A lot of personal information is getting stored on cell phones.

A lot of personal information is getting stored on cell phones. People say you simply remove the memory chip, but the chip is usually a copy of the information stored on the phone.  Moving the chip does not delete the old info.  What is the best way to dispose of a cell phone without leaving your personal information out there for someone else to access?  In this disposable society, cell phones are good for about 2 years and then you buy a new one. The old one usually ends up in the trash. Are they biodegradable.