Transistors — BJT and MOSFET Fundamentals

Section Overview

The diode was your first semiconductor; the transistor is the one that changed the world. Its single, powerful idea is control: a small signal at one terminal controls a much larger current through the other two. That one ability underlies the two things a transistor does — act as a switch (fully on or fully off, the basis of all digital logic and power switching) and act as an amplifier (a small input variation producing a larger output, the basis of audio, radio, and sensing). This section introduces the transistor through its two great families: the bipolar junction transistor (BJT), which is current-controlled, and the MOSFET, which is voltage-controlled. You'll learn their terminals, how they differ, what current gain means, how to identify their packages, and how to test one — building directly on the PN-junction diode from Section 4.4.

Why This Matters

Almost everything active in electronics is built from transistors — billions of them in a single processor, and discrete ones throughout every power supply, amplifier, and control circuit. For a repair technician, transistors are frequent and important failure points: a shorted power transistor in a supply or amplifier is one of the most common serious faults, often taking a fuse or other parts with it. Understanding whether a transistor is a switch or an amplifier, whether it's a BJT or a MOSFET, and how to identify and test it is central to diagnosing active circuits. And because the transistor builds directly on the diode junction you just learned, it's a natural next step — the BJT is, for testing purposes, a pair of diode junctions sharing a terminal. This section gives you the working understanding that the rest of active electronics depends on.

Required Prerequisites

Diodes — Function and Types — the PN junction and diode-mode testing; a BJT behaves like two diode junctions, and this section builds on that directly.
Current — The Flow of Electrons — a BJT controls current with current, so the idea of current flow is central.
Ohm's Law — The Foundation of Circuit Analysis — the voltages and currents around a transistor are reasoned with Ohm's law.

Recommended Consumables

No consumables required. A few transistors — new or salvaged — and their datasheets make ideal, reusable study material.

Recommended Practice Hardware

An assortment of transistors: a small-signal BJT (like a 2N3904 NPN or 2N3906 PNP), a power transistor in a TO-220 package, and a MOSFET if you have one
A multimeter with a diode mode for junction testing, and — if available — an inexpensive component/transistor tester, which identifies type, pinout, and gain automatically
Internet or datasheet access to look up pinouts, since they vary by part

No powered circuit is needed; you identify and test loose or de-energized transistors.

Real-World Applications

Transistors are everywhere active happens. As switches, they form the logic gates of every processor and memory, and they switch power in supplies, motor drivers, and LED drivers — a MOSFET turning a load on and off thousands of times a second in a switch-mode supply. As amplifiers, they boost the tiny signal from a microphone, antenna, or sensor to a usable level in audio gear, radios, and instrumentation. Discrete power transistors drive speakers and motors; small-signal transistors switch and buffer throughout a board. When a device with active circuitry fails — a dead amplifier channel, a supply that won't start, a control that won't switch — a shorted or open transistor is a leading suspect, which is why identifying and testing them is a core repair skill.

Common Challenges

Confusing the two families' terminals and control. A BJT has a base, collector, and emitter and is controlled by base current; a MOSFET has a gate, drain, and source and is controlled by gate voltage. Mixing up the terminal names or the control mechanism is the most common beginner confusion.
Assuming a pinout. Transistor pinouts are not standardized — two parts in the same package can have different pin orders. You must identify the specific part number and check its datasheet; guessing damages parts.
Forgetting MOSFET static sensitivity. A MOSFET's gate is insulated and easily damaged by static electricity (ESD). Handling one carelessly can destroy it before it's even installed.

Safety Notes

Risk Level: Low. Identifying and testing loose or de-energized transistors is low-risk. Two cautions apply.

Professional Tips Before Starting

Decide BJT-or-MOSFET and switch-or-amplifier first. Those two questions frame everything else: a BJT is current-controlled with base/collector/emitter; a MOSFET is voltage-controlled with gate/drain/source; a switch runs fully on or off; an amplifier runs in between.
Never trust a pinout from memory. Read the part number off the package and look up the datasheet — pin order differs even within the same package style. A component tester will also report the pinout and type automatically.
For a BJT, think "two diodes" when testing: the base-emitter and base-collector junctions each read like a diode in diode mode. That mental model makes a basic multimeter a useful transistor checker.

Understanding, Identifying, and Testing Transistors

The Core Idea: Control, as Switch and Amplifier

Every transistor does one fundamental thing: a small signal on its control terminal governs a much larger current through its other two terminals. That control is the source of two roles. As a switch, the transistor is driven fully on (conducting freely, low resistance — called saturation) or fully off (blocking, called cutoff); this is how digital logic and power switching work, with the transistor acting like a fast, controllable relay with no moving parts. As an amplifier, the transistor is operated in between — in its linear or active region — so that a small variation in the input produces a larger, faithful variation in the output; this is how a microphone's millivolts become a speaker's watts. Switching and amplifying are the same control ability used two different ways.

