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Measuring IV charachterstics at LN2 temperatures (Help!)

Hi I am trying to obtain the IDS versus VDS of a MESFET at cold temperatures. I have made a small circuit with a drain resistor 200 ohms (common source) and some bias elements. I have normal stranded wires connected for Gate bias,. drain bias and 1 to measure the drain voltage. Theoritically and from other publications the drain current values for Liquid nitrogen temperatures should be more than at room temperature. However, i am seeing a reverse trend. For a given applied Gate bias the drain currents are smaller for a particular VDS. I am not sure why is that. The only thing i can think of is the thermo couple affect - is it messing my readings. My wires are running from room temperature power supplies to the preamp which is dipped inside liquid nitrogen. Please help, i have very less experience in cryogenic measurements. Thank you

Reply to
Arch
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Is there a better way to do this - the right way?

Reply to
Arch

Are you sure your resistor is resisting the low temps?

Reply to
Ian Stirling

The temperature coeficient of the drain current for a fixed gate voltage varies with current and goes from positive at low current to negative at high current. at least thats what most of the datasheets show at more normal temperatures. this would sugest to me you are running the device at a low current.

Colin =^.^=

Reply to
colin

A possible source of problems could be the bonding.

Rene

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Reply to
Rene Tschaggelar

The resistors i am using are metal film and are supposed to be fine upto 4 Kelvin. I am taking them only to 77 Kelvin, in my present experiments. The drain current goes upto 25 mA for some values of VGS. Rene, by bonding you mean the connections? The solder joints?

Reply to
Arch

Almost. You're using a FET and this is a piece of semiconductor in a plastic case. The pins are connected to the semiconductor through bond wires, usually gold. This connection, between the semiconductor and gold, could break from thermal strain.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

Thanks Rene for the reply. No, I don't think there is a bad connection as i can still turn off the FET by applying a higher negative gate voltage, -1.2 in my case. But you have a point the thermal strain due to unequal expansion may cause a bad connection. I have to keep that in mind.

Reply to
Arch

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