VTU Module - 3 | Bipolar Junction Transistor

18EC33 | ELECTRONIC DEVICES | Module-3 VTU Notes

1. Fundamentals of BJT operation:

The "Fundamentals of BJT Operation" is essential in electronics and electrical engineering, especially at the university level. Bipolar Junction Transistors (BJTs) are essential semiconductor devices used in electronics. Electrical engineering, electronics, and related students must understand its operation.

BJT functioning controls current flow between the emitter and collector through a third area, the base. NPN and PNP BJTs use different charge carriers (electrons and holes).

The three main modes of BJT operation are:

• Cut-off Mode: The transistor is off and there is little or no current between the collector and emitter. When the base-emitter junction is reverse-biased.
• Saturation Mode: The transistor is fully on, allowing maximal collector-emitter current. This state is attained when the transistor is "on" and the base-emitter junction is forward-biased.
• Active Mode: The transistor is in this mode when it is in between on and off. A tiny base terminal current controls a big collector-to-emitter current. This mode is most typically used for electronic circuit amplification and signal processing.

Students learning BJT operation examine transistor biassing, small-signal analysis, and amplifier circuits. From amplifiers to digital logic circuits, developing and analysing electronic circuits requires a good understanding of these fundamentals.

BJT functioning provides the foundation for many advanced electronic devices and systems, including computers, communication systems, and power electronics, hence it's essential for electronics and electrical engineering students.

2. Amplification with BJTS:

BJTs are essential electrical devices used in many applications, especially analogue electronics. Amplification with BJTs involves increasing the strength or amplitude of an electrical signal.

NPN and PNP BJTs have different amplification characteristics. BJTs can boost weak input signals into stronger output signals when biased and coupled in a circuit.

The transistor's emitter, base, and collector layers govern current flow to amplify BJTs. By adding a little input current or voltage at the base terminal, the collector to emitter current increases. BJTs can be used as voltage or current amplifiers depending on circuit layout and components.

Audio amplifiers, RF amplifiers, and signal processing circuits use BJT amplification. Electrical and electronics engineering students and engineers must understand how BJTs amplify signals to construct and analyse analogue circuits in everyday electronic systems.

3. BJT Fabrication:

Bipolar Junction Transistor Fabrication is essential to semiconductor and electronics engineering. It involves the complex creation of Bipolar Junction Transistors (BJTs), which are used in many electrical circuits for signal amplification and switching.

The BJT manufacturing process includes numerous steps:

• Substrate Selection: The process begins with selecting a semiconductor substrate material, usually silicon due to its superb semiconductor properties.
• Epitaxial Growth: The substrate is epitaxially grown with a thin semiconductor layer. This layer controls transistor electrical characteristics.
• Base and Emitter Formation: Dopant materials are selectively inserted into the epitaxial layer to form the base and emitter. These zones are essential for transistor control.
• Collector Formation: The base and emitter collector region is doped to establish electrical properties.
• Oxide Layer Deposition: A thin oxide layer protects and isolates the semiconductor.
• Metalization: Transistor areas with metal contacts are electrically connected.
• Passivation and Packaging: BJTs are passivated to protect them from environmental influences and packed into electronic circuit-compatible enclosures.

Electronic engineers and scientists must understand BJT fabrication to design and build many electronic devices. This notion helps create sophisticated semiconductor technology and shapes the modern electronics industry, making it an important subject in university-level electronics and semiconductor engineering programmes.

4. The coupled Diode model (Ebers-Moll Model):

The linked Diode model, also known as the Ebers-Moll Model, is a basic electronic circuit model for bipolar junction transistors. This model, developed by John Ebers and James Moll in the 1950s, simplifies but accurately represents transistor action, making it a staple in semiconductor device analysis and design.

The emitter-base and collector-base diodes of a bipolar transistor are back-to-back in the Ebers-Moll Model. These diodes represent the transistor's electron and hole flow, allowing engineers and researchers to quantitatively analyse it.

Two equations describe the transistor's behaviour in this model:

1. Emitter Diode Equation: Current from the transistor's emitter to base is usually expressed as:

E_E = I_{Es} - 1 (e^{\frac{V_{BE}}{V_T}}).Where: $$

Current from the emitter is $I_E$.

- $I_{Es}$ represents the reverse saturation current of the emitter-base diode.

- $V_{BE}$ represents base-emitter junction voltage.

V_T is the thermal voltage, about 26 mV at room temperature.

2. Collector Current Equation: Current flowing from collector to emitter is represented by: $$I_C = I_{Cs} \left( e^{\frac{V_{BE}}{V_T}} - 1 \right).Where: $$

$I_C$ is collector current.

The reverse saturation current of the collector-base diode is $I_{Cs}$.

- $V_{BE}$ matches the emitter diode equation.

The thermal voltage is $V_T$.

The Ebers-Moll Model helps engineers design amplifiers, switches, and other electronic circuits by predicting and optimising transistor performance. It simplifies transistor behaviour into manageable formulae, enabling efficient electronic system creation and advancing current technology.

