Dr. Smith is consenting a subject using an electronic informed consent form on a tablet device. If the subjects sign their name electronically on the tablet, will that be a legally valid signature?

Yes, it is considered a legally valid signature. Instead of using the manual method that requires signing the consent form using a pen, the electronic method simply required you to sign your name in full on the tablet and if this is obtained in the right way, it can be seen as legally valid and original for record keeping purposes.