Transistor

Table of Contents

1. Introduction to Transistors

2. Bipolar Junction Transistors (BJT)

   • 2.1 NPN vs PNP Structure

   • 2.2 BJT Operating Modes

   • 2.3 BJT Characteristics and Parameters

   • 2.4 BJT Applications

3. Field Effect Transistors (FET)

   • 3.1 Junction FET (JFET)

   • 3.2 Metal-Oxide-Semiconductor FET (MOSFET)

   • 3.3 FET Characteristics and Parameters

   • 3.4 FET Applications

4. Specialized Transistor Types

   • 4.1 Darlington Transistor

   • 4.2 Phototransistor

   • 4.3 Unijunction Transistor (UJT)

   • 4.4 Insulated Gate Bipolar Transistor (IGBT)

5. Comparison of Transistor Types

6. Selection Criteria

7. Testing and Troubleshooting

8. Future Trends

9. Conclusion

Introduction to Transistors

Transistors are fundamental semiconductor devices that revolutionized electronics and made modern computing possible. Invented in 1947 at Bell Labs, transistors serve three primary functions: amplification, switching, and signal modulation. They form the building blocks of all modern electronic devices, from simple radios to complex microprocessors.

Basic Transistor Symbol

    Collector (C)

         |

         ▼

    B ──►|──── (Arrow indicates current direction)

         |

         ▼

     Emitter (E)

A transistor essentially acts as a current-controlled switch or amplifier, where a small input signal can control a much larger output signal, providing gain and switching capabilities essential for electronic circuits.

Bipolar Junction Transistors (BJT)

NPN vs PNP Structure

BJTs are three-layer semiconductor devices with two PN junctions. They come in two complementary types:

NPN Transistor Structure:

• Emitter: N-type (heavily doped)

• Base: P-type (lightly doped, very thin)

• Collector: N-type (moderately doped)

PNP Transistor Structure:

• Emitter: P-type (heavily doped)

• Base: N-type (lightly doped, very thin)

• Collector: P-type (moderately doped)

BJT Symbol Representation

NPN:           PNP:

  C              C

  |              |

  ▼              ▼

B►|◄           B►|◄

  |              |

  ▼              ▲

  E              E

BJT Operating Modes

Operating Mode

Base-Emitter Junction

Base-Collector Junction

Application

Active (Forward)

Forward Biased

Reverse Biased

Amplification

Saturation

Forward Biased

Forward Biased

Switching (ON)

Cutoff

Reverse Biased

Reverse Biased

Switching (OFF)

Reverse Active

Reverse Biased

Forward Biased

Rarely used

BJT Characteristics and Parameters

Key Parameters:

Parameter Symbol Description Typical Range

Current Gain β or hFE IC/IB ratio 50-800

Collector Current IC Main output current µA to A

Base Current IB Control current µA to mA

Emitter Current IE IE = IC + IB µA to A

Collector-Emitter Voltage VCE Voltage across C-E 0.1V to breakdown

Base-Emitter Voltage VBE Forward bias voltage ~0.7V (Si), ~0.3V (Ge)

Fundamental BJT Equations:

• IE = IC + IB

• IC = β × IB

• α = IC/IE (typically 0.95-0.99)

BJT Applications

BJTs excel in applications requiring high current gain and fast switching:

1. Audio Amplifiers - High gain and good linearity

2. RF Amplifiers - High frequency performance

3. Switching Circuits - Fast switching speeds

4. Current Sources - Stable current regulation

5. Oscillators - Phase shift capabilities

Field Effect Transistors (FET)

FETs are voltage-controlled devices with three terminals: Gate (G), Drain (D), and Source (S). Unlike BJTs, FETs have extremely high input impedance and are unipolar devices.

Junction FET (JFET)

JFETs use a reverse-biased PN junction to control current flow through a channel.

