Overview
This lecture explains the main differences between BJT (Bipolar Junction Transistor) and MOSFET (Metal Oxide Semiconductor Field Effect Transistor), covering their working principles, types, advantages, disadvantages, and typical applications.
Introduction to Transistors
- Transistors amplify or switch electrical signals using small input changes to create large output changes.
- The two primary types discussed are BJT and MOSFET.
BJT (Bipolar Junction Transistor)
- BJT is a current-controlled semiconductor device with three terminals: emitter, base, and collector.
- Operates using charge carriers (electrons and holes); two types are NPN and PNP.
- Output current is a function of base current, making it suitable for low current applications.
- Working principle: base-emitter voltage controls collector current.
MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
- MOSFET is a voltage-controlled device with three terminals: gate, source, and drain.
- Controlled by gate voltage, enabling high input impedance and low power consumption.
- Types include N-channel and P-channel, each having enhancement and depletion modes.
- Used in digital CMOS logic due to efficient power usage and high switching speeds.
- Working principle: gate voltage modifies channel conductivity between source and drain.
Key Differences Between BJT and MOSFET
- BJT is current-controlled; MOSFET is voltage-controlled.
- BJT uses electrons and holes as carriers; MOSFET uses majority carriers.
- BJT: lower input resistance, slower switching, less suitable for high power.
- MOSFET: higher input resistance, faster switching, better at high power and frequency.
- BJT shows negative temperature coefficient; MOSFET shows positive temperature coefficient.
- MOSFETs generally occupy less space and create less noise.
Advantages & Disadvantages
BJT Advantages
- Better performance under high load and higher frequencies.
- Higher fidelity and gain in linear regions.
- Good for voltage and low power applications.
BJT Disadvantages
- Sensitive to radiation and noise.
- Less thermal stability and more complex base control.
- Lower switching frequency and slower operation.
MOSFET Advantages
- Smaller size, simple manufacturing, and high input impedance.
- Supports high-speed operation with low power consumption.
- Suitable for digital circuitry and large-scale integration.
MOSFET Disadvantages
- Shorter lifespan and frequent need for calibration.
- Highly sensitive to overload voltage, requiring careful handling.
Key Terms & Definitions
- BJT — Bipolar Junction Transistor, a current-controlled semiconductor device.
- MOSFET — Metal Oxide Semiconductor Field Effect Transistor, a voltage-controlled semiconductor device.
- NPN/PNP — Types of BJT based on majority charge carriers.
- Enhancement/Depletion Mode — MOSFET operation modes defining conductivity based on gate voltage.
- Input Impedance — Resistance seen by input signal at the device terminals.
- Temperature Coefficient — Measure of how device parameters change with temperature.
Action Items / Next Steps
- Review and compare the characteristics of BJT and MOSFET.
- Prepare examples of applications for both transistors.
- Read further on different MOSFET types and their uses.