Bipolar Junction Transistor (N-P-N And P-N-P Transistor)

Imagine a traffic controller at a busy intersection, managing the flow of vehicles to ensure smooth movement and prevent congestion. A Bipolar Junction Transistor (BJT) works in a similar way, controlling the flow of electrical current in a circuit. There are two main types of BJTs: N-P-N and P-N-P transistors, each designed to handle current flow differently.

This Story also Contains

1. Junction Transistor
2. N-P-N Transistor
3. P-N-P Transistor
4. Basic Transistor Circuit Configurations
5. Solved Example Based On Bipolar Junction Transistor (N-P-N And P-N-P Transistor)
6. Summary:

In an N-P-N transistor, current flows from the collector to the emitter when a small current is applied to the base. Conversely, in a P-N-P transistor, current flows from the emitter to the collector when a small current is applied to the base. These transistors act as switches or amplifiers in electronic devices, regulating current flow with precision, much like a traffic controller manages the flow of vehicles. Understanding the roles of N-P-N and P-N-P transistors is crucial for designing and maintaining efficient electronic circuits that power everything from simple gadgets to complex systems. In this article, we are going to cover Junction Transistors, n-p-n transistors, p-n-p Transistors, and Basic transistor circuit configurations and also cover some solved examples based on these concepts.

Junction Transistor

A junction transistor, also known as a bipolar junction transistor (BJT), is a type of semiconductor device that consists of three layers of doped material, forming two p-n junctions. It has three terminals: the emitter, base, and collector. There are two types of junction transistors: NPN and PNP, which refer to the arrangement of the n-type and p-type materials.

In an NPN transistor, the structure is an n-type material (emitter) connected to a p-type material (base), followed by another n-type material (collector). Conversely, in a PNP transistor, the arrangement is p-type (emitter) to n-type (base) to p-type (collector).

Bipolar Junction Transistor (BJT)

It is a three-terminal electronic device that amplifies the flow of current. It has three doped regions ( emitter, base, and collector) forming two p-n junctions between them.

Based on their construction, Bipolar junction transistors are classified into two types as

1) n-p-n transistor

2) p-n-p transistor-

All three segments (emitter, base, and collector) of a transistor have different thickness and their doping levels are also different.

The schematic symbols of both these transistors are given in the below figure.

In the schematic symbols, as shown in the above figure, the arrowhead shows the direction of conventional current in the transistor.

The three segments of a transistor:

• Emitter- This segment is on one side of the transistor. It is of moderate size and heavily doped causing it to supply a large number of carriers for the flow of current.
• Base: This is the central segment. It is very thin and lightly doped.
• Collector: This segment collects a major portion of the majority of carriers supplied by the emitter. The collector side is moderately doped and larger in size as compared to the emitter.

Working of a Junction Transistor

In the case of a junction transistor, the depletion regions are formed at the emitter-base junction and the base-collector junction.

A junction transistor works as an amplifier when

1) The emitter-base junction is forward-biased and

2) The base-collector junction is reverse-biased.

Some Terminologies-

• V_EB= The voltage between the emitter and base
• V_CB= The voltage between the collector and base
• V_EE = Power supply connected between the emitter and base.
• V_CC = Power supply connected between the collector and base

N-P-N Transistor

In this, a p-type semiconductor (base) separates two segments of the n-type semiconductor (emitter and collector) as shown in the below figure. In an NPN transistor electrons are the majority charge carriers and flow from emitter to base.

Working of an n-p-n Transistor

Circuit diagram of NPN transistor

OR

In an NPN transistor, 5% emitter electrons combine with the holes in the base region resulting in a small base current.
The remaining 95 % of electrons enter the collector region. i.e i.e $I_e>I_c$

And according to Kirchhoff’s law $I_e=I_b+I_c$

P-N-P Transistor

In this, an n-type semiconductor (base) separates two segments of the p-type semiconductor (emitter and collector). As shown in the below figure. In PNP transistor holes are majority charge carriers and flow from emitter to base.