The BJT (Bipolar Junction Transistor)

The bipolar junction transistor has three terminals — the base (control), the collector, and the emitter — and comes in two polarities, NPN and PNP (the more common NPN conducts collector-to-emitter when the base is driven positive). Its defining feature is that it is current-controlled: a small current into the base controls a much larger current from collector to emitter. The base-emitter path is a PN junction and behaves like a diode, needing about 0.6 to 0.7 V to turn on — the same forward drop you met in Section 4.4.

The multiplication factor is the current gain, written hFE or the Greek letter beta (β): the collector current is beta times the base current. So with a beta of 100, a base current of 1 mA produces a collector current of about:

Ic = β × Ib = 100 × 1 mA = 100 mA

That's the amplification in numbers — a small base current commanding a hundred-times-larger collector current. Typical betas run from around 20 for power transistors to several hundred for small-signal types; it's an approximate, part-dependent figure, not a precise constant, but the idea is exact: small base current in, larger collector current out.

The MOSFET (Field-Effect Transistor)

The MOSFET — metal-oxide-semiconductor field-effect transistor — also has three terminals, but with different names and a different control mechanism: the gate (control), the drain, and the source. It comes in N-channel and P-channel types. Its defining feature is that it is voltage-controlled: a voltage on the gate (not a current) controls the current from drain to source. The gate is separated from the rest of the device by a thin insulating layer, so it draws almost no current — the MOSFET has a very high input impedance. That makes MOSFETs extremely efficient switches (the gate takes almost no power to hold on or off), which is why they dominate power switching in modern supplies and motor drives. The trade-off is that the insulated gate is delicate: a static discharge can puncture it, so MOSFETs are ESD-sensitive and need anti-static handling.

The headline contrast to keep is simply this: a BJT is current-controlled (base current), a MOSFET is voltage-controlled (gate voltage) — two routes to the same control-a-large-current-with-a-small-signal idea.

Packages and Pinouts

Transistors come in standard packages you'll learn to recognize: TO-92 (a small plastic half-cylinder, for small-signal parts), TO-220 (a larger plastic body with a metal tab for bolting to a heatsink, for power parts), and SOT-23 (a tiny surface-mount package), among others. But a crucial warning: the pinout is not fixed by the package. Two TO-92 transistors can have entirely different base/collector/emitter orders, and MOSFET and BJT pin assignments differ. You must read the specific part number printed on the body and consult its datasheet (or use a component tester) to get the pinout right — never assume it from the package shape.

Testing a Transistor

A BJT is testable with a multimeter's diode mode, because it behaves like two diode junctions sharing the base. For an NPN, the base is the common anode of two junctions: put the red (+) probe on the base and the black on the emitter, then on the collector, and each reads a forward drop of about 0.6 to 0.7 V; reverse the probes and both read OL. For a PNP it's the mirror image — the base is the common cathode, so it reads forward (about 0.7 V) with the black probe on the base. A good BJT shows two healthy junctions this way; a shorted transistor reads low (near 0) across terminals that should block — often collector-to-emitter — and an open one fails to show the expected junction. MOSFETs are harder to check reliably with a basic meter, because turning the channel on requires charging the gate. One quick basic-meter check is the MOSFET's intrinsic body diode: in diode mode it reads a diode drop one way (source to drain) and OL the other, like an ordinary diode — useful for spotting a dead part, though it does not confirm the gate works. For a full check, a dedicated component/transistor tester — which identifies type, pinout, and gain automatically — is the practical tool for MOSFETs and a great convenience for BJTs too.

Common Mistakes

Swapping the terminal sets. BJT = base/collector/emitter (current-controlled); MOSFET = gate/drain/source (voltage-controlled). Don't cross them.
Assuming a pinout from the package. Pin order varies by part number; always check the datasheet or use a tester. Guessing can destroy the part or the circuit.
Handling a MOSFET without ESD care. The insulated gate is static-sensitive; careless handling can kill a MOSFET before use.
Expecting exact gain. Beta varies part to part and with conditions; treat it as approximate. The control relationship is reliable; the precise number is not.

Troubleshooting Guidance

Transistors are high-value diagnostic targets in active circuits. A shorted transistor — most often collector-to-emitter (BJT) or drain-to-source (MOSFET) — is a common serious failure: a shorted power transistor in a supply or amplifier can blow fuses, damage adjacent parts, and leave a stage dead, so when an amplifier channel is silent or a supply won't start, test the power transistors early. Use diode mode on a de-energized BJT to check its two junctions: two good ~0.7 V junctions (one way) with OL the other way indicates a healthy device, while a near-zero reading across collector-emitter signals a short and a missing junction signals an open. Because in-circuit readings are muddied by surrounding parts, a suspect transistor often has to be removed to test cleanly, and a component tester makes this far quicker for both BJTs and MOSFETs. When replacing, identify the exact part from its markings, get the type and pinout from the datasheet, and fit the same part or a proper equivalent — matching polarity (NPN/PNP or N/P-channel), voltage and current ratings, and gain class. Orientation and pinout matter as much as with any directional part: a transistor installed with the wrong pin order will not work and may be damaged.

