Father and son John Hiett (senior) and John Hiatt (junior) have profiles which currently lack much in the way of primary source documentation.
Senior is first documented in Bucks County, Pennsylvania 14 of 8th 1700 by a land transaction. A collection of numerous independent 'family traditions' published 1951 (well-researched) hold that Hiatt came from England with Penn. But, records of the family before the 1700 land transaction are so far elusive.
Gilbert Cope extensively researched Quaker records in search of this family. A summary of what he searched can be found here. Quaker records for this time and place are certainly not complete or perhaps Hiatt was disowned at some point.
Speculation is made that the family originates Somerset but this appears to be based solely upon an entry in Besse's "Sufferings" [I:634] of a John Hyott who cannot so far be further traced (this is noted in the Research Notes of John Hiatt junior's profile).
Of the research on the immigrant Hiatt/Hiett available online currently, that of William Johnson can be found here. A later publication linked on the profile of Hiatt junior appears to have been withdrawn from the Family Search and FHC libraries (or at least I cannot find it, only found available print edition only via Google Books).