how can i show my ads on mobile devices only
Hence in order to run ads only on mobile devices, choose the minimum bid for your ad groups and then use the bid multiplier which could be taken as 300% (which is maximum) for mobiles. Doing this will keep your ad in a very low position and it might not be eligible for the ad auction on desktops, laptops, but it will be very much eligible for ad auction on mobile devices.
Make sure that when you are creating the ads, you are putting a check mark in front of the Device preference so that Adwords give preference to those ads while displaying on mobile devices.
There is no direct way you can accomplish that especially for Search network campaigns. So you need to keep the minimum bid and then use the Device level bid adjustment and increase it to max of 300% for mobiles so that your ads show primarily on mobile devices. However it doesn't mean that your ads will not show on other devices, but the possibility will be low. Also remember to select ad preference as mobile while creating your ad copies.
In case of Display network campaign, you do have the option to select particular mobile devices and tables, Operating Systems etc...for targeting. More details here: https://support.google.com/adwords/answer/2404245?hl=en
Unfortunately there is no clear cut way to do this. The option to target mobile specifically is no longer available.
The only work around that has had any kind of success is the one that has been suggested here. You need to create a normal search campaign, set your bids to 1/3 of the recommended first page bid estimate and then set mobile bid adjustment to 300%.
This will mean that the majority of your traffic will be mobile as your ads on desktop and tablet will not have high enough ad rank to show for most of the search queries.
It is not a perfect solution, but is workable if you have to use it.