electrical composition and construction, however, both are used for amplification and switching purpose in one way or the other.
In this tutorial, we’ll look at the NPN transistor, how it works, circuit diagram, output characteristics curve, and applications.
Introduction to NPN Transistor
- The NPN transistor is a bipolar junction transistor mainly used for amplification and switching purpose. As the name suggests, conduction is carried out by the movement of both charge carriers i.e. electrons and holes, however, electrons are major charge carriers in NPN transistor.
- It is composed of three layers i.e two N-doped layers and one P-doped layer. The P-doped layer represents the base of the transistor while other two layers represent emitter and collector respectively.
- NPN transistor comes with three terminals called emitter, base, and collector which are used for external connection with the circuits where small current at the base side is used to control the large current at the collector and emitter side.
- All three terminals in NPN transistor are different in terms of their size and doping concentration. Emitter terminal is highly doped and exhibits 100% of current flowing through the device. While the base is very thin and lightly doped which controls the number of electrons and collector is moderately doped which collects the number of electrons.
- The NPN transistor is nothing but a combination of two diodes which are joined together back to back.
- The NPN transistor comes with two junctions called emitter-base junction and collector-base junction. The NPN transistor sets in operating condition when an emitter-base junction is forward biased and the collector-base junction is reverse biased and enough current is present at the base terminal. In order to make emitter-base junction forward biased, a positive voltage is applied at the base side and a negative voltage is applied at the emitter side.
- Similarly, in order to make emitter-base junction reverse biased, collector voltage must be kept more positive than base and collector.
- This NPN transistor is one of the basic electronic components used for circuit designing.
Following figure shows the circuit diagram of the NPN transistor.
- We can see from the diagram, voltage and resistive loads are applied at the terminals of the transistor.
- A negative voltage is connected to the emitter while a positive voltage is connected to the base terminals.
- The base is more positive with respect to the emitter.
- The resistive load is applied at the base terminal which limits the current produced in this terminal.
- The positive voltage is applied at the collector terminal and load resistance is applied at this terminal that limits the electrons entering at this terminal.
- The base is responsible for initiating transistor action. When a voltage is applied at the base, it gets biased and draws small current which is then used to control large current at the collector and emitter side.
- The base action is considered as an ON-OFF valve which generates current when a proper bias voltage is applied at this terminal.
- The small change at the voltage applied at the base terminal shows a large impact at the output terminals. Actually, base acts as an input and collector acts as an output in NPN transistor.
- In case of silicon transistor emitter-base junction draws voltage around 0.7 when there is no voltage at the base terminal, in order to initiate the electron action and put the transistor in running condition, the base voltage must be greater than 0.7 voltage in case of silicon transistor and 0.3 in case of germanium transistor.
- In the N-side of the transistor which represents emitter, the electrons act as the majority charge carriers which are then diffused into the base when a suitable voltage is applied at the base terminal. These electrons act as minority charge carriers when they enter the base terminal, where they join with holes present in the base. Not all electrons join with the holes present at the base terminal. Some of them join with the holes, and the resulting electron-hole pair disappears. Most of the electrons leave the base terminal and then enter the collector region where they again act as a majority charge carriers.
- When a voltage is applied across the base terminal, the base current is given by;
- Collector current is directly related to base current times a constant factor.
- In order to increase the efficiency of the NPN transistor, the base is made very thin and a collector is made thick for two reasons i.e collector can handle more heat and accept more electrons diffused through the base terminal.
Current Gains and Relation between Them
- Current gains play an important role in the amplification process. The common emitter current gain is a ratio between collector current and base current. It is called beta and denoted by β. It is also known as an amplification factor which defines the amount of current being amplified.
- Beta is a ratio between two currents, so it features no unit. The beta value is always greater than unity and ranges between 20 to 1000 – 20 for high power transistors and 1000 for low power transistors, however, most of the time its value is taken as 50.
- Similarly, a common base current gain is another important factor which is a ratio between collector current and emitter current. It is called alpha and denoted by α. An alpha value ranges between 0.95 and 0.99, however, most of the time its value is taken as unity.
- Following figure shows the relation between two current gains.
- An alpha value cannot exceed from unity, because it is a ratio between collector current and emitter current i.e emitter current always remains greater than collector current because it exhibits 100% current of the transistor and is equal to the sum of collector current and base current.
