Joan Rivers -- Personal Doctor Took Selfie While She Was Under

Joan Rivers' personal doctor took a selfie while the comedienne was unconscious during a biopsy ... shortly before her heart stopped beating ... this according to a new report.

The doctor performing the endoscopy had just finished when Joan's personal ear nose and throat doctor began performing a biopsy which was done without her consent -- this according to the CNN report.

A staffer at the Yorkville Endoscopy clinic reportedly told investigators Joan's doc stopped to take the selfie.

As we previously reported ... Joan went into respiratory and cardiac arrest and never regained consciousness.

The clinic has denied a biopsy was performed.