Verification & Testing Methods

Check your understanding before moving on:

[ ] Explain how a transistor uses a small signal to control a larger current, and describe the switch and amplifier roles.
[ ] Name a BJT's three terminals, state that it is current-controlled, and use Ic = β × Ib for a given beta and base current.
[ ] Name a MOSFET's three terminals, state that it is voltage-controlled, and explain why its gate is ESD-sensitive.
[ ] Describe how to test an NPN BJT's junctions in diode mode and why you must consult a datasheet for the pinout.

Then try the practice exercises below — identifying and reasoning about loose, de-energized transistors.

Practice Exercises

Name the family (5 minutes, reasoning). For each description, state whether it's a BJT or a MOSFET and name its three terminals: (a) current-controlled, with a base; (b) voltage-controlled, with a gate; (c) has a drain and a source; (d) has a collector and an emitter.
Gain arithmetic (5 minutes, pen and paper). A BJT has a beta of 200. (a) What collector current results from a base current of 0.5 mA? (b) A different transistor with beta 100 has a base current of 0.1 mA — what is its collector current? Use Ic = β × Ib.
Test an NPN (10 minutes, with a transistor and meter). Using diode mode on an NPN BJT, put the red probe on the base and measure to the emitter and to the collector; then reverse. Record the readings and explain why the base is the "common anode" and what a good result looks like.
Plan a replacement (10 minutes, reasoning). A power transistor in a TO-220 package is shorted. Describe the steps to identify it and choose a replacement — what you read off the part, what you look up, and which ratings and properties the replacement must match. Explain why you cannot assume its pinout from the package.

These core ideas — the control idea and the switch/amplifier roles, the BJT (terminals, current-controlled, beta), the MOSFET (terminals, voltage-controlled, ESD), packages/pinouts, and diode-mode testing — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

A transistor uses a small signal to control a much larger current, giving it two roles: a switch (fully on/saturation or off/cutoff — digital logic and power switching) and an amplifier (linear region — a small input makes a larger output).
A bipolar junction transistor (BJT) has a base, collector, and emitter, comes as NPN and PNP, and is current-controlled: a small base current controls a larger collector current, with the base-emitter junction acting like a ~0.7 V diode.
Current gain (hFE or beta, β) is the multiplier: Ic = β × Ib, so β = 100 and Ib = 1 mA give Ic ≈ 100 mA — approximate and part-dependent, but the control relationship is exact.
A MOSFET has a gate, drain, and source, comes as N- and P-channel, and is voltage-controlled with a very high input impedance (the gate draws almost no current), making it an efficient switch — but its insulated gate is ESD-sensitive.
Packages (TO-92, TO-220, SOT-23) identify the physical part, but pinouts vary by part number — always consult the datasheet or a component tester; never assume pin order.
A BJT tests like two diode junctions in diode mode (NPN base = common anode, PNP base = common cathode); a shorted transistor (often collector-emitter or drain-source) is the common serious failure; MOSFETs are best checked with a component tester.

Skills Learned

You can now explain the transistor's control idea and its switch and amplifier roles.
You can now name a BJT's terminals, state that it's current-controlled, and apply Ic = β × Ib.
You can now name a MOSFET's terminals, explain why it's voltage-controlled, and handle it with ESD care.
You can now identify common packages, look up a pinout, and test a BJT's junctions in diode mode.
You can now recognize a shorted transistor as a common active-circuit failure and plan a correct replacement.

Glossary Additions

transistor — a semiconductor component that uses a small signal to control a much larger current, enabling both switching (fully on or off) and amplification (a small input producing a larger output); the fundamental building block of active electronics.
bipolar junction transistor — a transistor with base, collector, and emitter terminals, made as NPN or PNP, that is current-controlled: a small base current controls a larger collector current, with a current gain called hFE or beta. Abbreviated BJT.
MOSFET — a metal-oxide-semiconductor field-effect transistor, with gate, drain, and source terminals, made as N- or P-channel, that is voltage-controlled: a gate voltage controls the drain-source current. Its insulated gate gives a very high input impedance but is sensitive to static (ESD).
current gain — for a BJT, the factor (hFE or beta, β) by which base current is multiplied to give collector current: Ic = β × Ib; an approximate, part-dependent value typically from around 20 (power) to several hundred (small-signal).

Suggested Next Sections

Must read next:

Voltage Regulators — Linear and Switching — from raw transistors to the ICs built around them: how a voltage regulator holds a steady output, the linear and switching approaches, and how to identify and check one.

Recommended:

Diodes — Function and Types — the PN junction and diode-mode testing that a BJT's two-junction behavior builds on.
Resistors — Types, Values, and Markings — the identification approach that started this chapter, now applied to active parts.