documentclass[compsoc, conference, letterpaper, 10pt, times]{IEEEtran}
ifCLASSOPTIONcompsoc
  % IEEE Computer Society needs nocompress option
  % requires cite.sty v4.0 or later (November 2003)
  usepackage[nocompress]{cite}
else
  % normal IEEE

  usepackage{cite}
fi
ifCLASSINFOpdf
else
fi

usepackage{colortbl}
usepackage{array}
usepackage{booktabs}
usepackage{multirow}
newcommand{head}[1]{textnormal{textbf{#1}}}
newcommand{normal}[1]{multicolumn{1}{l}{#1}}
usepackage{amssymb}% http://ctan.org/pkg/amssymb
usepackage{pifont}% http://ctan.org/pkg/pifont
newcommand{cmark}{ding{51}}%
newcommand{xmark}{ding{55}}%
usepackage{tablefootnote}
usepackage{lipsum}

begin{document}
title{Test}
lipsum
maketitle

begin{table}
caption{mytable}
label{table:mytable}
centering
begin{tabular}{ cccc}
toprule
&   multicolumn{3}{c}{B} \
cmidrule(lr){2-4}
multicolumn{1}{c}{multirow{-2}{*}[0.5ex]{A}}
&   AAA & BBB & CCC \
midrule
rowcolor[gray]{.9}
Text1tablefootnote{label{note1}footnote} &   cmark & cmark & cmark \
Text2textsuperscript{ref{note1}} &   xmark & cmark & xmark \
rowcolor[gray]{.9}
Text3 &   cmark & xmark & xmark \
Text4 &   cmark & cmark & cmark \
bottomrule
end{tabular}

lipsum
end{table}
end{document}

documentclass{article}
usepackage{threeparttable}

begin{document}
begin{threeparttable}
    caption{A table}
    begin{tabular}{lll}
        hline\
        A columntnote{a}&another column&column 3\
        hline\
        1tnote{b}&2&3\
        4&5tnote{b}&6\
        7&8&9\
        hline\
    end{tabular}
    begin{tablenotes}
        raggedright
        item[a] A table note.
        item[b] A rather longer table note referred to in multiple places.
    end{tablenotes}
end{threeparttable}
end{document}

documentclass{article}
usepackage{threeparttable}
usepackage{siunitx}
usepackage{etoolbox}
usepackage{booktabs}
usepackage{hyphenat}
usepackage{multicol}
DeclareRobustCommand{Wmk}{si{wattpermetreperkelvin}}
newcommand*rfrac[2]{{}^{#1}!/_{#2}}%running fraction with slash - requires math mode.
robustifytnote %for using siunitx and threeparttable
apptoTPTnoteSettings{footnotesize} %and for making threeparttable's footnotes footnotesize
sisetup{tight-spacing=true,scientific-notation=false}%

begin{document}
begin{threeparttable}
    caption[Highhyp{}frequencyhyp{}related material parameters.]{label{tab_Theory_Materials}Highhyp{}frequencyhyp{}related material parameters.  Based on a table in reference emph{dummy}, some data from reference emph{dummy}.}
    small{
    begin{tabular}{@{}l@{}S@{}S@{}S@{}S@{}S@{}S@{}S@{}S@{}}
        toprule
        &multicolumn{1}{c}{$E_g$tnote{a}}&multicolumn{1}{c}{$n_i$tnote{b}}&multicolumn{1}{c}{$epsilon_r$tnote{c}}&multicolumn{1}{c}{$mu_n$tnote{d}}& multicolumn{1}{c}{$v_mathrm{sat}$tnote{e}}&multicolumn{1}{c}{$E_mathrm{br}$tnote{f}}&multicolumn{1}{c}{$mathit{TC}$tnote{g}}& multicolumn{1}{c}{$mathit{JM}$tnote{h}}\
        &multicolumn{1}{c}{small{si{eV}}}&multicolumn{1}{c}{small{si{cm^{-3}}}}&multicolumn{1}{c}{}&multicolumn{1}{c}{small{si{cmpervoltpers}}}& multicolumn{1}{c}{small{$times 10^7$si{cmpers}}}&multicolumn{1}{c}{small{si{megavoltpercm}}}&multicolumn{1}{c}{small{Wmk}}&multicolumn{1}{c}{}\
        midrule\
        Si&1.1&1.5e10&11.8&1350&1.0&0.3&150&1\
        GaAs&1.42&1.5e5&13.1&8500&1.0&0.6&43&2.7\
        SiC (4H)&3.26&8.2e-9&10&700&2.0&3.0&450tnote{~i}&20\
        GaN&3.4&1.9e-10&9.0&2000tnote{~j}&2.5&3.3&130&27.5\
        Diamond&5.4&1.6e-27&5.5&1900&2.7&5.6&2000&50\
        bottomrule
    end{tabular}}
    begin{tablenotes}
        setlength{columnsep}{0.8cm}
        setlength{multicolsep}{0cm}
        begin{multicols}{2}raggedright
            item[a] Bandgap.
            item[b] Intrinsic carrier density at room temperature.
            item[c] Relative permittivity.
            item[d] Electron mobility.
            item[e] Saturation velocity.
            item[f] Breakdown field.
            item[g] Thermal conductivity at room temperature. The thermal conductivity reduces with temperature at typical operating temperatures. For GaN $mathit{TC}propto T^{-1.4}$ is often used, though values vary.
            item[h] Johnson figure of merit, which compares the powerhyp{}frequency performance of materials, normalised to the value for Si. $JM=rfrac{E_mathrm{br}v_mathrm{sat}}{2pi}$.
            item[i] In-plane; parallel to the $c$-axis the thermal conductivity is 330~Wmk.
            item[j] Within the 2DEG; the bulk value is SI[scientific-notation=false]{1200}{cmpervoltpers}.
        end{multicols}
    end{tablenotes}
end{threeparttable}
end{document}