What is a transistor?

A transistor is a three-terminal semiconductor device that switches or amplifies an electrical signal - a small input controlling a much larger output. Since the invention of the first working transistor in 1947, it has become the basic building block of almost all modern electronic devices and electronic circuits.

For a full introduction to what transistors are and where they are used, see what is a transistor. This article focuses on the physics of how they work.

Transistors as switches and amplifiers

A basic transistor performs one of two jobs: switching or amplifying. As a switch, the transistor behaves like an electronic switch: fully off it is an open circuit (no current flows), and fully on it acts like a closed path - close to a short circuit - letting current through. As an amplifier, a small input signal controls a much larger output, boosting weak signals. Billions of these switches strung together form the logic gates inside computer chips.

Explanation of semiconductors

Transistors are made up of silicon or other semiconductor materials, like germanium, gallium nitride (GaN) or silicon carbide (SiC). Unlike conductors (metal) and insulators (rubber), semiconductors allow or block electricity based on specific conditions.

Their conductivity can be precisely controlled through temperature, light, or by adding impurities, making them perfect for controlling electrical signals.

Doping means intentionally adding small amounts of foreign atoms to silicon to alter its electrical properties.

  • N-type silicon: Doped with phosphorus or arsenic, which donate extra free electrons (negative charge carriers)
  • P-type silicon: Doped with boron, which creates "holes" - absence of electrons that behave as positive charge carriers

When N-type and P-type materials meet, they form a PN junction. This junction creates a depletion region, where mobile carriers are reduced, developing an internal electric field.

Why this matters: The PN junction is not just a transistor concept — it is the foundation of every diode, solar cell, LED, photodetector, and thyristor. Understanding junction physics gives you an intuitive model for the behaviour of virtually all active semiconductor devices.

This region establishes a built-in electric field (contact potential, ~0.6–0.7 V for silicon) that opposes further diffusion. Applying an external voltage can either forward-bias the junction (allowing current to flow) or reverse-bias it (blocking current). This behavior is the electrical foundation of all diodes and transistors.

Electric current in semiconductors is carried by two types of charge carriers:

  • Electrons (negative charge) in N-type material
  • Holes (effective positive charge) in P-type material

Devices that rely on both types of carrier, such as BJTs, are called bipolar. Devices that use only one carrier type, such as MOSFETs, are called unipolar.

Transistors are made up of silicon or other semiconductor materials, like germanium, gallium nitride (GaN) or silicon carbide (SiC). Unlike conductors (metal) and insulators (rubber), semiconductors allow or block electricity based on specific conditions.

Their conductivity can be precisely controlled through temperature, light, or by adding impurities, making them perfect for controlling electrical signals.

Doping means intentionally adding small amounts of foreign atoms to silicon to alter its electrical properties.

  • N-type silicon: Doped with phosphorus or arsenic, which donate extra free electrons (negative charge carriers)
  • P-type silicon: Doped with boron, which creates "holes" - absence of electrons that behave as positive charge carriers

When N-type and P-type materials meet, they form a PN junction. This junction creates a depletion region, where mobile carriers are reduced, developing an internal electric field.

Why this matters: The PN junction is not just a transistor concept — it is the foundation of every diode, solar cell, LED, photodetector, and thyristor. Understanding junction physics gives you an intuitive model for the behaviour of virtually all active semiconductor devices.

This region establishes a built-in electric field (contact potential, ~0.6–0.7 V for silicon) that opposes further diffusion. Applying an external voltage can either forward-bias the junction (allowing current to flow) or reverse-bias it (blocking current). This behavior is the electrical foundation of all diodes and transistors.

Electric current in semiconductors is carried by two types of charge carriers:

  • Electrons (negative charge) in N-type material
  • Holes (effective positive charge) in P-type material

Devices that rely on both types of carrier, such as BJTs, are called bipolar. Devices that use only one carrier type, such as MOSFETs, are called unipolar.

The two main types of transistors: BJTs and FETs

The most common transistors fall into two broad families that work on different principles:

  • Bipolar junction transistors (BJTs) are current-controlled and conduct using both electrons and holes (bipolar).
  • Field effect transistors (FETs), which include the MOSFET and JFET, are voltage-controlled and use only one carrier type (unipolar).

