semiconductor physics and Devices

Week 7: Semiconductor Devices – Applications of Transistors

Lecture Topics:

1. Transistor as an Amplifier

   • Recap of transistors (BJTs and FETs) functioning as amplifiers.

   • Small-signal model for analyzing transistor amplifiers.

     • BJT amplifier: Current amplification through the collector-emitter junction, with the base as the control terminal.

     • MOSFET amplifier: Voltage amplification, with the gate controlling the current through the drain-source junction.

   • Common amplifier configurations:

     • Common-emitter (BJT) or Common-source (FET): Provides both voltage and current gain.

     • Common-base (BJT) or Common-gate (FET): Used in high-frequency applications, provides voltage gain without current gain.

     • Common-collector (BJT) or Common-drain (FET) (also known as emitter follower/source follower): Provides current gain but no voltage gain.

   • Voltage gain (A_v): 

where gmg_mgm​ is the transconductance, and RC​ is the load resistance.

Power gain: Combining voltage and current gain for overall power amplification.

• 1. Frequency Response of Transistor Amplifiers

     1. Low-frequency response: Dominated by capacitive coupling and bypass capacitors.

     2. High-frequency response: Limited by parasitic capacitances (e.g., junction capacitance in BJTs, gate capacitance in MOSFETs).

     3. Cutoff frequency: The frequency at which the gain drops to 70% of the maximum value.

     4. Bandwidth: The frequencies over which the amplifier maintains a significant gain.

  2. Multistage Amplifiers

     1. Combining multiple amplifiers to achieve higher gain, improved impedance matching, or specific frequency responses.

     2. Cascaded amplifiers: Connecting amplifiers in series, with the output of one stage driving the input of the next stage.

     3. Darlington pair (BJT): Two BJTs combined to provide high current gain.

     4. Cascode configuration (BJT/FET): Combines a common-emitter (common-source) and common-base (common-gate) amplifier to improve gain and bandwidth.

  3. Transistor as a Switch

     1. Recap of how transistors (BJTs and MOSFETs) are used as switches in digital circuits.

     2. Switching characteristics: Switching speed depends on factors such as carrier mobility, capacitance, and threshold voltage.

     3. On state (saturation mode): Transistor allows current to flow, acting as a closed switch.

     4. Off state (cutoff mode): The transistor blocks current, acting as an open switch.

     5. Applications of transistors as switches:

        1. Logic gates: Transistors form the building blocks of digital logic circuits (AND, OR, NOT gates).

        2. Pulse-width modulation (PWM): Used in motor control and power supply regulation by switching transistors rapidly between on and off states.

        3. Power transistors: MOSFETs and BJTs are used in high-power applications such as power supplies, motor drivers, and RF amplifiers.

  4. CMOS Technology

     1. CMOS (Complementary Metal-Oxide-Semiconductor) technology combines n-channel and p-channel MOSFETs in a complementary fashion.

     2. CMOS inverter: The fundamental building block of CMOS logic circuits, consisting of an n-channel and a p-channel MOSFET.

        1. In the "off" state, one transistor is fully off while the other is on, minimizing power consumption.

        2. In the "on" state, both transistors briefly conduct, leading to fast switching and low static power dissipation.

     3. Advantages of CMOS:

        1. High noise immunity.

        2. Low static power consumption (important for battery-operated devices).

        3. High switching speed.

     4. Applications: CMOS technology is the backbone of integrated circuits used in microprocessors, memory, and digital electronics.

  5. Digital Logic Circuits

     1. Transistor-transistor logic (TTL): Early digital logic circuits based on BJTs.

     2. CMOS logic circuits: Widely used in modern digital circuits due to their low power consumption and high integration.

     3. Design of basic logic gates using transistors:

        1. Inverter (NOT gate): A single transistor (BJT or MOSFET) in a common-emitter (common-source) configuration.

        2. AND/OR gates: Built using combinations of transistors that control current flow based on input signals.

        3. NAND/NOR gates: Fundamental gates are used in digital logic design and are often implemented in CMOS technology.

  6. Power Amplifiers

     1. Classes of power amplifiers:

        1. Class A: High linearity but low efficiency. Used in low-power applications.

        2. Class B/AB: Higher efficiency, with push-pull configuration to reduce distortion.

        3. Class C: Very high efficiency, used in RF amplifiers but unsuitable for audio applications due to high distortion.

     2. Efficiency and distortion trade-offs in power amplifiers.

     3. Heat dissipation: Managing power loss as heat in high-power transistors using heat sinks and cooling mechanisms.

Examples:

• Design and analysis of a common-emitter BJT amplifier, calculating voltage gain and input/output impedance.

• Plotting the frequency response of a transistor amplifier and identifying the bandwidth.

• Design a CMOS inverter circuit and explain its operation in switching logic.

• Analysis of a Darlington pair to calculate overall current gain.

Homework/Exercises:

1. Design a common-source MOSFET amplifier and calculate the voltage gain, input impedance, and output impedance.

2. Explain the frequency response of a common-emitter amplifier and calculate the cutoff frequency for a given configuration.

3. Plot the I-V characteristics of a CMOS inverter and explain how it operates as a logic gate.

4. Compare the efficiency and power output of Class A and Class B amplifiers and describe their applications.

Suggested Reading:

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

Key Takeaways:

• Transistors are versatile components used in a wide variety of applications, including amplification, switching, and logic circuits.

• Amplifiers based on BJTs and FETs are used to increase signal strength, and their frequency response determines their application range.

• MOSFETs in CMOS technology are the foundation of modern digital electronics, offering high efficiency and fast switching times.

• Understanding transistor operation in both analog and digital circuits is essential for designing complex systems like amplifiers, logic gates, and power devices.

This week focuses on practical applications of transistors in amplification, switching, and digital logic circuits. By understanding the operation of both BJTs and FETs in these contexts, students will gain a deeper understanding of how semiconductor devices are used in real-world electronics.