We are doing some DC characterizing of HEMTs. Because we do some measurements at different temperatures, the devices are mounted on a Kovar carrier and the carrier is placed on a heating block. The device is connected to the outside world though gold wirebonds and gapwelded gold ribbon. Measurements are taken using an HP4145 or HP4156 semiconductor parameter analyzer. The question is, if the device is measured like that, it will on occasion, oscillate. What is observed is, when the standard Ids-Vds transfer curves are measured, you will see a series of "cracked-out" sharp turns and jumps at maybe 0.5-1.0 volts Vds at ( I believe) all gate voltages. These are depletion mode MESFETs so applying a bias to the gate turns the device off. Why does it do this? there are ways to get around this by dampening the so called oscillations (RC network, ferrite beads...), however, it's a DC measurement! There have been various explanations such as long wires re-radiating some high frequency components back to the gate or drain connectors; the high frequency components coming from the semiconductor parameter analyzer when it steps up (step functions have a lot of high frequency components)...another is that the transistor has very high gain and high gain devices are prone to oscillation...

I've never heard a satisfactory explanation for this? Can anyone help?

Felix

 

hi felix,

"These are depletion mode MESFETs so applying a bias to the gate turns the device off. Why does it do this?"

have you read any application notes on deplection mosfets? check out application notes on depletion mosfets from Fairchild or Texas Inst. or STMicroelectronics.

 

sorry, I thought that might have been misinterpreted...I know why they turn off when you apply a bias...anyone who has studied transistor circuits and device physics will know this.
I meant to ask why they oscillate when doing DC characterization. I've asked several PhDs who've done their theses on high speed transistors but they don't seem to have a good idea, partly because they don't mount them on a separate carrier for probing. Other PhDs don't seem to know either because they always give a hand waving explanation. It seems to be a phenomena in the industry that you can avoid and so people do what they can to avoid it...