A Buying Guide for Transistors

Whether you're building, repairing, or maintaining electronics, you'll end up needing transistors. Universally useful and adaptable, the transistor is an essential component of electronic circuitry. Wherever you need to control the flow of electricity in a device - from transistor radios, to diodes, to circuit boards - you can find a great collection of transistors on eBay.

What types of transistors are there?
  • Switch - At its most basic, a transistor is a switch. It allows the flow of electrons only under certain conditions. Those conditions are determined by how the semiconductor material, silicon, is doped and layered.
  • Amplifier - The semiconductor nature of silicon as a raw material can be transformed to create an amplifier which encourages voltage flow along the circuit in a device.
  • NPN - The most common type of transistor is bipolar, and NPN is one sub-type. It allows for quick flow along the circuit from the voltage collector to emitter.
  • PNP - This is another type of bipolar transistor that functions in reverse of NPN. The power flows along the circuit from emitter to collector. Its default state is ON when there is no voltage flowing through the device.
What are some gains of using a transistor?
  • Acts as an amplifier - The gain is a rating of how much the transistor will amplify, or increase the voltage, as power passes through it.
  • Allows circuit control - You need to know how much power will flow through your circuit in order for your electronics to function properly. Knowing the gain of your semiconductor component allows you greater control since you can calculate precisely what voltage is delivered.
How does a transistor act as a switch?
  • Function - Voltage flows through the transistor in a certain way, determined by how the semiconductor material is doped and layered. The most common way that this functions is to signal ON or OFF, 1 or 0. Power either flows through or it doesn't. Based on this, your electronics system knows whether the circuit is signaling ON.
  • NPN or PNP - Common configurations of semiconductor material within the device, NPN or PNP, signal differently. In NPN, the power is OFF when no voltage is flowing through the circuit, and vice versa.
  • Binary application - Generally, there are two states in a switch: ON or OFF. This translates to 0 or 1, when the circuit is interpreted by your electronics. Data can be stored as 0's and 1's within the device. This is the basis for all of your electronic devices.
How do you select a transistor?

When selecting a new or gently used transistor on eBay, consider the functions offered by the device. Based on functionality, you can select a small signal transistor, power transistor, or high frequency/RF transistor. Some of the brands that make transistors include Fairchild Industries, Motorola, RCA, STMicroelectronics, International Rectifier, Toshiba, and Vishay. There are unbranded transistors as well. Consider your needs and the intended use before making a final selection.