Test Yourself: Transistor Technology Basics

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Did you know…

  1. There are no known organisms that can see radio waves, which is probably a good thing. Our vision would be a constant haze from all of the radio waves we put out into the world.
  2. The microwave oven in your house and your Wi-Fi router produce the same frequency of waves, 2.4GHz – and that is 120,000 times higher of a frequency than the average human can hear, 20,000 Hz.

Aren’t microwaves and radio frequencies fun? Although the relationship between electricity and magnetism was discovered  in 1820s, the power of microwave and radio frequencies for electronic devices was truly harnessed with the invention of the transistor.

Today, we’ll be discussing different types of transistors – how far they have come and the pros and cons of each. Lumped into two large categories, transistors are either electromechanical or solid state, the older of the two being electromechanical.

Electromechanical Switches

Electromechanical switches rely on a mechanism to make contact with electrical actuators, as assumed by the name. Until physical contact is made, there can be no diverting of power. Because of the physicality and nature of these switches, switching speed is slowest with electromechanical switches (we’re talking milliseconds verses micro- and nanoseconds of course).

The other downside to electromechanical switches, and partially why engineers looked to make newer switches, is the mechanical vibrations and the shorter lifespan of these switches. Routing high frequencies over a long period of time can damage electromechanical switches.

As manufacturing improved throughout the 20th century, we saw the rise of microelectromechanical switches (frequently abbreviated MEMS). Although the general principle was the same as their predecessor, the electromechanical switch, changes in manufacturing made these switches smaller, lighter, more durable, and more precise.

Solid State Switches

Solid state switches opened the door to a diversity of different internal uses for transistors. Because they’re smaller, less prone to being environmentally effected, and unlike electromechanical switches they don’t spark, solid state switches can be used in computers and mobile devices, as well as explosion hazardous devices.

But this is not to say that solid state switches are without flaw. First, they can produce a lot of heat. Over time and depending on what sorts of materials are used to manufacture and attach the transistor, the heat produced can damage a solid state switch. Extended use can also cause spurious switching due to voltage transients.

Field Effect: Field effect switches are first on the list of solid state switches because they existed in theory before other solid state switches although they could not be manufactured due to lack of semiconductor materials. Problems of insertion loss and mechanical vibration are eliminated due to the field effect switch’s use of an electric field to control the shape and route of radio frequencies or power.

Field effect transmitters are the masters of frequency range, handling everything from DC to tens of kilowatts, depending on their size, but have better isolation at low frequencies.

Speed, reliability, repeatability — field effect switches are leaps and bounds ahead of electromechanical switches, but are unipolar and are thereby limited to single-carrier type applications only.

PIN-diode: PIN-diode switches are next on the list of solid state switches. What separates these from field effect switches is their miraculous switching speeds (we’re down to nanoseconds) as well as their power-handling capabilities, which are less than the field effect switch, ranging from milliwatts to tens of kilowatts of average power handling, and maintain good isolation at high frequencies.

Hybrid Switches: The theory behind hybrid switches was to take the best things from field effect and PIN-diode and blend them. Sounds good, right? And it works to some degree in that way. A series of field effect switches can be joined together by PIN-diode switches. The result is slightly less power than if the field effect switches had been used alone, but with the high frequency isolation capabilities that the field effect switches were missing.

Testing Transistors

There are different types of transistor tests:

Quick-check in-circuit checker: This circuit tester is an easy and quick way to determine whether a transistor is still operational. While the transistor is still in the circuit, the technician tests the transistor’s ability to amplify and takes that as a rough index of performance.

Service type tester: This tester performs three different types of checks: forward-current gain, or beta of transistor, base-to-collector leakage current with emitter open (ico) and short circuits from collector to emitter and base. Occasionally, the tester will measure to see when the current-gain of the transistor has reached a certain threshold. Although it can be useful, keep in mind that current-gain is highly dependent on the integrated circuit used and, in real world applications, can vary from test results significantly.

Laboratory-standard tester: This is the most absolute and accurate tester for transistors usually measuring Icbo collector current with emitter open (common base), ac beta (common emitter) and Rin (input resistance). It measures these parameters dynamically under various operating conditions.

Do your applications rely on a single or multiple types of transistors? How are you testing them for performance and reliability?

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