How do I make an unbalanced binary tree?

You can use missing children (Section 18.5.3 Missing Children in the pgf manual):

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\node[circle,draw](z){$30$}
  child[missing]{}
  child{
    node[circle,draw]{40} child{node[circle,draw] {20}} child[missing] };
\end{tikzpicture}

\end{document}

You can also do this with tikz-qtree, which has a simpler syntax, especially for large trees and if most of your nodes are balanced, and only a few aren't.

\documentclass{article}
\usepackage{tikz-qtree}
\begin{document}
\tikzset{every tree node/.style={minimum width=2em,draw,circle},
         blank/.style={draw=none},
         edge from parent/.style=
         {draw, edge from parent path={(\tikzparentnode) -- (\tikzchildnode)}},
         level distance=1.5cm}
\begin{tikzpicture}
\Tree
[.50     
    [.60 ]
    [.30  
    \edge[blank]; \node[blank]{};
    \edge[]; [.40
             \edge[]; {20}
             \edge[blank]; \node[blank]{};
         ]
    ]
]
\end{tikzpicture}
\end{document}

There is also Forest, of course. This permits defining a couple of simple styles enabling a more concise tree specification.

\begin{forest}
  gappy tree
  [
    [, c phantom]
    [
      []
      [, c phantom]
    ]
  ]
\end{forest}

Complete code:

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\forestset{%
  gappy tree/.style={
    for tree={
      circle,
      draw,
      s sep'+=10pt,
      fit=band,
    },
  },
  c phantom/.style={draw=none, no edge},
}
\begin{document}
\begin{forest}
  gappy tree
  [
    [, c phantom]
    [
      []
      [, c phantom]
    ]
  ]
\end{forest}
\end{document}