If you are planning to resign from your job in a professional way it is better to give some notice period for your employer so that they have the time to transfer your responsibilities to some other person. It not only helps the employer but also you in collecting your dues properly. So it is better to give a two weeks notice letter to your employer to enable them to organize themselves in your absence. Without this if you resign your job without giving notice some of the employers need not pay you the amount for the period from your last pay check and the time you resigned. There are some things that you should know when you write a two weeks notice letter. That letter is going to be a cold one in some manner as you are resigning in a short notice. Do give it to your immediate boss and to the HR department if needed.
To soften the impact you might write a short letter which has the minimum impact. You might write a brief reason for your leaving the company. It would help the employer, if they need to improve some aspect in their company. Give your current contact number in the letter so that they can contact you if they need your help in some tasks. Who knows you might even land up getting some small contract jobs from them! Mention that you are available to them in case of emergencies and you are ready to help to train somebody if needed for your job. These would make it a professional letter.