EE6312: Homework Assignment 2

For this assignment use the same devices as you used in the previous one.

Create a grid of devices sweeping W and L. Use minimum W, 3x min W, 9x min W and use min L, 3x min L, 9x min L. You have a 3x3 grid of nine devices.

CVN Students: Use the 1.8V nominal Vt device, and use min W, 3x min W, 9x min W for a length of 3x min L.

Plots and Estimations:

Now for each of the above devices plot the following:

1. log(gm) vs log(Id) for the different devices. (Show three plots - each for one length. Each of these plots will have three curves for the three widths.) Use a fixed V_DS of 1V.
2. gm/I_D vs inversion coefficient. Use a fixed V_DS of 1V. (You can estimate the inversion coefficient as I_D/I_Z which you estimated in the earlier assignment. Make sure that the x and y axes are in log scale, i.e 0.1, 1, 10 are equally spaced.)
3. gds as a function of V_DS. Use a V_GS of V_T+0.5V. (You have already computed V_T of the device in the previous homework.) From this plot, obtain V_A of the device. (V_A is the characteristic voltage in the first order channel length modulation formula.)
4. Cgs, Cgd, Cbs, Cbd, Cgb as a function of V_DS. Use a V_SB of 0 Volts and a V_GS of V_T+0.5V. (Hand in separate plots for each individual device, each plot containing all the curves. 9 plots.)
5. Plot ft of the device as a function of the inversion coefficient. (ft is the frequency at which the current gain of the transistor is equal to 1.)

References:

Notes:

Save the dc operating point information of your simulation. Now in the calculator when you click on "op" a small window will pop up asking you to choose the device. Choose your device - select the parameter that you want to evaluate from the list. Now you can plot this parameter in a window. If you now do a parametric analyses, you can plot this parameter over a parameter sweep.

You will notice that the capacitance values are negative! Don't panic, this is actually correct. According to spectre's definition of capacitance, Cxy = d(Q_x)/d(V_y) when all other voltages are constant. The definitions traditionally used in class, in books and in this homework assignment are: Cxy = -d(Q_x)/d(V_y) when x is not the same as y, and Cxx = +d(Q_x)/d(V_x). So when you send in the plots, for cross capacitances, negate the sign and then hand it in!

MOS capacitances can be intrinsic or extrinsic. You can play with the number of fingers of a device when you want to tweak the extrinsic capacitance. The extrinsic cap is a result of AD, AS, PD, PS. Before you play with the number of fingers, create a directory called "skill" in your home directory, and copy this code into a file in that directory. Add the following line in your ".cdsinit" file:
load("/my/home/directory/skill/fet_cdf.il")
For this homework, use only one finger for every device - so you need not worry about this. But this will come in later again and again and you need to have correct values for AD, AS, PD, PS. In your free time you can play with the fingering and observe the capacitances Cjs and Cjd.

Submission:

Due Date: