how to include the Subfig captions below the main figure caption in LaTex?

I am preparing a manuscript for an IET journal. I have used the following packages for figure:

\documentclass[10pt]{article}
\usepackage{graphicx}
\usepackage{caption}
\usepackage[caption=false,font=footnotesize,labelformat=simple]{subfig}


then I used the following code snippet:

\begin{figure}[tb]
\centering
\subfloat[Tunneling and thermionic component of current]{
\includegraphics[width=0.48\linewidth]{It+Ith}
\label{It_Ith}
}
\subfloat[Total drain current]{
\includegraphics[width=0.48\linewidth]{Ids}
\label{I_ds}
}
\label{I_V}
\caption{I-V characterstics in the subthreshold regime}
\end{figure}


and the sub-captions are below the respective figures like this:

But I want the subfig captions to be under the main caption like this:

Note that, all the captions are left aligned and the sub-captions are not italic. How to generate this type of formatting?

You can achieve this using empty subcaptions and then reference them with \protect\subref* within the main caption (the * removes the parantheses - without it, you get (a) instead of a):

\documentclass[10pt]{article}
\usepackage{graphicx}
\usepackage{caption}
\usepackage[caption=false,font=footnotesize,labelformat=simple]{subfig}

\begin{document}

\begin{figure}[tb]
\centering
\subfloat[]{
\includegraphics[width=0.48\linewidth]{It+Ith}
\label{It_Ith}
}
\subfloat[]{
\includegraphics[width=0.48\linewidth]{Ids}
\label{I_ds}
}
\label{I_V}
\caption{I-V characterstics in the subthreshold regime\\[2ex]
\protect\subref*{It_Ith} \textnormal{Tunneling and thermionic component of current}\\
\protect\subref*{I_ds} \textnormal{Total drain current}
}
\end{figure}

\end{document}


The normal font can be used by \textnormal.