# High frequency devices

## Main Question or Discussion Point

Can someone explain me what do high frequency semiconductor devoices mean? how to differentiate between low and high frequency devices? Thanks

## Answers and Replies

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berkeman
Mentor
Can someone explain me what do high frequency semiconductor devoices mean? how to differentiate between low and high frequency devices? Thanks
Welcome to the PF.

What is the context of your question? Is this for schoolwork? I can move the thread to the Homework Help forums if so.

And what do you mean by "high frequency"? What frequency range do you consider "high"?

It is not related to homework. for curiousity, i started to learn high level physics and i heard a basic of semiconductor lecture on internet which say high frequency devices. I want to know what high frequency is referred to? and what kind of dveices it is referred in semiconductors? Thanks

In general, I'd say high-frequency devices operate in the GHz range. But it keeps getting higher and higher. It's pushing into THz now.

The higher-frequency devices are usually made from III-V compound semiconductors, like GaAs and InP (and other III-V materials like InGaAs etc.) They can operate at higher frequencies because electrons have a much higher mobility than in Si devices.

The 2 most common devices are:
HEMT: high-electron mobility transistor (and various subsets)
HBT: heterojunction bipolar transistor

@caffenta pretty much nails it.

Another "subtle" (well not that subtle) aspect is Lumped vs. Distributed circuit models. Most low frequency devices are Lumped circuits while most high frequency devices are Distributed circuits. The ways you analyze, simulate and use the two types are different.

Lumped circuits are ones where the components are largely independently acting and can be described by "lumpy" parameters

Distributed circuits are closer to Maxwell's Equations and involve necessarily concepts of transmission lines with reflected and incident power instead.

Thanks.

vk6kro
If you are talking about transistors, you can get the data sheet for each type of transistor (using Google) and you can look for an entry that says something like
fT = 300 MHz

This would be the frequency at which the transistor has a gain of 1, but you could just read it as the upper limit in frequency for that transistor. It is called the gain-bandwidth product.

Whether this would be called high frequency or not, probably doesn't matter. I think it is.
If you want it to work as an amplifier at 3 MHz, it will work, but any bypassing and circuit layout has to be suitable for up to 300 MHz, or else the transistor may oscillate without you knowing it.