"“current limit” " means "compliance" or "range"?

[4135SMU]

Santhosh,

a good example of unknown device's behavior can be like what I described in another topic:

 if the device's current is 1nA at 1V and 2mA at 2V, then 1nA again at 3V.   if you set the current range as mA, then the system's resolution would not be good enough to tell the nA. if you set the current range as uA, which should have enough resolution to tell nA, then it can not handle the mA readings. and in a lot of situation, you don't know at what voltage the current will reach the peak,

I saw case like the above many times in my 15 years of device-testing practice. Keithley system could handle them with a snap. but from your reply, it seems NI's system could not handle it. I wonder what NI guys would say. 

regards,

tao

[santo_13]

I believe even in a Keithley SMU case, the same limitation applies. For each of the swept voltage range, if the current measurement range is not set appropriately, the measurement will be saturated (over the measurement range). The same applies to NI instruments too, in addition, since the compliance value should be within the measurement range, it is a safety mechanism for the measurement circuit to avoid over ranging situation where the current sense resistors could be damaged due to over range.

Example,

Characterizing a diode using a Keithley SMU where the voltage is swept from 0V to 2V and the current measurement range is set to 20uA and compliance set at 100mA. Now at some voltage, the current through the diode will be higher than 20uA and voltage continues to sweep since compliance is not reached. Now the current is going to increase above 20uA while flowing through a high precision current sense resistor which was designed to handle effectively upto 20uA. Doesn't this case damages the current sense resistor since the compliance is only at 100mA?

[santo_13]

On investigating Keithley2400 SMU specifications, I would like to state that NI SMUs features are same as Keithley 2400 SMU,
Keithley 2400 compliance

If you observe in the above image, the compliance limits are same as measurement range.

Example, for a current measure range of 10uA, compliance can be set upto 10.5uA.

In NI SMU, for a current measure range of 10uA, compliance can be set upto 10uA.

Based on these data, I believe that NI did not miss any of the industry standard features.

Please let me know if any of the SMUs that you have used allows the compliance to be way higher than the measurement range.

[4135SMU]

Santhosh,

  Thanks for the reply.

  when I say " Keithley system", I did not mean "Keithley 2400" SMU, My bad; what I refer to is " Keithley 4200-SCS(Semiconductor Characterization System). It is a big fellow.  4200-SCS is a electrical characterization system including 2-6 SMUs and the control software. the software are called KITE(Keithley interactive testing environment). you set-up and execute your measurements in the KITE. for example: output voltage and measure the current.  I wish I was in the university lab so that I could get a screenshot but I cut a piece from their software manual and attached. To set up the  "measurement range", the KITE software give you several options: "Auto"(dynamically optimized range), " Limited Auto" (dynamically optimized range above a specific lower limit) , or manually specified numeric range. when I do the current measurement, I always set the range as "Auto". I don't know how the "dynamically optimized range" works. but in the same IV scan, the Keithley system  give us accurate current readings no matter the values are in the pA or in the uA range or mA.

   so when the range is set as "Auto", in my example "if the device's current is 1nA at 1V and 2mA at 2V, then 1nA again at 3V...", you just click "run" and Keithley 4200SCS will do the job.

back to your example, "where the voltage is swept from 0V to 2V and the current measurement range is set to 20uA and compliance set at 100mA...", I think Keitley's engineers must figure out a way to jump from one sense resistor to the next one when the current is about to reach the maximum allowed value on the 1st sense resistor.  this is probably what is called "dynamically optimized".  but this is not my job to develop or explain, I am just a user. NI guys should work on it. 

Regards,

Ted

[4135SMU]

 when I started to do electrical measurements 15 years ago, I started with Agilent  4155C Semiconductor Parameter Analyzer; then moved to Keithley 4200SCS, then this year NI-PIXe SMUs. I am a simple user, not affiliated with any of the manufacturers and  I just share my experience and questions. 

regards,

Ted

[santo_13]

Ted,

Thanks for clarifying your use case. Keithley 4200SCS seems to be a complex, highly specialized instrumentation in the hardware aspects. I believe that the NI SMUs are intended for the industry standard SMU like Keithley 24xx or any other equivalent.

I am interested to know more about the complex instruments that you work with. I have experience working with NI STS and Teradyne microFlex ATEs used for semiconductor production testing.

I hope someone from NI R&D will be able to support you on this scenario.

Regards,

Santhosh

[Brad_O]

I'm a Keithley guy but I'm pretty sure NI and Keithley SMUs handle compliance the same:

There are 2 types of compliance, "real" compliance and "range" compliance.  Range compliance is shown on the chart you posted Santhosh and is to protect the instrument.  Real compliance is settable by the user to protect the device under test.  When manual ranging, real compliance must be less than or equal to the range compliance.  The instrument will always clamp to the lower compliance. 

When auto ranging, you can set a real compliance anywhere within the instrument's specs.  As the measured value increases, the SMU will range up until the measurement hits the real compliance.  For lower ranges, there's no compliance since the instrument will range up if a measurement gets close to a range compliance.  Some Keithley SMUs (I'm not sure if NI SMUs do this too) let you set the real compliance anywhere even if you're manually ranging, but if your range is lower than what you set real compliance to, range compliance is used.  

Both 2400 SMUs and the 4200SCS support autoranging. I'm think the 4135 does too, but it might not be obvious in this .vi   

[4135SMU]

Brad,

  thanks for the reply. 

  "Both (Keithley) 2400 SMUs and the 4200SCS support autoranging. I'm think the (National Instruments) 4135 does too, BUT it might not be obvious in this .vi". Actually this is the  point of my post--- NI need to figure out how to achieve "Auto-range" soon. 

Thinking about an example: during an instrument demonstration, a customer handle over you a device whose current flowing through  is no more than 1mA between 0-10V, but its current is not monotonically increasing or decreasing. the high current  can be as high as 1mA, and the low can be as low as 1pA.  In Keithley 4200SCS, with "Auto range", it is one click in KITE and you get the accurate IV curve. but with NI's current system which does not have anto-range, how many IV scans you have to try before obtaining the accurate IV curve over 0-10V?  

this 4135 is NI's newest SMU, and this specific vi.(NI DCPower hard-timed voltage sweep) is an example vi from NI too. there is no one else to blame.  As a customer, I am kind enough to point it out and next thing is for NI to fix the problem and update their driver/example Vis.  

regards,

ted 

[4135SMU]

Santhosh,

  thanks for the constant help. I don't know how the duties are divided within NI, or this Auto-range thing now becomes a development issue (software or hardware?). Please let the right department know,   just hope it could be solved soon.

regards,

ted

[santo_13]

Ted,

As far as I know from NI R&D contacts during my work with NI STS projects, NI DCPower driver handles AutoRange feature i.e., only on the software side whenever you change the current limit or voltage limit. So far, the SMUs used in NI Semiconductor Test System does not implement a hardware auto range feature where it can dynamically change ranges as per the load.