Rule of thumb for the length and the current density of a transistor

[mixed_signals]

Hello All,

What is a good rule of thumb for selecting the channel length and the current density of a transistor?

For example, I use 1uA_current/1um_width for a 1um_length transistor but I do not know whether there is a better way or not? Also, Is your choise of the current density vary with the chosen channel length?

 

[BigBoss]

There is no such rule of thumb for current density.
The foundry defines max. current density for given W/L ratio. There should be charts about them in their Bluebook.

 

[mixed_signals]

There is no such rule of thumb for current density.
The foundry defines max. current density for given W/L ratio. There should be charts about them in their Bluebook.

Thank you for your response.

Yes, I know that the maximum current density (I/W) is set by the foundry but my question is what is the proper current density I should choose below the max? Should I choose the max density or is there a good rule of thumb for that?

 

[dick_freebird]

You set operating point based on what you want from
the device - gm, rout, linearity, ron... thumb doesn't know
Jack about your care-abouts.

 

[vivekroy]

Entirely depends on the BEOL of the process. There are many things that define the 'thumb' rule you are talking about:

1. Electromigration rules surrounding the metal lines that connect to the source and drain

2. The amount of IR drop that can be tolerated by your transistor. That depends on how the source and drain are connected to the metals (number of contacts, number of metals stacked, their sheet resistance)

3. Self heating is another consideration.

 

[mixed_signals]

Entirely depends on the BEOL of the process. There are many things that define the 'thumb' rule you are talking about:

1. Electromigration rules surrounding the metal lines that connect to the source and drain

2. The amount of IR drop that can be tolerated by your transistor. That depends on how the source and drain are connected to the metals (number of contacts, number of metals stacked, their sheet resistance)

3. Self heating is another consideration.

Thank you for your response.

I know that max current density depends on what you said. Please refer to my reply above on @BigBoss to clearly understand my inquiry.

Thank you for your response.

Yes, I know that the maximum current density (I/W) is set by the foundry but my question is what is the proper current density I should choose below the max? Should I choose the max density or is there a good rule of thumb for that?

 

[vivekroy]

Thank you for your response.

I know that max current density depends on what you said. Please refer to my reply above on @BigBoss to clearly understand my inquiry.

You can absolutely set it to the max if all the reliability concerns are met. Now there is a bit more caveat to it:

1. Check the mission profile for the part. Automotive parts work at elevated temperatures and needs higher reliability lifetime than consumer or industrial parts. You need to make sure you don't exceed the ratings for the mission profile of your part. BEOL reliability can be more or less scaled quite easily.

2. HCI, NBTI etc will need proper simulations or recommendation from the foundry. Different processes have different doping profiles which impact FEOL reliability differently.

Please ask the foundry for the guidelines. There is no rule of thumb. We design as per the guidelines (or simulation).

 

[mixed_signals]

You can absolutely set it to the max if all the reliability concerns are met. Now there is a bit more caveat to it:

1. Check the mission profile for the part. Automotive parts work at elevated temperatures and needs higher reliability lifetime than consumer or industrial parts. You need to make sure you don't exceed the ratings for the mission profile of your part. BEOL reliability can be more or less scaled quite easily.

2. HCI, NBTI etc will need proper simulations or recommendation from the foundry. Different processes have different doping profiles which impact FEOL reliability differently.

Please ask the foundry for the guidelines. There is no rule of thumb. We design as per the guidelines (or simulation).

Thank you for your detailed response.