What are the Operation Region of Transistors in Cadence

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How to know the region of operation of a single transistor in Cadence? What are the meaning of 0, 1, 2, 3 and 4 Regions in terms of cutt-off, saturation, sub-threshold, etc

The Virtuoso tool tells us valuable information about the operation region of any transistor in the circuit. For example, if a transistor is working in the saturation or in the triode region.

There is a standard operating point parameter named “region”. Although it depends on each specific technology, most of them are using it.

The region of operation of the transistors are coded in Virtuoso with numbers:

0 cut-off
1 triode / linear
2 saturation
3 subthreshold
4 breakdown

The condition for saturation region in based on the zeroth approximation:

|Vds|>=|Vds,sat|≈|Vgs-Vth|

What is the Meaning of the Operation Region?

The region parameter don’t tell you the complete information!!!

Historically, circuits were designed to operate in saturation (Strong Inversion) and what we want to bias our transistors in saturation all of the time. BUT in new designs, a new subthreshold circuit era is starting, sometimes circuits are biased to operate in the subthreshold region, using the weak inversion. Before, circuits were operating almost exclusively in saturation.

As low-voltage and low-power designs become more relevant, designers are starting to design circuits in Weak and Moderate Inversion.

The region parameter doesn’t give you the complete information if your device is working in Weak Inversion. You must take a look of Vds to see if you are in triode or saturation.

Vds and Vgs have completely 2 different roles:

The Vgs parameter is determining the inversion level. How much inversion you have in the channel: Weak or Strong Inversion.
The Vds parameter is determining the behaviour: Triode or Saturation. If the circuit is behaving as a Voltage-Controlled Resistor (VCR) in Triode or as a Voltage-Controlled Current Source (VCCS) in Saturation.