Ultimate Transistor Guide

Introduction to Transistors

Transistors are fundamental semiconductor devices used to amplify and switch electronic signals. They have three terminals and can control current flow in a circuit. Transistors revolutionized electronics, enabling everything from simple switches to high-speed digital computing. They form the building blocks of analog circuits, digital circuits, and power electronics.

The first transistor was invented in 1947 at Bell Labs by John Bardeen, Walter Brattain, and William Shockley. Its invention replaced bulky vacuum tubes and made modern electronics possible. Transistors are categorized into bipolar junction transistors (BJT) and field-effect transistors (FET), each having distinct properties and applications.

• Bipolar Junction Transistor (BJT): Current-controlled, three layers (emitter, base, collector).
• Field Effect Transistor (FET): Voltage-controlled, three terminals (source, gate, drain).
• MOSFET (Metal Oxide Semiconductor FET): A type of FET with insulated gate, high input impedance, and wide usage in digital electronics.
• IGBT (Insulated Gate Bipolar Transistor): Combines MOSFET gate control with high-power BJT characteristics.
• Phototransistors: Light-sensitive devices for sensing and opto-isolation.

Bipolar Junction Transistors (BJT)

BJTs are three-layer semiconductor devices with two PN junctions. They are classified into two types:

• NPN: Electrons are the majority carriers, current flows from collector to emitter.
• PNP: Holes are the majority carriers, current flows from emitter to collector.

BJT Terminals

• Emitter: Heavily doped terminal that emits charge carriers into the base.
• Base: Thin, lightly doped region that controls the transistor’s operation.
• Collector: Collects carriers from the base and provides output current.

BJT Operation

In forward active mode, the base-emitter junction is forward-biased, and the base-collector junction is reverse-biased. The base current (Ib) controls the collector current (Ic) through the current gain β (Ic = β × Ib). The emitter current (Ie) is the sum of Ic and Ib (Ie = Ic + Ib).

Cutoff, Active, and Saturation

• Cutoff: Both junctions reverse-biased, transistor OFF.
• Active: Base-emitter forward biased, base-collector reverse biased, transistor amplifies.
• Saturation: Both junctions forward biased, transistor fully ON, acts as a switch.

BJT Configurations

BJT can be connected in three main configurations:

• Common-Emitter (CE): Most widely used, provides voltage and current gain.
• Common-Base (CB): Input at emitter, output at collector, low input impedance, high voltage gain.
• Common-Collector (CC): Also called emitter follower, voltage gain ≈ 1, high input impedance, low output impedance.

Common-Emitter Amplifier

The CE amplifier provides significant voltage gain. The input signal is applied between base and emitter, and output is taken from collector and emitter. Biasing resistors ensure proper operating point.

Common-Base Amplifier

CB amplifier has low input impedance, suitable for high-frequency applications. Current gain < 1, voltage gain high.

Common-Collector Amplifier

CC or emitter follower has unity voltage gain but provides impedance matching between high-impedance sources and low-impedance loads.

BJT Biasing Techniques

Proper biasing stabilizes transistor operation in the active region:

• Fixed bias: Simple but unstable with temperature changes.
• Collector-to-base bias: Slightly more stable.
• Voltage divider bias: Most widely used, stable operating point.
• Emitter-stabilized bias: Improves thermal stability by adding emitter resistor.

Field Effect Transistors (FET)

FETs are voltage-controlled devices. Current flows from drain to source depending on gate voltage.

JFET

• Gate-source reverse-biased controls channel conduction.
• High input impedance.
• Pinch-off voltage defines the saturation region.

MOSFET

• Gate insulated from channel, very high input impedance.
• Enhancement-mode: Normally OFF, conducts when Vgs exceeds threshold.
• Depletion-mode: Normally ON, turns OFF when Vgs applied.
• Rds(on) critical for conduction losses.

Transistor Amplifiers

Transistors amplify voltage, current, and power. Key formulas:

Amplifiers can be:

• Small signal amplifiers
• Power amplifiers
• RF amplifiers
• Audio amplifiers
• Operational amplifier input stages

Transistor Switching

• Used as electronic switches
• Cutoff = OFF, Saturation = ON
• Switching speed affected by junction capacitances and charge storage
• Used in relay drivers, PWM circuits, logic gates

Power Transistors

Power transistors handle high currents and voltages. Cooling is critical. Darlington pairs increase current gain. IGBTs combine MOSFET gate control with BJT current handling.

• Applications: Motor control, inverters, SMPS, amplifiers
• Need heatsinks and thermal management
• High voltage and current ratings

Advanced Transistor Topics

• Frequency response: fT, high-frequency limitations
• Noise: Shot noise, thermal noise, flicker noise
• Temperature effects: Thermal runaway, compensation techniques
• Transistor matching in differential amplifiers
• Current mirrors and active loads
• ESD protection in MOSFET gates
• Switching times: Rise, fall, storage, delay

Practical Applications

• Audio amplifiers: CE and MOSFET circuits
• Digital switching: CMOS logic, TTL circuits
• Motor drivers and relays
• Voltage regulators and linear regulators
• PWM circuits for LEDs and motors
• RF amplifiers and oscillators
• Temperature sensing and thermal control circuits

Transistor Circuits and Applications

Transistor Quick Reference

Choosing Transistors for Projects