The second issue is that todays small semiconductors have temperature time constants which are 10-100x faster than 10us. So the curves are distorted by the various temperature effects.
It is fascinating to see inside this instrument and to see the accuracy they can measure to for dc - but for many semiconductor devices such as FETs in particular the resultant curves are only useful for estimating a dc bias point.
Many devices have charge trapped in deep energy levels (ironically generally associated physically with the surface of the semiconductor) which have time constants much slower than RF so when the device is operated under RF the curves it follows can be completely different from those measured under slow dc conditions.
This is a subject close to my heart because the small company I started with two others joined with another small company to develop a pulsed measurement system to take measurements faster than the time constants.
The difference can be spectacular, on some SiC power devices charge trapped in surface states led to the nicely spaced dc curves (as would be measured on a SMU) collapsing more or less into a single curve as the gate voltage was pinned by the surface charge.
So making super accurate dc measurements of semiconductor devices and then using them to fit a large-signal spice like model or even just to estimate the class A power you might get out can lead to spectacular errors much larger than those of using a cheap bench power supply and pocket DVMs to get the same curves!
The characteristic curves depend on where the device is biased and at the rate at which you then measure them.
This is not true of all devices, some we looked at showed very close agreement between pulsed and dc measurements. There will always be some difference because of thermal effects.
Home
Help
Search
Login
Register
