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Measuring IV charachterstics at LN2 temperatures (Help!)

A

Arch

Jan 1, 1970
0
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
 
A

Arch

Jan 1, 1970
0
Is there a better way to do this - the right way?
 
I

Ian Stirling

Jan 1, 1970
0
Arch said:
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.

Are you sure your resistor is resisting the low temps?
 
C

colin

Jan 1, 1970
0
Arch said:
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

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 =^.^=
 
R

Rene Tschaggelar

Jan 1, 1970
0
Arch said:
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.



A possible source of problems could be the bonding.

Rene
 
A

Arch

Jan 1, 1970
0
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?
 
R

Rene Tschaggelar

Jan 1, 1970
0
Arch said:
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?

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
 
A

Arch

Jan 1, 1970
0
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.
 
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