Lab

Lab 5 - EE 421L

Authored by Jesus Flores-Arellano

Email: florej29@unlv.nevada.edu

October 10, 2023

Lab description

-Design, layout, and simulation of a CMOS inverter

Prelab Tasks

- Go through Tutorial 3

Prelab

To start off this prelab, we made the schamatic of an inverter using a 6u/6n (W/L) NMOS and (12u/6n) PMOS connected as seen in the shematic below:

We then also make the symbol view:

Following the schematic and symbol view of the inverter, we proceed to the layout. We instantiate an NMOS and PMOS with the same specifications as above, a m1_poly to connect our polys layers, ntap for the PMOS Vdd connection, and ptap for the NMOS ground connection. We then connect them with metal1 layer, and assign the pin assignment accordingly, while not forgetting that the left pin is an input pin, and right pin is the output pin, ntap and ptap pin is assigned as input/output. This step is importnat for the LVS process. We then get the following:

We then DRC our layout

Now that we have a clean DRC, we can LVS our layout and schematic, results are seen below:

End of prelab, Tutorial 3.

Lab Objectives

Draft schematics, layouts, and symbols for two inverters having sizes of:

12u/6u (= width of the PMOS / width of the NMOS with both devices having minimum lengths of 0.6u)

48u/24u where the devices use a multiplier, M = 4 (set along with the width and length of the MOSFET, image), as seen below

Using SPICE simulate the operation of both of your inverters showing each driving a 100 fF, 1 pF, 10 pF, and 100 pF capacitive load

Comment, in your report, on the results

Use UltraSim (Cadence's fast SPICE simulator for larger circuits at the cost of accuracy) and repeat the above simulations

Use Setup -> Simulator/Directory/Host and select UltraSim as seen below
You'll also have to point to the MOSFET models again as seen below
Note that UltraSim only performs transient simulations (not AC, Noise, DC, operating point, etc.)

Not knowing this last item will lead to wasted time if trying to use UltraSim exclusively for simulations

Lab

12u/6u Inverter

Schematics of 12u width of the PMOS & 6u width of the NMOS with both devices having minimum lengths of 0.6u, are can be seen below:

The symbol for schematic seen above :

Layout view:

DRC and LVS:

48u/24u Inverter

Similarly, the following process was the same , but with a multiplier of 4 ( m = 4) for the NMOS and PMOS, which gives us a 48u/6n, 24u/6n, PMOS and NMOS, respectively.

You will find the schematic, and Symbol view in the images below.

For the layout we can start by instantiating a 12u/6n PMOS and 6u/6n NMOS and both with multipliers of 4 (i.e m = 4) as seeen below. This will obviously give us the desired value of our 48u/24u Inverter.

After, we layout the metal1, poly, and pin connections as per our schematic. We should end up with the following below:

DRC pass for the finished layout:

Once we pass the DRC, we can LVS now:

Now that we have both Inverters done (i.e 12u/6u and 48u/23u ) we can move onto the simulations.

Simulations

For these next simulations, the inverters will be driving a 100fF, 1pF, 10pF, and 100pF capacitive load.

100fF Capacitive Load

For the following simulation below , we can observe there to be slightly sharp edges that are in the Vout waveform. We will keep in this mind for when we change the capcaitive load.

1pF Cacpacitive Load

In the following simulation we can analyze that the wavceform for Vout is smoother at one end of the graph while the other end is slightly sharper.

10pF Capacitive Load

During this next simulation, we will see that that Vout is taking a longer time to reach the desired value of the inverter.

100pF Capacitive Load

Here we can see that the pattern continues as the previous results. At this point the output just follows the input of the inverter, rather than giving the inverse value.

In sumamry, I will provide all simulations in a single graph to compare all capacitive load situations (i.e 100fF, 1pF, 10pF, 100pF). A small little walk through for my reference will be provided below.

Let us define the capacitors' value as a variable, since this will be the value we want to change in the transient simulation.

We then define this variable the ADE L window. Go to Variables -> Edit, and then add the name of the variable, in this case "CAP", we can set the value expression to 0. Click Ok.

Assuming we have everything else setup for simulation, we can then go to Tools -> Parametric Analysis, Change Run Mode to Parametric Set, Add the variable "Cap" and add the list of values that we want as seen below:

We can then run the simulation with the green play button next to Parametric Set.

48u/24u Inverter Simulation
I will do the same procedure to simulate the 48u/24u inverter to save time and less pictures.

All combination for the capacitive loads will be given in the simulation below:

As we can see, with the inverter with a higher multiplier, or more MOSFETs, allows the capacitor to more effeciently charge and discharge.

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