5. Switching operation of a transistor:

The notion of "Switching Operation of a Transistor" is crucial to electronics and electrical engineering. Electronic signals are controlled and amplified by transistors. From simple on/off switches to complicated digital logic gates, designing and implementing electronic circuits requires understanding their switching mechanism.

In its simplest form, a transistor switches between two states: "off" (an open circuit that allows no current flow) and "on" (a closed circuit that enables current flow). This switching capacity is used to generate digital logic gates and complex electrical circuits for binary data processing in computers and other devices.

The two main switching transistors are NPN (negative-positive-negative) and PNP. Changing the transistor's base terminal voltage or current controls whether it conducts or blocks current between its collector and emitter terminals. Binary representation enables digital computations and data storage by switching states.

Pulse-width modulation (PWM) techniques, which govern motor speed, LED brightness, and other variable output applications, are based on transistors' switching action. High-frequency applications like RF amplification and microwave communication require transistors' quick switching.

In conclusion, understanding transistor switching allows the invention of digital logic, precise control in numerous applications, and the foundation of our technologically advanced world. Electronics and electrical engineering students will study transistor-based circuits and its many applications in today's technology-driven culture.

6. Cutoff, Saturation, Switching Cycle, Specifications:

Understanding basic electrical concepts is essential for circuit design and analysis. Every university student studying electronics should understand these four key concepts:

• Cutoff: Cutoff is a crucial notion in electronic devices like transistors and operational amplifiers. A gadget is non-conductive or off in this condition. A transistor's cutoff zone occurs when it is biased to inhibit current passage between its collector and emitter. Understanding cutoff is crucial for building circuits that turn electronic components on and off.
• Saturation: Saturation is the reverse of cutoff and equally important in electronics. A transistor or other electronic component is most conductive when saturated, allowing maximum current flow. Amplification and signal processing require full device saturation to get desired results.
• Switching Cycle: The switching cycle is the process of switching an electronic component, such as a transistor or diode, from one state to another and back again. The time it takes a device to move from cutoff to saturation (or vice versa) and return to its initial condition. Designing signal processing, pulse shaping, and frequency modulation circuits requires understanding switching cycles.
• Specifications: Details in parameters and characteristics define the performance and behaviour of electrical components and systems. These characteristics include voltage, current, frequency response, and temperature range. Students must read and work with requirements when choosing circuit components or evaluating a design.

Cutoff, saturation, switching cycle, and requirements are the foundation of electronics engineering. Students must master these ideas to design, analyse, and troubleshoot electrical circuits for successful careers in this dynamic area.

7. Drift in the base region:

"Drift in the Base Region" is a key concept in semiconductor physics and electrical devices, especially bipolar junction transistors. This notion involves electron and hole migration in a BJT's base area.

The base region controls current flow between the emitter and collector regions in a BJT. The transistor's voltage generates an electric field across the base, causing drift in the base region. This electric field controls charge carrier movement.

Electrons injected from the emitter into the base area of an NPN transistor drift. The base's electric field guides these electrons to the collector, where they cross the base-collector junction and amplify the transistor's current.

Optimising bipolar transistor performance requires understanding and managing drift in the base region, which affects gain, speed, and efficiency. To efficiently drift charge carriers, engineers and researchers create transistor architectures and doping profiles to precisely control electronic circuit current flow. This notion underpins current semiconductor devices and electronics technologies.

8. Base narrowing:

"Base narrowing" is essential to mathematics, economics, and computer science. This idea refers to limiting options inside a context or framework.

Number systems and mathematical bases are regularly narrowed in mathematics. It reduces a base's value range, which can affect number theory, coding theory, and computer science. Base narrowing reduces the amount of binary digits in a sequence, making data storage and transmission more efficient in binary code.

Base narrowing in economics refers to strategies to narrow a product's target market. Businesses can optimise resources and boost success by targeting a more narrow consumer segment.

Base narrowing is used in algorithms and data structures in computer science. It reduces the search space or collection of probable solutions for computing problems, making algorithms faster and more efficient.

Base narrowing emphasises the need of narrowing the scope of study in numerous fields to achieve specific goals, whether in mathematics, economics, or computer science. It is useful in problem-solving and decision-making across fields.

9. Avalanche breakdown:

Avalanche breakdown is crucial to semiconductor physics and electronics. When the applied voltage crosses a crucial threshold, a reverse-biased diode or semiconductor junction experiences a sudden and rapid rise in current flow. Impact ionisation generates electron-hole pairs, causing a chain reaction or avalanche effect.

A reverse bias voltage on a semiconductor junction provides a large electric field over the depletion zone. This powerful electric field accelerates electrons and holes. When these carriers hit with crystal lattice atoms or electrons, they gather energy to release more charge carriers, forming an electron-hole pair cascade. This cascade rapidly increases diode current, making it conduct forward-biased.

Avalanche photodiodes in optical communication systems and Zener diodes in voltage regulators depend on avalanche breakdown. Designing reliable semiconductor devices and circuits and preventing electronic component breakage and damage require understanding and regulating this phenomenon.