18-09-2015, 04:54 PM
You ar quite correct. Electron velocity in valves sets the ultimate limit to upper operating frequency. Hence high frequency valves have smaller electrode structures. In fact the actual transit time isn't a fundamental limit - radio waves travel through the atmosphere for many times their wavelength without losing their shape. The trick of klystrons etc is to ensure that the waves travel coherently through the device, despite the deveice being longer than a wavelength.
Transistors start off with the advantage of being small so the distances the carriers have to move is small too. Not sure how fast the carriers (remember you can have electrons and holes as carriers) actually move but again the smaller the device that higher the frequency it can do. The transistors in a modern PCU or memory chip are truly minute, down to a few 10s of nanometres, and so can switch very fast.
All of this causes a conflict if you want high power (large device) and high frequency (small device) at the same time. A lot of effort has gone into resolving this.
The important term in semiconductors is CARRIER MOBILITY. It's higher in specialist materials such as Gallium Arsenide than in ordinary silicon. This article isn't bad but you'll find it very hard going: https://en.wikipedia.org/wiki/Electron_mobility Google carrier mobility and you might find something a bit easier to follow.
It's worth knowing that the electrons in a conductor such as copper move very slowly indeed, snails pace gives you some idea. But think of it like a set of billiard balls bumping into each other. Each ball may not move far or fast but the shove you put in at one end of the line can appear at the other pretty quickly.
Transistors start off with the advantage of being small so the distances the carriers have to move is small too. Not sure how fast the carriers (remember you can have electrons and holes as carriers) actually move but again the smaller the device that higher the frequency it can do. The transistors in a modern PCU or memory chip are truly minute, down to a few 10s of nanometres, and so can switch very fast.
All of this causes a conflict if you want high power (large device) and high frequency (small device) at the same time. A lot of effort has gone into resolving this.
The important term in semiconductors is CARRIER MOBILITY. It's higher in specialist materials such as Gallium Arsenide than in ordinary silicon. This article isn't bad but you'll find it very hard going: https://en.wikipedia.org/wiki/Electron_mobility Google carrier mobility and you might find something a bit easier to follow.
It's worth knowing that the electrons in a conductor such as copper move very slowly indeed, snails pace gives you some idea. But think of it like a set of billiard balls bumping into each other. Each ball may not move far or fast but the shove you put in at one end of the line can appear at the other pretty quickly.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
