Transistors: Function of Switches & Amps
- Sylvia Rose
- Mar 19
- 3 min read
A transistor is a semiconductor device. It can operate as either a switch or an amplifier. Made from materials like silicon or germanium, transistors conduct or pause electricity under specific conditions.
By manipulating materials, engineers create two types of charge carriers. These are electrons, which carry a negative charge, and holes, which carry a positive charge.
Where they meet creates a p-n junction, through which transistors modulate electrical signals. Silicon makes up about 95% of the semiconductor industry due to excellent conductivity when doped (treated) with other elements.
Design
Transistors consist of three terminals:
Base (or Gate): This terminal controls the flow of current between the other two.
Collector (or Drain): This is where the current flows from, in a Bipolar Junction Transistor (BJT) or the positive terminal in a Field-Effect Transistor (FET).
Emitter (or Source): This is where the current flows to, in a BJT or the negative terminal in a FET.
There are two main types of transistors, using different mechanisms to control the current flow:
Bipolar Junction Transistor (BJT)
BJTs come in two varieties: NPN and PNP, differing in the doping of the semiconductor layers.
BJTs are composed of three layers of semiconductor material and can be configured as either NPN (negative-positive-negative) or PNP (positive-negative-positive).
NPN Transistors: A small current at the base allows a larger current to flow between collector and emitter. In a regular NPN transistor, a base current of only 5 mA enables a collector current of 100 mA, a current gain of 20.
PNP Transistors: These operate similarly, but use holes as the primary charge carriers.
In an NPN transistor, a small electrical current flows into the base, allowing a larger current to pass from the collector to the emitter.
This occurs because the base-emitter junction is forward-biased, allowing electrons (n) to flow into the base and interact with holes (p). In audio equipment, transistors can amplify signals from a mic to drive speakers.
Field-Effect Transistor (FET)
FETs use an electric field applied to the gate terminal to control flow of current between source and drain. FETs are voltage-controlled devices. FETs use an electric field to control conductivity of the semiconductor.
There are two families of FETs: Junction Field-Effect Transistors (JFETs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). MOSFETs are the most widely used transistor type in digital circuits due to low power consumption and high density.
A MOSFET may control power usage by operating at voltages as low as 1 volt, efficient and ideal for battery-operated devices. In an N-channel MOSFET, a positive voltage is applied to the gate terminal.
This creates an electric field and attracts electrons to the channel between the drain and source terminals. It allows current to flow easily. A well-designed MOSFET can achieve on-resistance as low as 0.1 ohms.
Amplification and Switching
The power of the transistor lies in its ability to either amplify or switch signals.
Amplification: When a small signal is applied to the base (BJT) or gate (FET) terminal, it can control a much larger current flowing between the collector and emitter (BJT) or drain and source (FET).
The transistor boosts weak signals. A typical guitar amplifier uses transistors to boost electric signal from the guitar pickup, allowing the sound to be heard loudly through speakers.
Switching: By applying sufficient voltage to the base or gate terminal, the transistor rapidly switches between fully on and fully off states, allowing or blocking the flow of current. This ability is the foundation of digital logic.
The "on" state represents a "1" and the "off" state represents a "0." Computers can perform complex calculations using binary code.
Modern CPUs use billions of transistors. They're able to process more than 3 billion instructions per second.
The invention of the transistor leads to microchips or integrated circuits (ICs), which can contain millions of them on a single chip. An integrated circuit once requiring an entire room can now fit into a chip smaller than a fingernail. Most smartphones have over 10 billion transistors.
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