How NPN and PNP Junction Transistors Work in Circuits
NPN and PNP Transistors Definition
Semiconductors are basically used in manufacturing two devices that are diodes and transistors. With the help of diodes, amplification of the signal is not possible and that is the reason we use transistors. In a transistor, one semiconductor is sandwiched from both sides by another semiconductor. The transistor has three regions that are emitter, base, collector. The emitter emits the charges and these charges are collected by the collector. Thee are two types of transistors NPN and PNP transistor. In PNP transistor they are two differences between both P, that is doping and the size of both p is different.
PNP Transistor Definition
A transistor in which there is one n-type semiconductor that is doped by two p-type semiconductors from both sides is called PNP transistor. The PNP transistor turns on when there is no current at the base of the transistor.
In PNP transistors instead of electrons, the emitter emits holes and they are collected by the collector. Here the collector and the emitter old of p-type and the base are of n-type. The base size is small and it is lightly doped, the emitter size is moderate (between the base size and the collector size) and the doping is strong, collector size is large as compared to base and emitter and the doping is moderate (between the base and emitter doping).
Symbol of the transistor is
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Here, which has an arrow is known as emitter, the base is always in the center.
The arrow shows the direction of the electric current and the electric current always flows from positive to negative. To identify whether it is NPN or PNP we use this arrow.
If the arrow is pointing outside then it is NPN transistor and if it is pointing inwards then it is PNP transistor.
NPN Transistor Definition
A transistor, in which there is one p-type semiconductor that is doped by two n-type semiconductors from both sides are called NPN transistors. The NPN transistor turns on when the current flow through the base of the transistor. The current flows in the direction from the collector to the emitter.
Usually, NPN transistors are used because it is easy to design them. In NPN transistors the majority charge carriers are electrons and in PNP transistors the majority charge carriers are holes.
The mobility of electrons is better than the mobility of holes, therefore, NPN transistor works faster as compared to PNP transistors. So, we usually prefer NPN transistors for better results.
Transistor Action
Each NPN junction has two terminals we connect a cell between them do there is a potential difference across it.
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There is a rule for the emitter and the base that is the battery connected should always be in forward biased. And in the base and collector circuit, it is always reverse biased.
In the above circuit diagram, the base terminal is common for the emitter and the collector so, therefore, it is called a common base configuration. Similarly, we can have a common emitter configuration and a common collector configuration.
But mostly we use a common emitter configuration because the most popular use of transistor is to make the amplifier and the best amplification is done by a common emitter configuration. Below is the circuit diagram of common emitter configuration.
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After the circuit is connected the depletion layer of the emitter-base is decreased whereas the depletion region of the collector- base is increased.
The current flow is more in the emitter-base side current decreases in the collector- base side.
Resistance is decreased in the emitter-base side and becomes conductor, whereas resistance is increased in the collector-base side due to this the current majority stops in the collector-base side but the reverse current continues to flow.
The resistance is transformed from one side to another side; therefore, it is called a transistor.
More than 95℅ emitted electrons enter into the collector and only 5℅ recombine with the holes in the base, therefore, the collector current IC is much larger than the base current IB.
The transistor is also called a bipolar transistor because both the polarity carriers are present that are electrons and holes.
Solved Problems
1. In a collector-emitter connection, the current amplification factor is 0.9. If the emitter current is 1mA determine the value of the base current.
Solution- α = 0.9, IE = 1mA
To find IB
We know that,
α=IC / IE
IC= IE× α
IC= 0.9×1 = 0.9mA
Now, IE= IB +IC
IB= IE -IC
= 1-0.9
IB= 0.1mA
The base current is 0.1 mA.
2. In a collector base connection IE = 1mA, IC = 0.95mA. Calculate the value of IB.
Solution- We know that,
IE= IB + IC
IB= IE – IC
IB= 1 – 0.95
IB= 0.05 mA.
FAQs on Junction Transistors: NPN and PNP Types
1. What is the fundamental difference between an NPN and a PNP transistor?
The primary difference lies in their construction and the type of charge carriers. In an NPN transistor, a thin layer of P-type semiconductor is sandwiched between two layers of N-type semiconductor, with electrons as the majority charge carriers. Conversely, a PNP transistor has an N-type layer between two P-type layers, and holes are the majority charge carriers. This also affects the biasing, where a PNP transistor requires a negative voltage at the base to turn on, while an NPN requires a positive voltage.
2. How can you identify NPN and PNP transistors from their circuit symbols?
You can identify them by the direction of the arrow on the emitter terminal. The rule is "Not Pointing In" for NPN.
- For an NPN transistor, the arrow on the emitter points outward, away from the base.
- For a PNP transistor, the arrow on the emitter points inward, towards the base.
3. What are the common applications of NPN and PNP transistors in electronics?
Both NPN and PNP transistors are fundamental components used for two main purposes:
- Amplification: They can amplify weak electrical signals, which is crucial in audio amplifiers, radio receivers, and sensor circuits.
- Switching: They can act as fast electronic switches to turn a circuit on or off, used in digital logic gates, microcontrollers, and power control systems like relays.
4. What are the current gain factors, alpha (α) and beta (β), in a transistor?
Alpha and beta are parameters that describe the current amplification capability of a transistor.
- Alpha (α) is the common-base current gain, defined as the ratio of the collector current (IC) to the emitter current (IE). Its value is always slightly less than 1. Formula: α = IC / IE.
- Beta (β) is the common-emitter current gain, defined as the ratio of the collector current (IC) to the base current (IB). Its value is typically high (e.g., 50 to 400). Formula: β = IC / IB.
5. How does a transistor function as a simple switch?
A transistor acts as a switch by operating in two extreme states: the cut-off region (OFF) and the saturation region (ON).
- OFF State (Cut-off): When a very small or zero voltage is applied to the base, almost no base current flows. This prevents a larger current from flowing from the collector to the emitter, effectively opening the switch.
- ON State (Saturation): When a sufficient voltage is applied to the base, a small base current (IB) flows. This allows a much larger collector current (IC) to flow, effectively closing the switch and turning on the connected load (like an LED or a motor).
6. Why is the base region of a transistor made very thin and lightly doped?
The base region is engineered this way to maximise the transistor's efficiency. Its thinness ensures that the majority carriers (electrons in NPN, holes in PNP) coming from the heavily doped emitter can easily cross the base and reach the collector with minimal recombination. Light doping in the base means there are fewer majority carriers in the base to recombine with the carriers from the emitter. This design ensures that almost all the emitter current becomes collector current, leading to a high current gain (β), which is the primary goal of transistor action.
7. For a transistor to act as an amplifier, why must the emitter-base junction be forward-biased and the collector-base junction reverse-biased?
This specific biasing arrangement, known as the active region, is critical for amplification.
- Forward-biased Emitter-Base Junction: This allows a large number of charge carriers from the heavily doped emitter to be injected into the thin base region. It essentially 'turns on' the flow of charge.
- Reverse-biased Collector-Base Junction: This creates a strong electric field that sweeps or 'collects' the charge carriers that have crossed the base into the collector circuit. The reverse bias also provides a high output resistance, which is essential for achieving significant voltage gain across a load resistor.
8. Why are NPN transistors more commonly used than PNP transistors in electronic circuits?
NPN transistors are generally preferred over PNP transistors primarily because the majority charge carriers in NPN transistors are electrons, whereas in PNP transistors, they are holes. The mobility of electrons in silicon is significantly higher (about two to three times) than the mobility of holes. This higher mobility allows NPN transistors to operate at higher speeds and provide better high-frequency performance, making them more suitable for a wider range of applications, especially in fast switching and amplification circuits.
