semiconductor physics and Devices

Week 5: Transistors - Bipolar Junction Transistors (BJTs)

Lecture Topics:

1. Introduction to Transistors

   • A transistor is a semiconductor device used to amplify or switch electronic signals.

   • Bipolar Junction Transistors (BJTs) are the most fundamental transistor type, consisting of three layers of semiconductor material.

   • BJTs are used in various applications, including amplifiers, switches, and oscillators.

2. BJT Structure and Types

   • The BJT has three regions:

     • Emitter (E): A heavily doped region that injects charge carriers (electrons or holes).

     • Base (B): Thin, lightly doped region through which carriers pass.

     • Collector (C): A moderately doped region that collects the carriers.

   • Types of BJTs:

     • NPN transistor: Consists of two n-type regions (emitter and collector) with a p-type base.

     • PNP transistor: Consists of two p-type regions (emitter and collector) with an n-type base.

3. BJT Operating Modes

   • BJTs can operate in four different regions depending on the biasing of the emitter-base and collector-base junctions:

     • Active mode: The transistor is used as an amplifier. The emitter-base junction is forward-biased, and the collector-base junction is reverse-biased.

     • Cutoff mode: The transistor is off, with no current flow. Both junctions are reverse-biased.

     • Saturation mode: The transistor is fully on and used as a switch. Both junctions are forward-biased.

     • Reverse-active mode: Rarely used; the collector and emitter functions are reversed.

4. BJT Current Flow

   • In the NPN transistor:

     • Forward active mode: Electrons flow from the emitter to the base, and the majority are swept into the collector due to the reverse bias between the collector and base.

     • Collector current (IC): Most of the electrons from the emitter are collected, contributing to the main current flow.

     • Base current (IB): A small fraction of the electrons recombine in the base region.

     • Emitter current (IE): The sum of the base and collector currents: 

IE​=IC​+IB​

5. BJT Current Relationships

   • Current gain (β): The ratio of the collector current to the base current: 

β=IC/IB

5. • A BJT's large signal model can approximate the device's behavior in the active mode, with the emitter current driving the collector and base currents.

6. BJT as an Amplifier

   • The BJT is widely used as an amplifier in analog circuits, where a small input current at the base controls a larger current through the collector.

   • Common-emitter configuration: The most commonly used amplifier configuration, where the input is applied between the base and emitter, and the output is between the collector and emitter.

   • Voltage gain: The ratio of the output voltage to the input voltage is influenced by the current gain β\betaβ and the resistances in the circuit.

7. BJT as a Switch

   • The BJT can also be used as a switch in digital circuits.

   • Cutoff mode: The transistor is off, and no current flows between the collector and emitter.

   • Saturation mode: The transistor is fully on, allowing current to flow freely from the collector to the emitter.

   • Switching applications include motor control, logic gates, and digital circuits.

8. BJT I-V Characteristics

   • Input characteristics: The relationship between the base current IB and the base-emitter voltage VBE​.

   • Output characteristics: The relationship between the collector current IC​ and the collector-emitter voltage VCE​ for different values of base current IB​.

   • Plotting the output characteristics helps understand the BJT's operation in the active, cutoff, and saturation regions.

Examples:

• Calculate the collector current given the base current and current gain β in a BJT.

• Plotting the input and output characteristics of an NPN transistor.

• Designing a simple common-emitter amplifier circuit and calculating the voltage gain.

Homework/Exercises:

1. Given a base current of 20 µA and a current gain β of 100, calculate the collector current and emitter current in an NPN transistor.

2. Explain the operation of a BJT in active mode and how it can be used as an amplifier.

3. Design a common-emitter amplifier circuit with given parameters and calculate the voltage gain.

4. Plot the output characteristics of an NPN transistor for different base currents and explain the regions of operation (cutoff, active, and saturation).

Suggested Reading:

• Charles Kittel, Introduction to Solid State Physics, Chapter 8: Semiconductors (continued).

Key Takeaways:

• BJTs are fundamental components in electronic devices and circuits used for amplification and switching.

• The operation of a BJT depends on the biasing of the emitter-base and collector-base junctions, leading to different operating modes (active, cutoff, and saturation).

• In amplification, a small base current controls a larger current between the collector and emitter.

• Understanding the I-V characteristics of a BJT is essential for analyzing its behavior in circuits.

This week introduces the operation and applications of Bipolar Junction Transistors (BJTs), focusing on how they function as amplifiers and switches. Understanding the current relationships, operating modes, and characteristics is crucial for designing and analyzing transistor-based circuits.