Write a resignation letter to your manager.How was the experience.Explain why you are leaving.

Dear Manager,

This to notify you that I would like to pursue opportunities outside this organization, and I am aspiring to move up in the technical career ladder.

It has been immense pleasure working with this organization, and especially under your leadership. I have developed skills to overcome the obstacles in the project work, and I learnt how to give estimates to track and reach each milestone. I have been involved in various types of work, which made me acquire diversified skill set.

However, I had to make a choice between making progress in career ladder and acquiring plenty of skills by staying in the same position. I chose to move up in the ladder by accepting the higher designation in a promising start-up. I shall transfer my knowledge to fellow engineer before the completion of notice period service so that there will not be any unwanted interruptions in the progress of the project.

I hope you understand my career goals and give positive feedback if I give you as reference.

Regards
John