Sweep-out experiments: A new spectroscopy for the electronic density of states in doped semiconductors

The forward-bias transient response of i–n–i and i–p–i semiconductor structures is analysed in terms of space-charge-limited and emission-limited currents. It is shown that an initial space-charge-limited current governed by the i-layer characteristics gives way at later times to an emission-limited current from the doped layer. It is shown that the emission-limited current response can be used as a new spectroscopy for determining the electronic density of states near the mobility edge of doped semiconductors. The analysis is applied to the previously reported experimental data on hydrogenated amorphous silicon (a-Si:H). It is concluded that n-type a-Si:H has a sharp peak within 0.3 eV of the conduction-band mobility edge, while slowly cooled p-type a-Si:H has a broader peak within 0.4 eV of the valence edge.