The two sections below explain how each works. For the FET family in depth - MOSFETs, JFETs, and their characteristics - see what is a field effect transistor (FET).

How BJTs work

A BJT consists of three semiconductor layers forming two PN junctions. It is a current-controlled device: a small current flowing into (or out of) the transistor base controls a much larger current between the collector and emitter. The voltage from base to emitter decides whether it conducts, so a tiny base current commands a large current through the load — steering the flow of electricity through the device.

The key figure of merit is current gain (β or hFE) - the ratio of collector current to base current: β = IC / IB

Typical values range from 20 to 500 depending on the device. A β of 100 means 1 mA of base current enables 100 mA of collector current.

NPN transistors are the most common type. Current flows from collector to emitter when the base is driven with a positive voltage relative to the emitter. PNP transistors operate with reversed polarity - the base must be pulled below the emitter voltage to conduct. PNPs are commonly used in high-side switching and complementary push-pull output stages.

BJTs excel in:

  • Precision linear amplification
  • Low-cost general-purpose switching
  • Current mirror circuits
  • Audio amplification stages
  • Applications requiring predictable VBE characteristics (e.g., temperature sensing)

Their main limitation is the continuous base current requirement - the driver must constantly supply IB to keep the BJT conducting, which adds to system power consumption.

For bipolar transistor products, see Infineon's bipolar transistor portfolio.

How MOSFETs work as a switch and amplifier

A MOSFET transistor is the most common field effect transistor and works the opposite way to a BJT: it is voltage-controlled. A voltage on the gate sets up an electric field that forms a conducting channel between the drain and source, while almost no current flows into the gate itself. This makes MOSFETs very efficient to drive and extremely fast to switch - which is why, in CMOS form, billions of them make up the logic on a single chip.

For the full working principle - structure, how it is controlled, the current flow, and what makes it efficient - see how MOSFETs work .

To explore available devices, browse Infineon's MOSFET portfolio.

Additional documents to continue exploring

Frequently asked questions about how transistors work

What does a transistor do?

A transistor does one of two jobs in a transistor circuit: it switches an electrical signal on and off, or it amplifies a weak signal into a stronger one. A small input at one terminal controls a much larger current between the other two.

A transistor operates by using a small signal at its control terminal to govern a much larger current through the device. In a BJT, a base current controls the collector-emitter current; in a FET, a gate voltage controls the drain-source current. Either way, a tiny input commands a large output.

NPN turns on when the base is driven positive relative to the emitter; PNP turns on when the base is pulled below the emitter. NPN is the default choice; PNP is used in high-side switches and complementary output stages.

Selecting the right transistor involves evaluating several factors:

  • Voltage and current requirements
  • Switching speed or frequency
  • Power dissipation capability
  • Package type and thermal limits
  • Gate or base drive requirements

Engineers typically consult the transistor datasheet and application notes to ensure compatibility with the target circuit.

It depends on the job. Use a BJT for low-cost switching and precise linear amplification, a MOSFET for efficient power switching, motor control, and digital logic, and a JFET for low-noise analog work. 

How you drive a transistor depends on its type. A BJT is current-driven: you supply a continuous base current - roughly the load current divided by the gain β - to keep it on. A MOSFET is voltage-driven: you apply a gate voltage above its threshold, and only have to charge the gate capacitance, with no steady current. 

For driving MOSFETs in detail, see how does a MOSFET work.

It depends on the type. A BJT needs a continuous base current - roughly the collector (load) current divided by the current gain β, so a higher-gain device needs less drive current. A MOSFET needs almost no steady current at all: the gate only has to be charged and discharged, so the "drive current" is the brief pulse that moves the gate charge each switching cycle.

Transistors come in many packages suited to different power levels - from small surface-mount types such as SOT-23 to through-hole power packages such as TO-220, with larger packages able to dissipate more heat. 

See all Infineon packages

Yes - Infineon provides online product finders and parametric search tools that let you filter its transistor and power MOSFET portfolio by voltage, current, package, RDS(on), and other parameters to quickly shortlist the right device for your application.

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