Working of a p-n-p Transistor

Circuit diagram of PNP transistor

OR

In the PNP transistor, 5% emitter holes combine with the electrons in the base region resulting in a small base current.
The remaining 95% of holes enter the collector region. i.e $I_e>I_c$

According to Kirchhoff’s law $I_e=I_b+I_c$

Basic Transistor Circuit Configurations

A transistor can be connected in a circuit in the following three different configurations.

Common base (CB), Common emitter (CE) and Common collector (CC) configuration.

(1) CB configurations

In these configurations, the Base is common to both the emitter and collector as shown in the below figure.

For the above figure Input current = I_e and Input voltage = V_EB

whereas Output voltage = V_CB and Output current = I_c

With a small increase in emitter-base voltage V_EB, the emitter current I_e increases rapidly due to small input resistance.

• Input characteristics-

If V_CB= constant, the curve between I_e and V_EB (as shown in the below figure) is known as input characteristics. It is also known as emitter characteristics.

The Dynamic input resistance of a transistor is given by

$R_i=\left(\frac{\Delta V_{E B}}{\Delta I_e}\right)_{V_{C B}=\text { constant }}$

and

$\mathrm{R}_{\mathrm{i}}$ is of the order of $100 \Omega$.

• Output characteristics

Taking the emitter current i_e constant, the curve drawn between I_C and V_CB (as shown in the below figure) is known as the output characteristics of CB configuration.

The Dynamic output resistance of a transistor is given by

$R_0=\left(\frac{\Delta V_{C B}}{\Delta I_C}\right)_{i_e=\text { constant }}$

(2) CE configurations

In these configurations, the Emitter is common to both the base and collector as shown in the below figure.

The graphs between voltages and currents, when the emitter of a transistor is common to input and output circuits, are known as CE characteristics of a transistor.

Input characteristics: The input characteristic curve is drawn between base current I_b and emitter-base voltage V_EB , at constant collector-emitter voltage V_CE(as shown in the below figure).

The Dynamic input resistance of a transistor is given by

$R_{i}=\left(\frac{\Delta V_{ BE}}{\Delta I_{B}}\right)_{V_{C E}=\mathrm{constant}}$

Output characteristics-

Variation of collector current I_C with V_CE can be noticed for V_CE between 0 to 1 V only (as shown in the below figure).

The value of V_CE up to which the I_C changes with V_CE is called knee voltage. And The transistor is operated in the the region above knee voltage.

The Dynamic output resistance of a transistor is given by

$R_0=\left(\frac{\Delta V_{C E}}{\Delta I_C}\right)_{i_B=\text { constant }}$

Solved Example Based On Bipolar Junction Transistor (N-P-N And P-N-P Transistor)

Example 1: When the transistor is used as an amplifier :

1) electrons move from base to collector

2) holes move from emitter to base

3) electrons move from the collector to the base

4) holes move from base to emitter.

Solution:

Transistor -

Three-layered semiconducting device.

NPN or PNP

- where

1. Emitter is heavily doped

2. The collector is moderately doped.

3. Base is lightly doped & very thin

In the NPN junction, the Emitter-Base is forward-biased. So electrons will move from the emitter to the Base. Base and collector are at reverse bias hence electrons will move from Base to collector.

Hence, the answer is option (1).

Example 2: An n-p-n transistor has three leads A, B and C. Connecting B and C by moist fingers, A to the positive lead of an ammeter, and C to the negative lead of the ammeter, one finds large deflection. Then, A, B and C refer respectively to :

1) Emitter, base and collector

2) Base, emitter and collector

3) Base, collector and emitter

4) Collector, emitter and base.

Solution:

In an n-p-n transistor, the leads are identified as the emitter (E), base (B), and collector (C). When the base and collector are connected using moist fingers, a path with a relatively low resistance is created due to the moisture. If you connect the positive lead of an ammeter to one lead and the negative lead to another, you can identify the leads based on the current flow.

Given the setup:

• A is connected to the positive lead of the ammeter.
• C is connected to the negative lead of the ammeter.
• B and C are connected via moist fingers.

