How can I switch applications on iPhone or iPad by keyboard like "Windows key + tab" on Windows? I like typing by keyboard while using iPhone and iPad. But when I switch applications(e.g. between Safari and memo application), I have to the push home putton on the devices'. It is very frustrating. Does anybody know how to switch applications by keyboard like "Windows key + tab" on Windows?
For iPad, ensure that multitasking gestures are turned on in "Settings > General > Multitasking Gestures". Instructions to switch between applications are right below the button.

IPhones do not support task switching with a Bluetooth keyboard.

On iPhones equipped with a 3D Touch screen, you can switch applications by force pressing on the left-most corner of the screen. This will open the task switching view.