JFET Types:

• N-Channel JFET: Current flows through N-type channel

• P-Channel JFET: Current flows through P-type channel

JFET Symbol

N-Channel:        P-Channel:

    D                 D

    |                 |

    ▼                 ▼

G ──►|◄            G ──►|◄

    |                 |

    ▼                 ▲

    S                 S

JFET Characteristics:

• Depletion mode operation only

• Negative gate voltage for N-channel (positive for P-channel)

• High input impedance (>10^9 Ω)

• Low noise characteristics

Metal-Oxide-Semiconductor FET (MOSFET)

MOSFETs are the most widely used transistors in digital circuits and power applications.

MOSFET Types:

Type Mode Channel Gate Voltage Applications

Enhancement N-Channel Normally OFF N-type Positive to turn ON Logic circuits, switching

Enhancement P-Channel Normally OFF P-type Negative to turn ON Complementary circuits

Depletion N-Channel Normally ON N-type Negative to turn OFF Analog circuits

Depletion P-Channel Normally ON P-type Positive to turn OFF Analog circuits

MOSFET Symbol

Enhancement N-Channel:    Enhancement P-Channel:

       D                        D

       |                        |

       ▼                        ▼

G ──►||◄                  G ──►||◄

     ||                        ||

       |                        |

       ▼                        ▲

       S                        S

FET Characteristics and Parameters

Key MOSFET Parameters:

Parameter Symbol Description Importance

Threshold Voltage VTH Gate voltage to start conduction Turn-on characteristic

Transconductance gm Change in ID per change in VGS Amplification capability

Drain Current ID Current through drain-source Power handling

Gate-Source Voltage VGS Control voltage Input signal

Drain-Source Resistance RDS(on) ON-state resistance Power efficiency

Gate Capacitance Ciss Input capacitance Switching speed

FET Applications

FETs dominate in several key areas:

1. Digital Logic - Low power consumption, high integration density

2. Power Switching - High efficiency, low RDS(on)

3. RF Applications - High input impedance, low noise

4. Analog Amplifiers - High input impedance, voltage control

5. Motor Drives - High power handling, fast switching

Specialized Transistor Types

Darlington Transistor

A Darlington pair consists of two BJTs connected to provide very high current gain.

Characteristics:

• Current gain: β1 × β2 (typically 1000-50000)

• Higher VBE (≈1.4V for silicon)

• Slower switching speed

• Built-in base resistors in integrated versions

Applications:

• High current switching

• Power amplification

• Motor drivers

Phototransistor

A BJT designed to respond to light instead of base current.

Structure:

• Exposed base-collector junction acts as photodiode

• Light generates base current

• High sensitivity to light

Applications:

• Optical switches

• Light sensors

• Optocouplers

• Position sensing

Unijunction Transistor (UJT)

A three-terminal device with unique switching characteristics.

Structure:

• N-type bar with P-type emitter

• Two bases (B1, B2) and one emitter

• Negative resistance region

Applications:

• Oscillators

• Pulse generators

• Timing circuits

• SCR triggering

Insulated Gate Bipolar Transistor (IGBT)

Combines advantages of BJT and MOSFET technologies.

Characteristics:

• MOSFET input characteristics (high impedance)

• BJT output characteristics (low saturation voltage)

• High power handling capability

• Medium switching speed

Applications:

• Power inverters

• Motor drives

• Welding equipment

• Induction heating

Comparison of Transistor Types

Characteristic BJT JFET MOSFET IGBT

Control Type Current Voltage Voltage Voltage

Input Impedance Low (kΩ) Very High (GΩ) Very High (TΩ) High (MΩ)

Switching Speed Fast Medium Very Fast Medium

Power Handling Medium Low High Very High

Voltage Drive Low Medium Low Low

Cost Low Medium Low High

Noise Medium Low Medium Medium

Temperature Stability Poor Good Good Good

Performance Comparison Chart

Application Best Choice Second Choice Reason

High Frequency Amplification BJT MOSFET Fast switching, good gain

Power Switching MOSFET IGBT Low RDS(on), fast switching

Low Noise Amplification JFET MOSFET Low noise figure

Digital Logic MOSFET BJT Low power, high density

High Current Switching IGBT MOSFET High power capability

Selection Criteria

Choosing the Right Transistor

For Amplification Applications:

