edit: something like this:

\documentclass{article}\usepackage{tikz}\usetikzlibrary{trees, positioning, decorations.pathreplacing, calc}\begin{document}    \begin{tikzpicture}[ node distance = 2mm and 0mm,font=\footnotesize,          grow = down,sibling distance=22mm,level distance = 11mm,%      B/.style = {decorate,                  decoration={brace, amplitude=3pt,                  raise=1mm, mirror},                  thick},every node/.append style = {align=center, anchor=north},                    ]\node (TOO) {Theory of\\Operation } % root    child { node {Tunnelling}        child {node (NI) {Without \\Illumination}}        child {node (I) {Under\\ Illumination}}            }    child { coordinate (NEGFr) node (Geo) {Geometric}}    child { node {Hybrid}}    ;% end of basic treee\begin{scope}[every node/.append style={align=left}]    % Now we list all the nodes down of each tree branch\node[below right=of NI.south west] (Simmons)   {- Simmons\\[1ex]                                                 - Sanchez};\node[below right=of Simmons.south west] (TMM)  {- TMM\\[1ex]                                                 - QTBM};\node[below right=of I.south west] (PAT)        {- PAT\\[1ex]                                                 - Floquet};\node[below right=of PAT.south west] (Mayer)    {- Mayer};    \end{scope}% NEGF node\path (TMM.south west) -- coordinate (aux) (TMM.south -| Geo.east);\path   let \p1 = ($(TMM.west) - (TMM -| Geo.east)$),             \n1 = {veclen(\x1,\y1)} in        node (NEGF) [draw, minimum width=\n1, below=of aux] {- NEGF};  % explanatory text\draw[B]    (Simmons.north west) --    node[below=2mm,font=\tiny, sloped] {Single\\ barrier\\ insulator}                                    (Simmons.south west); \draw[B]    (TMM.north west) --    node[below=2mm,font=\tiny, sloped] {Single \& \\ multi barrier\\ insulator}                                    (TMM.west |- NEGF.south);    \end{tikzpicture}\end{document}

• Cheers Zarko, Very close with just one additon; I wish to add another node whose name is {-NEGF}, and is meant to horizontally span the tree such that it starts from the left side of the nodes below {-QTBM} and extends to below the right side of the {Geometric}. Of course, it should be the lowest item (i.e. comes below the {-QTBM} node. For that reason I used the \p & let comands to extract the x ccordinates of both boxes, start drawing the node from the middle x-space between them, but should be located with the y of the lowest node (i.e. (QTBM) one)

  Mazen

  – Mazen

  2017-08-20 21:40:04 +00:00

  CommentedAug 20, 2017 at 21:40
• @Mazen, thank you for comment. i correct mymwe accordingly

  Zarko

  – Zarko

  2017-08-21 01:29:13 +00:00

  CommentedAug 21, 2017 at 1:29
• Cheers Zarko now that works! I have altered this bit in my code just a little but all in all this is great! I will post the working code AW.

  Mazen

  – Mazen

  2017-08-21 10:16:29 +00:00

  CommentedAug 21, 2017 at 10:16

Something like this?

I used Forest because it makes doing things with trees much easier, generally. I usedfit to create the spanning node.

\documentclass[border=10pt]{standalone}\usepackage{forest}\usetikzlibrary{positioning,fit,calc}\begin{document}\begin{forest}  for tree={%    align=center,    font=\footnotesize,  },  before typesetting nodes={%    where={ > Ow+P {n children} {isodd(#1)} }{%      calign=child edge,      if n children=1{%        parent anchor=south west,        for children={%          no edge,          align=left,          content/.wrap value={\textbullet{} #1},          child anchor=north west,          anchor=north west,          before computing xy={l'=1.75\baselineskip},        },      }{%        calign child/.process={ Ow+n {n children} {(#1+1)/2} },      },    }{},  }  [Theory of\\Operation, name=to % names are optional - needed only if you want to refer to nodes by name later    [Tunnelling, name=t      [Without\\Illumination, name=wi        [Simmons, name=si          [Sanchez, name=sa            [TQM, name=tq              [QTBM, name=qtbm]            ]          ]        ]      ]      [Under\\Illumination, name=i        [PAT, name=p          [Floquet, name=f            [Mayer, name=m]          ]        ]      ]    ]    [Geometric, name=g]    [Hybrid, name=h]  ]  \path (current bounding box.south west) coordinate (b) -- (b -| g.east) node (n) [midway, below, font=\footnotesize] {\textbullet{} NEGF};  \node [draw, inner sep=0pt, fit=(n) (b) (b -| g.east)] {};  \begin{scope}[every node/.append style={font=\tiny, rotate=270, inner sep =0cm, align=center}]    \node (sb) [anchor=north east] at (sa.south west) {Single barrier\\insulator};    \node (sbm) [anchor=north west] at (tq.north west) {Single \& multi\\barrier insulator};  \end{scope}  \draw [densely dashed] (current bounding box.west |- {$(sa.south)!1/2!(tq.north)$}) coordinate (a) -- (wi.east |- a);\end{forest}\end{document}

