5.1 Design Phase

Table of contents

• 5.1.1 Background
• 5.1.2 Component Selection and Placement
• 5.1.3 Wiring the Components
• 5.1.4 Setting Transistor Parameters
  • Expected Characteristics of a Good Inverter Design:

5.1.1 Background

Start by opening the schematic editor Xschem. You can find installation instructions for the IIC OpenSource tools Docker container here.

To begin a new schematic, click the “+” icon.

Figure 5.1.1.1: Start a new schematic by clicking the "+" icon.

5.1.2 Component Selection and Placement

For the inverter circuit, you will need the following components:

• One PMOS transistor
• One NMOS transistor
• Two voltage sources
• Two code_shown components
• Ground pins

To insert these components, navigate to the xschem folder located inside the libs.tech directory of the IHP SG13G2 PDK.

Figure 5.1.2.1: Access the component symbols as shown.

Next, go to the sg13g2_pr directory:

Figure 5.1.2.2: Enter the folder containing primitive devices.

Select the low-voltage MOSFET components:

• sg13_lv_pmos.sym
• sg13_lv_nmos.sym

Place these components on the schematic canvas. You can rotate them using Shift + r.

Figure 5.1.2.3: PMOS and NMOS placed on the Xschem canvas.

Then, open the devices folder in the xschem_library.

Figure 5.1.2.4: Path to the devices folder in the Xschem library.

Select the following additional components:

• gnd.sym
• vsource.sym
• code_shown.sym
• lab_pin.sym

Use the “c” key to copy components on the canvas.

The completed component placement should look like this:

Figure 5.1.2.5: Final component placement.

5.1.3 Wiring the Components

Use the “w” key or the wire tool to connect components. To connect nodes without visible wires, you can use lab_pin, which gives a cleaner and more professional layout.

Figure 5.1.3.1: Insert wire tool in Xschem.

The complete wiring should look like this:

Figure 5.1.3.2: Final wiring of the inverter circuit.

The xschem .sch and .sym files can be found here and here, respectively.

5.1.4 Setting Transistor Parameters

To adjust the W/L ratios of the transistors, double-click on each MOSFET and modify the values in the pop-up window.

Figure 5.1.4.1: Adjust the width-to-length ratio of MOSFETs.

Expected Characteristics of a Good Inverter Design:

• Balanced rise and fall times
• Low propagation delay
• Low power consumption

How to choose W/L?

Choose W/L ratios based on the required drive strength and power supply voltage.

You may adjust these parameters after running simulations to meet your specific design goals. For example, increasing the W/L ratio enhances the transconductance (gₘ), which can help reduce delay and increase speed.

For this design:

• PMOS W/L = 20
• NMOS W/L = 10