1. Frequency Response - BJT for high frequency, FET for wide bandwidth

2. Gain Requirements - BJT for high current gain, FET for voltage amplification

3. Input Impedance - FET for high impedance sources

4. Noise Performance - JFET for low noise applications

For Switching Applications:

1. Power Level - MOSFET for medium power, IGBT for high power

2. Switching Speed - MOSFET for high frequency, BJT for fast rise times

3. Drive Requirements - MOSFET for low drive power

4. Cost Considerations - BJT for cost-sensitive applications

Design Considerations Table

Factor BJT Priority FET Priority

Power Consumption Medium Low (CMOS)

Component Count Higher (bias network) Lower

Temperature Drift Significant Minimal

ESD Sensitivity Low High (MOSFET)

Drive Complexity Higher Lower

Testing and Troubleshooting

Basic Transistor Testing

BJT Testing with Multimeter:

1. Diode Test Mode:

   • Forward bias B-E junction: ~0.7V

   • Forward bias B-C junction: ~0.7V

   • Reverse bias should show high resistance

2. Gain Testing:

   • Use transistor tester or curve tracer

   • Measure IC vs IB characteristics

FET Testing:

1. JFET Testing:

   • Gate-channel junctions should be high resistance

   • Pinch-off voltage measurement

2. MOSFET Testing:

   • Gate isolation check (infinite resistance)

   • Threshold voltage measurement

   • RDS(on) measurement

Common Failure Modes

Transistor Type Common Failures Symptoms Causes

BJT Junction damage No amplification Overvoltage, overcurrent

BJT Thermal runaway Excessive heating Poor heat sinking

MOSFET Gate oxide damage Gate short ESD, overvoltage

MOSFET Body diode failure Permanent conduction Reverse voltage

Future Trends

Emerging Transistor Technologies

Wide Bandgap Semiconductors:

• GaN (Gallium Nitride): Higher frequency, higher power density

• SiC (Silicon Carbide): High temperature, high voltage applications

Advanced MOSFET Structures:

• Super Junction MOSFETs: Lower RDS(on), higher voltage ratings

• Trench MOSFETs: Improved power density

• GaN HEMTs: Ultra-high frequency applications

Market Trends

Technology Growth Driver Applications

GaN 5G, Fast Charging RF amplifiers, Power adapters

SiC Electric Vehicles Inverters, Charging systems

Advanced Si Efficiency demands Server power, Motor drives

Conclusion

Transistors remain the fundamental building blocks of modern electronics, with each type offering unique advantages for specific applications. BJTs excel in high-frequency amplification and precise current control, while FETs dominate in digital applications and power switching due to their voltage control and high input impedance.

The choice between transistor types depends on specific application requirements including power levels, frequency response, control requirements, and cost constraints. As technology advances, new materials like GaN and SiC are pushing the boundaries of what's possible in terms of power density, switching speed, and operating temperature.

Understanding the characteristics and trade-offs of different transistor types is essential for effective circuit design and troubleshooting. Whether designing a simple amplifier or a complex power management system, selecting the appropriate transistor technology is crucial for optimal performance and reliability.

Key Takeaways

• BJTs are current-controlled devices ideal for linear amplification and high-frequency applications

• JFETs offer low noise and high input impedance for sensitive analog circuits

• MOSFETs provide efficient switching and are the backbone of digital electronics

• Specialized transistors like IGBTs and Darlington pairs serve specific high-power applications

• Future technologies like GaN and SiC are enabling new levels of performance and efficiency

The transistor revolution that began in 1947 continues to evolve, driving innovations in everything from smartphones to electric vehicles, making transistors one of the most important inventions in human history.

Transistor

Technical Specification:

• Type: NPN or PNP (BJT), N-channel or P-channel (MOSFET).

• Voltage Rating: 20V to 1,200V.

• Current Rating: 0.1A to 50A.

• Power Dissipation: Up to 150W.

• Switching Speed: 10ns to 100ns.