Citation with DOI that has underscore breaks functionality in BibDesk

For your particular example (of the unified.bst) style, it suffices to add the following line to your preamble (and load the hyperref package)

\newcommand{\doi}[1]{\textsc{doi}: \href{http://dx.doi.org/#1}{\nolinkurl{#1}}}

The MWE

\documentclass{article}

\usepackage{natbib}
\usepackage{hyperref}

\begin{filecontents}{\jobname.bib}
@article{chomsky1977b,
    Author = {Chomsky, Noam and Lasnik, Howard},
    Doi = {10.1007/978-94-015-6859-3_4},
    Journal = {Linguistic Inquiry},
    Keywords = {control theory, syntax},
    Number = {3},
    Pages = {425-504},
    Title = {Filters and Control},
    Volume = {8},
    Year = {1977}}
\end{filecontents}

\newcommand{\doi}[1]{\textsc{doi}: \href{http://dx.doi.org/#1}{\nolinkurl{#1}}}

\begin{document}

\citet{chomsky1977b}

\bibliographystyle{unified}
\bibliography{\jobname}

\end{document}

then yields

If you look at your .bst, i.e. the BibTeX style file, there should be a function formating the DOI output.

There you could use \hreffrom the hyperref package and \detokenize as mentioned here or here, you can do something like

FUNCTION {format.doilink}
{ duplicate$ empty$
{ pop$ "" }
{ doi empty$
    { skip$ }
    { "{,\ doi:~\href{http://dx.doi.org/" * doi * "}{\detokenize{" * doi * "}}}" * }
  if$
}
if$
}

which is based on the naming used in style files of the elsevier BibTeX files, so your function in the .bst file might differ in naming. The function above pouts a “doi:” in front, as your example suggests. So you could just search your style file for that and change the output after that.

There is a nice solution to similar issue by Roly in Missing $ inserted issue, who recommends to use \usepackage[strings]{underscore}.

The package arranges that, while in text, ‘_’ itself behaves as \textunderscore (the behaviour of _ in maths mode is not affected).