Adding a linear trendline to a semi-log scale in Microsoft Excel is a simple process. Here are the steps you need to follow:

1. Open the Excel file that contains the data you want to plot on a semi-log scale.
2. Select the data you want to plot on the chart.
3. Click on the "Insert" tab in the Excel ribbon.
4. Click on the "Scatter" chart type and select the chart subtype that has a semi-log scale.
5. Excel will create a chart with your data plotted on a semi-log scale.
6. Right-click on one of the data points in the chart and select "Add Trendline" from the context menu.
7. In the "Format Trendline" pane that appears on the right-hand side of the screen, select "Linear" as the trendline type.
8. Excel will add a linear trendline to your chart, which will be plotted on the semi-log scale.

That's it! You have successfully added a linear trendline to a semi-log scale in Microsoft Excel. You can customize the trendline further by adjusting its color, thickness, and other properties using the options in the "Format Trendline" pane.