• Many thanks cfr, The diagram is what I want. However, this is too advanced for the level of work im doing. I wish also to extend over the already available code becuase this is not the final form and there may be some alterations, and so I wish to have a scalable code I can understand. BTW: I have read Ow+P & Ow+n are arguments processors but wasn't able to understand what they do exactly.

  Mazen

  – Mazen

  2017-08-20 23:20:19 +00:00

  CommentedAug 20, 2017 at 23:20
• @Mazen Best use Zarko's code, then. I added this as an alternative since you already have an answer which modifies your existing code. Forest is great, but it isn't always the best tool, especially given an existing document which relies on an alternative. Yes, they are argument processors.Ow+P means the first argument is the name of an option, the next argument does something with the value of that option and processes it withpgfmath (e.g. checking if odd).Ow+n is the same, but the processing is done using TeX manipulation of number expressions (e.g. summation), which is faster.

  cfr

  – cfr

  2017-08-20 23:56:55 +00:00

  CommentedAug 20, 2017 at 23:56
• @Mazen Note that the code I gave for the node you wanted to add doesn't depend on Forest. You can use it with Zarko's code for the rest. It is just TikZ code at that point. Just use it after drawing the tree but before adding the annotations at the left side.

  cfr

  – cfr

  2017-08-20 23:58:39 +00:00

  CommentedAug 20, 2017 at 23:58

Following is the working code. Altered a tiny bit of that of @Zarko. A very enlighting note is gievn by @cfr. So many thanks to both of you.

\documentclass{article}\usepackage{tikz}\usepackage{lipsum}\usetikzlibrary{trees,positioning, graphs, calc}\begin{document}\tikz [font=\footnotesize, grow=down, level 1/.style={sibling distance=2.6cm},level 2/.style={sibling distance=2.5cm}, level distance=1cm]{    \node   [align =center](TOO){Theory of\\Operation } % root    child { node {Tunnelling}        child { coordinate (NEGFl) node [align=center](NI) {Without \\Illumination}}        child {node [align=center](I) {Under\\ Illumination}}    }    child { coordinate (NEGFr) node (Geo) {Geometric}    }    child { node {Hybrid}    }    ;% This comma is for the parent \node DON'T REMOVE    % Now we list all the nodes down of each tree branch    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (NI.south west)(Simmons){-Simmons};    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Simmons.south west)(Sanchez){-Sanchez};    \node[below right][inner sep = 0.4, minimum height =0.8cm] at (Sanchez.south west)(TMM){-TMM};    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (TMM.south west)(QTBM){-QTBM};    %\node[below right][inner sep = 0.4, minimum height =0.6cm, minimum width =10pt, fit = (NEGFl)(NEGFr)] at (QTBM.south west)(NEGF){-NEGF};    % Now let's list the nodes below the "With Illumination" option    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (I.south west)(PAT){-PAT};    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (PAT.south west)(Floquet){-Floquet};    \node[below right][inner sep = 0.4, minimum height =0.6cm] at (Floquet.south west)(Mayer){-Mayer};    \draw (QTBM.south west) -- coordinate (aux) (QTBM.south -| Geo.east);    \path   let                 \p1=($(QTBM.west) - (QTBM -| Geo.east)$),                \n1= {veclen(\x1, \y1)}            in                node (NEGF) [below right, draw, minimum width=\n1 ] at (QTBM.south west) {-NEGF}    ;    %Now let's put some explanatory text around    %Hint: You can use below above, right, left,... OR use anchor = east, west,...    %Single insulator models text    \node [below left, rotate=270, inner sep = 0cm, align= center, text width =1.5cm] at (Sanchez.south west)(Ss){\tiny Single barrier insulator};    %Multi insulator text    \node [below right, rotate=270, inner sep = 0cm, align= center, text width =2cm] at (Sanchez.south west)(Ms){\tiny Single \& multi barrier insulator};    %Draw dashed line to separate between single & Multi insulator models    \draw [dashed] (Ss.south east) -- ($(Sanchez.south east) + (0.5cm, 0)$);} %This bracket closes the \tikz command\end{document}

• tikz-pgf
• calc