Block from U+3100 to U+312F. This block was introduced in Unicode version 1.1 (1993). It contains 41 codepoints.
The Wikipedia provides the following information on block Bopomofo:
Bopomofo is a Unicode block containing phonetic characters for Chinese. It is based on the Chinese standard GB 2312. Additional Bopomofo characters can be found in the Bopomofo Extended block.