Hello,

I'm not quite sure about the specification of Gate-to-Source Forward Leakage (I_gss) given in many datasheets of MOS-FETs. I researched its definition and it seems to be the leakage current (for specific gate-source voltage), that flows from gate to source, while drain and source are shorted to ground.

E.g. the BSS138-G specifies 100nA. Does that also mean in the schematic attached there are 100nA flowing in/out of the gate of the MOS-FET? Since the current flowing in/out of the gate results in an error, given that the load current should equal 10mA. I would be interested to estimate that error. I know this should result in a tiny error in this case but for other designs it might be more significant.

Thanks for your help I did some research but I could not find the answer.

Best Regards John

 

What error do you mean?

 

E.g. the BSS138-G specifies 100nA. Does that also mean in the schematic attached there are 100nA flowing in/out of the gate of the MOS-FET? Since the current flowing in/out of the gate results in an error, given that the load current should equal 10mA. ... I know this should result in a tiny error in this case but for other designs it might be more significant.

If the total drain current is in the range of nanoamps or even microamps the gate leakage could conceivably cause a significant error, but I've never encountered such a case. And for the example you cite, 100nA (0.1μA) is 100,000 times less than the 10mA drain current-- and 0.001% is hardly a significant error.

I would be interested to estimate that error.

Estimate? You can calculate it directly, as I did above.

 

Thank you for your reply. I was thinking about calculating it like this, but I have concerns because the I_gss in the datasheet is specified at at Vgs = 20V and Vds = 0V. Also I feel like 100nA are a little much considering that many OpAmps have in input bias currents in the range of 100pA.

 

The mosfets in the input circuits of op amps are tiny little devices with very small gate areas, hence low leakage. Bipolar op amps with small input bias currents usually use some sort of bias current cancellation scheme.

Discrete mosfets such as the BSS138 and larger have much greater gate area, therefore higher leakage.