Semi-metals are materials that are simialr to metals except that they have poorer conductivity and are not malleable like metals. To explain these properties one must look at the bands of semi-metals. The band structure is shown below.
If the levels per unit energy is defined as the density of states, then it can be seen that the denstiy of states at the band gap is 0 since there are no energy levels in the band gap normally. However, if a full band and an empty band coincide in energy with a 0 density of states at their conjunciton, as in semi-metals, then there is only a few electrons to act as carriers, resulting in low conductivity. Since the bands are such, it is hard for the electron cloud to rearrange around defects, therefore they are not malleable. An example of this is graphite, which is a semi-metal in directions parallel to the sheets of carbon atoms.