calculation od RDS(on)

0Can anyone help on how to calculate the RDS(on) of LDMOS. in many journals they have given the dimensions of it as mohm.mm2. what are the length and width of the LDMOS which are to be considered while calculating the area of the ldmos

You usually use LDMOS as a switch, so you go with minimum length. Also, do note that for very low on resistance, the metal resistance may play a significant role compared to the silicon resistance.

Specific resistance of power devices is very often given in these strange units of r=mOhm*mm2 so that people can quickly estimate device area A for the required Rdson value:

Rdson = r / A

A more fundamental parameter is specific resistance of the device in Ohms per one micron of gate width - rch.
If you have SPICE model of the device, rch can be calculated as follows from simple DC SPICE simulation corresponding to operating point (Vds, Vgs):

rch=Vds/Ids*W,

where W is the gate width.

Areal specific device resistance r is related to the device linear specific resistance rch as follows:

rch=r*P

where P is the device pitch (period in the direction perpendicular to the poly gate, for multi-finger devices).

Some people/fabs do, some do not include the well tap area into account when calculating the pitch and areal device resistivity.

Also, as "checkmate" noted, these calculations give you just an estimate of the device Rdson, or lower bound for Rdson - you will need to add metallization resistance for source/drain nets, which can contribute a significant resistance value to the total Rdson (device + interconnect).

See this paper for more info on metallization resistance calculation (as well as for current density analysis, metal/via/wirebond layout optimization, sense device optimization, etc.):

https://www.siliconfrontline.com/fi...d_National_Semiconductor_ISPSD-2010_LV-P4.pdf

Max
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thank you very much

Re: calculation of RDS(on)

i am getting high values of on state resistance. is it due to low width of the devices. because in Silvaco they take the default value of the device as 1µm. please help me

Re: calculation of RDS(on)

georrose said:

i am getting high values of on state resistance. is it due to low width of the devices. because in Silvaco they take the default value of the device as 1µm. please help me

Click to expand...

"high value" as compared to what?
Are you doing measurements or simulation?
If you are simulating device with gate width of 1 um (popular default in many 2D TCAD simulation tools), you can estimate resistance for the gate width of W by simply dividing your Rdson resistance by the gate width (in um).

In your post, too little info is provided, and your question is vague...

Re: calculation of RDS(on)

thanks for the reply. i am doing a simulation. my RDS(ON) is of the range of k ohms(in some cases it even goes upto tens of kilo ohms). the gate width of some devices as referred in some journals are of the range of mms. is my high on state resistance due to the low default width. what correction can i do for the small width. i hope i am clear now.please do help me

Re: calculation of RDS(on)

georrose said:

thanks for the reply. i am doing a simulation. my RDS(ON) is of the range of k ohms(in some cases it even goes upto tens of kilo ohms). the gate width of some devices as referred in some journals are of the range of mms. is my high on state resistance due to the low default width. what correction can i do for the small width. i hope i am clear now.please do help me

Click to expand...

Your explanation is pretty clear now, thanks.

What you get from 2D TCAD simulation is true for 1 um of gate width - current, resistance, charge, etc. So, you can consider the Rdson (=Ids/Vds) value you are getting as specific resistance - i.e. resistance per one micron of gate width. The units os resistance is thus [Ohm*um].

Resistance for gate width W will be Rdson/W.

Typical values of specific device resistance is in the range 1kOhm*um to ~20 kOhm*um (for devices with breakdown voltage below ~100V). So, the values you are quoting looks do not look abnormal to me.

Typical values for gate width for power devices... anywhere from few microns to several meters (devices with gate widths of 2 meters are not unusual).

If you are getting, for example, Rdson = 10 kOhm*um, then for gate width of 2 meters you will get rdson=Rdson/W=1e4 Ohm*um/2e6 um = 5 mOhm.

I hope my explanation is clear enough.

Re: calculation of RDS(on)

Thanks a lot. I have been exploring all possible ways for quite a long time. Your reply has given me the exact answer. Thanks a lot again

Re: calculation of RDS(on)

georrose said:

Thanks a lot. I have been exploring all possible ways for quite a long time. Your reply has given me the exact answer. Thanks a lot again

Click to expand...

You are welcome.
(and thanks for the feedback - I am glad my comment was useful).