1. Introduction to BJT (Bipolar Junction Transistor)

The Bipolar Junction Transistor (BJT) is a three-layer, three-terminal semiconductor device capable of amplifying or switching electrical signals.
It was invented in 1947 at Bell Labs and has since been a cornerstone of analog electronics.

The term “bipolar” refers to the fact that both electrons and holes (two types of charge carriers) are involved in current conduction.

🔹 Definition:
A BJT is a current-controlled device where a small current at the Base terminal controls a larger current between the Emitter and Collector.

2. Structure of BJT

The BJT consists of three regions:

Region	Function
Emitter (E)	Heavily doped to inject carriers
Base (B)	Very thin and lightly doped to pass carriers
Collector (C)	Moderately doped to collect carriers

There are two types of BJTs based on the arrangement of these regions:

• NPN transistor
• PNP transistor

🎨 Diagram: BJT Structure

➡️ NPN Transistor Structure:

🎨 Symbol of BJT

🎨 Symbols:

NPN Transistor	PNP Transistor
The emitter arrow points inward	Emitter arrow points inward

4. Working Principle of BJT

When a small base current (Ib) is applied:

• It controls a much larger current flowing from Collector (Ic) to Emitter (Ie).

Mathematically:

I_E=I_B+I_C​

5. Modes of Operation

BJTs can operate in four modes:

Mode	Base-Emitter	Base-Collector	Use
Cutoff	Reverse biased	Reverse biased	OFF state (open switch)
Active	Forward biased	Reverse biased	Amplification
Saturation	Forward biased	Forward biased	ON state (closed switch)
Reverse Active	Reverse biased	Forward biased	Rarely used

6. Types of BJT

🔵 NPN Transistor:

• Electrons are majority carriers.
• Easier to fabricate.
• Used for positive voltages.

🔵 PNP Transistor:

• Holes are majority carriers.
• Operates with negative voltages.

Comparison Table:

Feature	NPN	PNP
Major Carrier	Electrons	Holes
Biasing	Positive voltage to base	Negative voltage to base
Current Direction	Collector to emitter	Emitter to collector

7. Characteristics of BJT

Parameter	Typical Values
Current Gain (β)	20–1000
Cut-off Voltage (Vbe)	0.6V–0.7V (Si), 0.2V (Ge)
Maximum Collector Current (Ic)	10mA – 100A
Power Dissipation	100mW – 500W
Frequency Response (fT)	Up to 300GHz

8. Applications of BJT

✅ Amplifiers — Audio systems, RF communication.
✅ Switching Circuits — Logic gates, relay drivers.
✅ Oscillators — Signal generation.
✅ Voltage Regulation — Power supplies.
✅ Motor Control — Robotics, drones.

Application	Device Example
Audio Amplifiers	2N3055 BJT
RF Circuits	BF199 BJT
Microcontrollers I/O Switching	BC547 NPN BJT

9. Advantages of BJTs

• High gain and linearity.
• Reliable and robust.
• Good low-frequency performance.
• Easily available and inexpensive.

10. Limitations of BJTs

• Lower input impedance compared to FETs.
• More power dissipation.
• Slower switching speed than MOSFETs.

11. Modern Advances in BJTs (2025)

• SiC BJTs now available for high-temperature and high-voltage applications.
• Heterojunction Bipolar Transistors (HBTs) used for high-frequency 5G and satellite communication.
• Organic BJTs researched for flexible electronics.

12. FAQs: BJT

❓ Why is BJT called bipolar?

Because both electrons and holes are involved in current conduction.

❓ Which BJT is more common: NPN or PNP?

NPN BJTs are more common because electron mobility is higher than hole mobility, making them more efficient.

❓ What happens if the base current is zero?

The transistor stays OFF — no current flows from Collector to Emitter (cutoff region).

❓ Why is the base made very thin?

A thin base region increases current gain by minimizing the recombination of carriers.

13. Conclusion

The Bipolar Junction Transistor (BJT) remains a fundamental building block of electronic circuits despite the emerging technologies of MOSFETs and IGBTs.
BJTs’ simplicity, amplification capabilities, and robust performance ensure that they are still widely used across industries ranging from audio systems to aerospace.

As technology evolves, SiC BJTs and heterojunction devices are expanding the horizons of where and how BJTs are applied in the modern world.