From PCG Blog:
The American Society for Engineering Education (ASEE) publishes reports and handbooks on a variety of topics related to engineering research at the university level. The following graph is a snapshot from the 2010 Profiles of Engineering and Engineering Technology Colleges book, which provides many graphs, charts and tables detailing the state of engineering education today.
The graph shows that the most popular engineering doctoral field in the US is Mechanical, followed by Electrical/Computer, Electrical, Computer Science and Chemical.
View more ASEE sample data here.