NPN Transistor Configurations
- This NPN transistor can be configured into three configurations called common emitter configuration, common collector configuration, and common base configuration.
- Common emitter configuration is mostly used for amplification purpose where base acts as an input, collector acts as an output while emitter acts as a common terminal between input and output.
- This common emitter configuration acts always operates in a linear region where small current at the base side is used to control large current at the collector side.
- The common emitter configuration used in the electronic circuits always produces inverted output that is highly affected by the bias voltage and temperature. This configuration is an ideal choice for ampliation circuits because it comes with high input impedance and low output impedance and produces the exact voltage and power gain required for amplification purpose.
- During common emitter configuration, transistor always operates between saturation and cut-off region that helps in amplifying the negative and positive cycles of the input signals. If the base terminal is not biased with the proper voltage, only half of the signal would be amplified.
Output Characteristics Curve of NPN Transistor
Following figure shows the output characteristic curve of the NPN bipolar transistor which is plotted between output collector current and the collector-emitter voltage with varying base current.
- As described earlier, there will be no output collector current if the applied voltage at the base terminal is zero. When proper bias voltage above 0.7 V, is applied at the base terminal, it gets biased and draws current that controls and effects the output collector current.
- We can see, Vce directly effects the value of output collector current as long as the applied voltage is 1 V. Above that value collector current no longer remains under the influence of Vce value. In that case, the collector current is widely dependent and controlled by the base current. A small change in the base current and bias voltage would produce a large change in the collector current.
- The load resistor applied at the collector terminal controls the amount of current entering the collector terminals. Keeping in the view of the load resistor and the voltage applied at the collector-emitter terminals, the collector current is given by;
- Straight load line between point A and B falls under active region when an emitter-base junction is forward biased and the transistor conducts where electrons are majority charge carriers.
- The Q point in the graph can be defined by the load line which is actually referred as an operating point of the transistor.
- The output characteristics curve of this NPN transistor is used to describe the collector current when base current and collector voltage is given.
- In order to conduct, collector voltage needs to be more positive than base and emitter.
- It is important to note that, when an emitter-base junction is not forward biased, Ic will be zero, no matter how much voltage is applied at the base terminals. When the emitter-base junction is forward biased and voltage is applied at the base terminal, it draws small current which is then used to control large current at other terminals.
Difference between NPN and PNP Transistors
- Both NPN and PNP transistors are different in terms of electrical construction and layers doping. NPN stands for negative-positive-negative and also known as sourcing device. While PNP stands for positive-negative-positive and also known as sinking device.
- In NPN transistor base is positive as compared to emitter and collector voltage is more positive as compared to both emitter and base. Similarly, in PNP transistor base is negative as compared to emitter and emitter voltage is much larger than collector voltage.
- The voltage polarities and current directions are reversed in both transistors.
- The NPN transistor conducts and initiates transistor action when a positive voltage is applied at the base terminal. The PNP transistor conducts when a negative voltage lower 0.7 V (for silicon) than emitter voltage is applied at the base terminal.
- The NPN transistor uses electrons as majority charge carriers for the conduction while PNP transistor uses holes as majority charge carriers for conduction process.
- In NPN transistor current flows from the collector to emitter while in case of PNP transistor current flows from emitter to collector terminal.
- Both transistors differ in terms of how they are powered on. The NPN transistor powers on when there is enough current present at the base terminal while PNP transistor powers on when there is no current at the base terminal.
- NPN transistors are mainly used for amplification and switching purpose.
- Used in logarithmic converters and high-frequency applications.
- Signal processing and radio transmission applications involve this NPN transistor.
- Darlington pair circuits make use of this NPN transistor for amplifying signals.
- Used in temperature sensor.
- Push-Pull amplifying circuits fall under the category of the classic amplifier circuit which makes use of this NPN transistor.
- In small quantities, transistors are used to make logic circuits and in the circuits where amplification is required. However, when thousands of transistors are joined together, they can be used to construct complex circuits like processors, computer memories circuits, and multiple ICs.
That’s all for today. I hope you have got clear what is NPN transistor and why it is used for. If you are unsure or have any question, you can approach me in the comment section below, I’d love to help you according to best of my expertise and knowledge. Feel free to keep us updated with your feedback and suggestions, they help us provide you quality content that aligns with your needs and requirements. Thanks for reading the article.