(Should you have any basic doubts, refer back to the previous posts.)
Transmission gate is basically used to avoid logic degradation. The nMOS passes logic low very efficiently but degrades high logic, where as pMOS passes logic high satisfactorily but degrades logic low.
To overcome the above problems we go for transmission gates. Here the nMOS and pMOS both are connected together with their source and drain terminals. A control signal switches on either of the devices (means either nMOS or pMOS depending on weather the logic is high or low).
Here is the schematic:
You can see that it requires an inverter to complement the control signal A.
Now we shall frame a vague problem definition at our hand. We need to design a TG and supply inverted signal to the nMOS and pMOS using an inverter.
Open our favorite window of Microwind.;)
So far we have drawn the nMOS on pMOS devices manually. Let us see how we can utilize the inbuilt function.
From the palette click on MOS generator. (looks like an nMOS symbol). A window will pop up. Here you can specify various parameters. For the present design, we will consider the default values. Note that nMOS is automatically selected in this dialogue box. Click on Generate device and place it by clicking on the substrate (black background). The red highlighted circles depict it.
In a similar manner we need to place a pMOS device. To do that, click on MOS generator, select pMOS, click on generate device and click somewhere above nMOS.
Aligning the devices.
In the part 1 of the tutorial I have shown how to move the devices or layouts from one position to another. That was a crude method. It does not have any control and things are done based upon the movement of mouse. Let us do it a systematic way.
From the top menu click on Edit>Move step by step. Now highlight the device which you need to move (hold the left mouse button and drag the mouse till the layout is completely selected). You should be able to see the following window:
Move left or right or up or down till they are properly aligned. Maintain a certain distance between the devices to obey the DRC. Make sure that you do DRC once after placing them.
Tip: You can directly pres Ctrl+M to move in this manner.
Half the design is done now. (Yes, so soon).
We need an inverter to give the direct and complemented signals to the TG. We have already drawn an inverter in Part 2 of the tutorial. We can import it over here and use it directly.
This step is very important. You will be using it more frequently as you will be drawing more complex layouts.
In the top menu got to File> Import Layout. A window will pop up. Browse to the destination where you have stored the file. Select and click open. Viola! Its very much there.
We can use this inverter for our design but I prefer taking width of pMOS twice that of nMOS (why?).
I included this step to make you familiar with the import command.
Let us build an inverter the same way we built the TG. That is using MOS generator. I won’t be going in detail of that.
Place the inverter to the left of TG. Make sure that the distance between the metals is at least 8λ.
Your design should look something like this:
Now we need to supply the direct and inverted signals. I have done it using poly. You can even do it using metal. But if metal is used more area will be consumed, you need to place the devices further apart and you need to use minimum of 3λ width metal.
After the connections (routing) the layout is:
You are almost done now. We just need to give a finishing touch to layout. Connect Vdd and Vss to the inverter. (Not to TG).
In Step 4, we have generated inverter. But again the p device and n device have same width; we need to make the n device with half of p device. Do it using the gun or shoot tool. (Delete some layout.).
Complete the missing metal and poly contacts. Do the DRC once more. Satisfy the design details.
Check if all the contacts are proper. (Explained in detail in Part 2).
Step 7: (Final Step).
Apply the clock signal to the input of inverter and next clock to the input of TG.
Apply an eye (Visible node) to the output of the TG. The complete TG is at your service!
I have shown the outputs. In the next simulation I have interchanged the clocks of TG and inverter to understand the working.
Have a nice day.