E C E4902 Lab 3

Simulation: D C Analysis

M OS Model Parameters: V T O Threshold Voltage U O Low-E-F ield Mobility NSUB Substrate Doping

The purpose of this lab is to compare simulation results with your measurements from circuits in Labs 2 and 3, while continuing to build up a set of model parameters for your CD4007 N-channel and P-channel MOSFETs. From Lab 2, you will use your extracted value of threshold voltage for the MOS parameter VTO. From Lab 3, you will use your measured data to estimate parameters UO and NSUB. From a DC sweep of the simulated circuit, you may need some iteration of the model parameter values to get a good match between your measured data and the simulation results. Be sure to have your lab data available for the following labs:

La DS(on) Lab 3: SPICE Parameter Extraction using Current Mirror

Following is an overview of the sections of this lab:

S3-1 Schematic Entry, NMOS

In this part you draw a schematic for simulating the DC VDS-ID characteristic from Lab 3. In Lab 3, you had to use different resistor values to change VDS; the nice thing about simulation is you just define a voltage source and sweep its value over whatever range you need.

S3-2. Model parameters, N-channel

In this part you add to the SPICE model parameter file you developed in the simulations for Lab 1, corresponding to the extracted parameters from your lab measurements.

S3-3. DC Simulation, your models, RB=5.1KOhm case

In this part you simulate the DC sweep of the VDS-ID characteristic circuit, using the model file you created corresponding to the extracted parameters from your lab measurements. The VGS used is from one of the data points, for the value of 5.1KOhm in the current mirror circuit

S3-4. DC Simulation, N-channel, RB=51KOhm case

The other VGS data point from Lab 3.

S3-5. Parameter Adjustment

Here you modify the parameter file to get good agreement between the simulated data and your measurements.

S3-6. P-channel

Repeat the DC sweep for the P-channel VDS-ID characteristic circuit.

S3-1. Schematic Entry, N M OS

In this part you draw a schematic for simulating the DC VDS-ID characteristic from Lab 3. In Lab 3, you had to use different resistor values to change VDS; the nice thing about simulation is you just define a voltage source and sweep its value over whatever range you need.

To avoid redefining the MOSFET as you had to do in Lab 1, you will make a copy of your circuit from Lab 1 and edit the copy.

1) Copying a cell

In the Library Manager window, click on the library name (CD4007) you created in Lab 1. Click on the name of the cell you created for the Lab 1 simulations. When it is selected, you should see "schematic" highlighted in the View section of the Library Manager Window:

Using the pull-down menu, select

Edit > copy ...

and you will get a "Copy View" dialogue box that allows you to specify the name of the new cell to which you will copy the existing cell view:

Type in the new cell name to copy the old schematic into. You will then edit this schematic to create the simulation for the Lab 3 circuit. Since this schematic already has the MOSFETs defined for the CD4007, you won't need to repeat the tedious process of changing the MOSFET properties to refer to your custom model parameters.

When you've finished typing in the new name, click on "OK". There will be a Copy Status window appearing for a little while; then you should see the new cell name in the Library Manager window. Click on the new file name, open the schematic, and you should see the same schematic that you finished from Lab 1:

You will be modifying this schematic to come up with a schematic that will allow you to perform a DC simulation sweeping the value of drain-source voltage VDS while holding the value of the gate-source voltage VGS constant. The new schematic will eventually look like:

Along the way toward creating this schematic, you will get to use a few new features of the software that will make this (and future) simulations easier. These features include:

The "stretch" command for moving symbols on a schematic while maintaining wire connectivity you've already defined

Rotating parts

Renaming nodes

Using variables for source values

Note: the following steps need not be followed exactly. If you're feeling confident with the schematic software, go ahead and try to get to the new schematic on your own. If you want more detailed step-by-step instructions ... follow along!) First, delete everything you don't need:

The pulse input source (we will be doing a DC sweep)

The resistor and capacitor

All the PMOS devices, and their associated wiring and grounds

Two of the NMOS devices, and their associated wiring

When you're done deleting, be sure to hit the "esc" key to get out of delete mode.

Now your schematic should look like this:

Stretch  Command  Select the vdd node, the V_DD source, and the ground symbol. They should be highlighted in white. Then from the pull-down menu, select Edit > stretch  

Then click in the window on the highlighted items, and move the cursor to the new location for the highlighted components. Click again to place them in the new location, which will be shown in yellow as you move the cursor:

In the new location, the connectivity will be maintained.

Rotating Next, select the MOSFET and use the stretch command to move it. This time, however, click on the "Rotate" box in the stretch window:

While you're at it, delete the wire connecting the drain to ground and connect the drain to the V_DD source. Also, select and delete the "vg" node name since vin will be wired directly to the gate. Use the Edit > Copy function to place a copy of the DC source V_DD at the gate. Then wire up the gate and drain terminals of the MOSFET. Now your schematic should look like

Renaming nodes Easy enough: select a node name, then hit the "q" key (or from the pull-down menu, Edit > Properties > Objects ... ) and enter the new name in the dialogue box. Rename the nodes vgs and vds so when you are plotting results it's easier to figure out what you're looking at.

Using variables in sources One way to give the simulator flexibility to control the value of sources is to define them with a variable, rather than a numeric value (such as +5V, for example).

Select the source at the gate and open the window to edit its properties. The first thing to do is to rename it V_GS, so you have a better idea of what it's there for.

Then, change the "DC voltage" value to a variable name, such as (for example) "vgs_in". In the Analog Environment simulator, you will set the value of this variable to the DC voltage(s) you measured for the various parts of your lab.

Finally, select the V_DD source and open the window to edit its properties and rename it V_DS. You don't need to give its value a variable name (although you could if you wanted to) since you can sweep the voltage value directly in the simulator.

Now your schematic should look like

Check and Save, and you're ready to simulate!

S3-2. Model parameters, N-channel

For your simulation results to match the lab measurements, you need to have the correct model parameters in the simulator. You need to modify your model parameter file, which is done in this part.

In the parameter file that was used in the previous lab, the only parameter you had was TOX, the oxide thickness (in meters m). You will need to add parameters VTO, UO, and NSUB. VTO is the threshold voltage, UO is the carrier mobility (in cm2/V.sec), and NSUB is the substrate doping (in /cm3) which is related to the channel length modulation parameter.

Here are links to the procedure for calculating VTO and UO and the procedure for calculating NSUB. In order for Spice to be able to use these values, you need to create a modified model file in the directory 'cadence/models/spectre' that you created earlier. Using any unix text editor you're comfortable with (preferably emacs), edit the

CD4007N.m file to have the following format (with your own value for TOX from Lab 1 substituted in):

.MODEL CD4007N NMOS ( LEVEL=3 & TOX=5e-8 & VT0=myvton & U0=myuon & NSUB=mynsubn & )

For the 5e-8 value above, substitute the value of TOX from your models from Lab 1. For the variable names

myvton, myuon, mynsubn

you will specify these values simulating the DC sweep. Note that when you repeat this process for the PMOS device, you will need different variable names for the PMOS device parameters since threshold voltage, mobility, and substrate doping are not the same for NMOS and PMOS devices.

CD4007N.m CD4007P.m In the text editor, save the model file and now you're ready to repeat the simulation using your parameters.

S3-3. D C Simulation, N-channel, RB

Here you simulate the acquisition of data points from parts L3-14, 15, 16 of lab 3. You will sweep VDS while holding VGS constant with the value of VGS set to the value you measured in the RB=5.1KOhm case. When you do the DC sweep and plot ID as a function of VDS, you should see a plot similar to what you measured in lab -- but the simulation results will not match your lab results yet, since the model parameters are different. That's OK, the point now is just to get the DC simulation working. You will change the model parameters in the next part.

1) Open simulation tool

To open the analog artist simulation tool, go to Tools, then Analog Environment in the schematic window. This will open the Analog Artist simulation tool.

2) Specify D C analysis and source to sweep

Go to the pull-down menu

Analyses > Choose,

and select DC in the Choosing Analyses window. We want to sweep VDS to duplicate in simulation the measurement you made in lab. So click on Component Parameter and then click on Select Component in the DC Analysis part of the Choosing Analyses window.

Go back to the schematic window, click to select the VDS voltage source. In the pop-up window, double click 'dc' (see below).

Find the Choosing Analyses window again.

Lastly, input the range for the sweep. We will use a range from 0V to 5V in steps of 0.01V each. (This is a much finer resolution than you did in lab - but it's easy to take many more points in simulation). The step size can be selected by changing 'sweep type' from 'automatic' to 'linear'. To do this:

Type 0 in the "Start" box Type 5 in the "Stop" box Select "Linear" from the Sweep Type menu Click on Step Size Enter 0.01 in the Step Size box

Click on OK to finish your work in the Choosing Analyses window.

3) Select outputs to plot We want to plot the drain current ID as VDS is swept. From the pull-down menu in the Analog Design Environment window, choose Outputs > To Be Plotted > Select On Schematic.

Then click the drain terminal of the NMOS which is indicated with the small red square on the boundary of its symbol. When you click a circle should appear around the pin as shown below. This means the current into that terminal (the drain current ID) has been selected to be plotted.

4) Set up the variables

Before running the simulation, you need to specify numeric values for the variable names you are using in the circuit. Some variables are in your model file, but there is also one in your circuit schematic.

To read in the variable names from the schematic use the Variables > Copy from Cellview pull-down.

Double-click on the variable name to open the "Editing Design Variables" window and change the variable name. While you're there, add variables for myvton, myuon, and mynsubn, and use the tentative numerical values you calculated from your lab measurements. Click "OK" when you've entered all the variables and values, and you should see the Analog Environments window updated with the information you just entered:

5) Start the simulation

Go back to the Analog Design Environment window, and choose

Simulation > Run

Then, after much thought on the part of the simulator, you will get an output waveform plot that looks something like the following:

The general shape should be OK, but when you use the cursors to check the values for current at the triode-active boundary, at VDS=+5V, and the slope in the active region, there will probably be some discrepancy.

Before modifying the variables for your model parameters, check the simulation results for the other value of VGS from the lab. Compare the measured data points from Lab 3 to your plot of the simulated current results. Two good VDS-IDS points to choose are

around the bounday of the triode-active region (VDS ~ 1V)

the maximumn VDS (VDS ~ 5V)

You can use the crosshair markers

Markers > Place > Trace Marker

to get numerical readings for points on the plot. Some parameters will probably need to be adjusted to get the measured and simulated data to agree; before you do that check the results from the 51KOhm case.

S3-4. D C Simulation, N-channel, RB

Change the value of the vgs_in variable to change the value of VGS to your measured value from the 51KOhm case.

Repeat the DC sweep, and again compare some of the measured data points from Lab 3 to your plot of the simulated current results.

S3-5. Parameter Adjustment

Adjust your simulation parameters as necessary to get reasonable agreement between your measured data and the simulated VDS-ID characteristics.

S3-6. P-channel

Repeat parts S3-1 through S3-5 for the P-channel device.