This has been a common problem that affects most phones and majority of phone users don't really care about it until the phone goes
completely bad. Truly, there are some very sensitive parts of your phone that are allergic to much heat. Therefore, reading this post carefully will definitely save you a lot of cash now or in future.
When phone gets hot, the usual suspects are;
• Wrong or bad charger
• Inadequate access to air and
• Prolonged usage or using too many applications at once
• As all manufacturers always warns, use original charger and accessories for your phone most especially smart phones and other android devices. The use of broken chargers or cracked charger for any of this devices can cause a big damage to the phone which is best noticed when the phone gets extremely hot after charging. Yes, broken chargers is the most common cause of phones getting hot because they channel the wrong voltage to the battery which in turn leads to excessive heat that's noticed allover the body of the phone
Solution: Go get the specified charger for your device before it costs you more.
•• Secondly, majority of phone users hardly remove their phones from their pockets, maybe due to the fear of loosing it to thieves or whatever but the fact is that phones also have processors just like computers. Phones are built in such a way that the heat are taken away from heat sensitive areas in the phone to were they can be easily dispersed like the case of your phone. Now imagine keeping that same phone in your pocket away from fresh air for a long time.... Really bad I guess
Solution: Learn to hold your phone in your hand at times and keeping it away from air tight places like your pocket from time to time.
••• Lastly, prolonged usage of your phone like chatting on different chat sites at the same time or any other application will also increase the heat of your phone through the processor. So, whenever you are using your phone for a long period of time, you should also expect a change in temperature of the phone
For more information: http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries