design CMOS inverter Electric !

[biolycans]

Hi every one !

I am using a BSIM3 Model level=8 for a C5 process. This model I took from Jacob Baker 's website. (in the attachment).

I decided first to understand the behaviour of the MOSFET. I am trying to design an inverter with a switching point = 2.5 V, where my Vdd is 5 V

I calculated a switching point in 2.5 V. The equation I used I took from baker´s book and other books of Digital VLSi desing.

Vsp = ( Vdd - Vtp + Vtn * (Bn/Bp)0.5 ) / ( 1 + (Bn/Bp)0.5 )

For a Vsp = 2.5 V, I calculated:

Bn / Bp = 0.744, where:

Bn = Uo * Cox * Wn/Ln; Uon * Cox = 110 uA/V2 (tox and Uo I took from C5_models.txt, it is in the attachment)
Bp = Uo * Cox * Wp/Lp; Uop * Cox = -51 uA/V2 (tox and Uo I took from C5_models.txt)

I took Vthn = 0.669 V and Vthp = -0.92 V from C5_models.txt too.

I chose

Wn = 6 um and WL= 0.6um
Wp = 2.052 um. and Wp = 0.6um

In the simulation I obtained:

For a Vin = 2.5 V, Vout = 4.3V aproximately.

Maybe I am not using the right equation for this model. View attachment C5_models.txtView attachment C5_models.txt

I would be very grateful if you can help me.

Regards,

Joaquín

 

[dick_freebird]

You will have to play with W, to get equal drive current
at Vgs=2.5V, Vds=2.5V for N and P devices. In all my
years I have never bothered to calculate stuff like this,
a looped simulation will put you right quicker than that
(provided your models are any good). And of course the
calculations have nothing for process, temperature, and
if you're back-figuring from somebody's SPICE model
anyhow, how can you possibly gain accuracy?

 

[biolycans]

I understood what you told me. But I think that there should be an equation where using the parameters from this model, I can have an approximately calculation with it's simulation.

Regards!

 

[erikl]

For a Vin = 2.5 V, Vout = 4.3V aproximately.

Actually you got a surprisingly good result: Vout is neither zero nor 5V. Think of the rather high open loop gain of an inverter in mid voltage range!

I think you can't do any better with such calculations. Look at the Vin-to-Vout transfer characteristic: a few tens of mV Vin or a few degrees of temperature change will throw Vout between 0 and 5V .

 

[biolycans]

Thank you for your response !. You are right ! Nowadays I am doing my thesis for the degree of biomedical engineer and it´s about Microelectronic (CMOS VLSI design). I want to design a current starved ring oscillator. For that reason I was trying to design first, an inverter with a switching point in 2.5 V. I know that some variations in temperature can affect the Vout voltage but I as a novice designer, I find it remarkable that with the model of the transistor and some equations I am using, I can`t have a simulation at least aproximate with my calculations. Maybe there is an equation where I can consider others parameter such as the variations of the temperature, and other values.

Regards !

Joaquin