For large deflection in the ammeter, a significant current must flow through the transistor. In an n-p-n transistor, this typically means that the current is flowing from the collector to the emitter (since the emitter is connected to the negative lead of the ammeter and the collector to the positive lead).

Thus, the arrangement corresponds to:

• A (positive ammeter lead) = Collector
• B (connected via moist fingers to C) = Base
• C (negative ammeter lead) = Emitter

Therefore, the correct answer is option (4) Collector, emitter and base.

Example 3: In the given figure, given that $\mathrm{V}_{\mathrm{BB}}$ supply can vary from 0 to 5.0 V
$ V_{C C}=5 V, \beta_{d c}=200, R_B=100 k \Omega, R_C=1 k \Omega \text { and } V_{B E}=1.0 \mathrm{~V}
$ The minimum base current and the input voltage at which the transistor will go to saturation will be, respectively :

1) $25 \mu \mathrm{A}$ and 3.5 V
2) $25 \mu \mathrm{A}$ and 2.8 V
3) $20 \mu \mathrm{A}$ and 2.8 V
4) $20 \mu \mathrm{A}$ and 3.5 V

Solution:

Relation between emitter current, Base current, collector current -

$
\begin{aligned}
& I_E=I_B+I_C \\
& \text {-wherein } \\
& I_E=\text { Emitter Current } \\
& I_B=\text { Base Current } \\
& I_C=\text { Collector Current }
\end{aligned}
$
- wherein

At saturation $V_{C E}=0$
$
\begin{aligned}
& V_{C E}=V_{C C}-I_C R_C \\
& \Rightarrow V_{C C}=I_C R_C \\
& I_C=\frac{5 \mathrm{~V}}{1 \times 10^3}=5 \times 10^{-3} \mathrm{~A} \\
& I_B=\frac{5 \times 10^{-3}}{200}=25 \mu \mathrm{A}
\end{aligned}
$

We knew that
$
\begin{aligned}
& V_{B B}=I_B R_B+V_{B E} \\
& R_B=100 K \Omega \\
& V_{B E}=1 V
\end{aligned}
$

So $V_{B B}=25 \times 10^{-6} \times 100 \times 10^3+1=2.5+1=3.5 \mathrm{~V}$
So $V_{B B}=3.5 \operatorname{Vand}_B=25 \mu \mathrm{A}$

Hence, the answer is option (1).

Example 4: In a common emitter amplifier circuit using an n-p-n transistor, the phase difference between the input and the output voltages will be :

1) $45^0$
2) $90^{\circ}$
3) $135^0$
4) $180^{\circ}$

Solution:

In such a circuit, output is amplified as compared to input.

Input Voltage must be forward-biased output voltage must be reverse-biased

So V input must be out of phase of V Output. In a common emitter amplifier, the output will be 180^o out of phase with the input voltage waveform. Thus the common emitter amplifier is called an inverting amplifier circuit.

Example 5 : The ratio (R) of output resistance $r_0$ and the input resistance $r_i$ in measurements of input and output characteristics of a transistor is typically in the range :
1) $R \sim 10^2-10^3$
2) $R \sim 1-10$
3) $R \sim 0.1-0.01$
4) $R \sim 0.1-1.0$

Solution:

Amplifier -

The transistor is used as an amplifier

wherein

in such a circuit, output is amplified as compared to input.

It's typically in the range of 1~10

Hence the answer is option (2).

Summary:

A Bipolar Junction Transistor (BJT) is a semiconductor device that can amplify or switch electrical signals. There are two types of BJTs: N-P-N and P-N-P. When a base is equipped with a positive voltage, the N-P-N transistors are activated and pass the current from the collector to the emitter. In contrast, the P-N-P transistors are triggered by a negative voltage at the base, which allows the current to flow from the emitter to the collector. Both types are crucial amplifiers in audio equipment, and digital circuits function as switches and devices for the regulation of currents in diverse electronic products, thus, they are the very basis of modern electronics.