VanderTiffany的部落格

5 Junctions
5.1 Introduction
5.2 Basis of photovoltaic action
5.3 Work function of material
5.4 Metal-semiconductor junction
5.5 Semiconductor-semiconductor junction
5.6 Electrochemical junction
5.7 Junctions in organic materials
5.8 Surface and interface states

5.2 p-i-n junction

The difference between p-n junction and p-i-n junction is that the layer between p-doped and n-doped layer has been left undoped (intrinsic, i). In this case, the built-in bias the same as in the p-n junction, but the electric field extends over a wider region. The depletion region is elongated. This is preferrable in materials with short minority carrier diffusion lengths, which leads to the minority carriers generated by incoming light being unlikely to contribute to the photocurrent.

Fig. 5.8(a): Band profiles of p-, i- and n-type semiconductor in isolation.

Fig. 5.8(b): Band profile of p-i-n junction in equilibrium.

5.5.3 p-n heterojunction

A heterojunction always consists of two different materials with different band gaps. These heterojunctions can also be either p-n or p-i-n junctions. Devices based on heterojunctions can improve carrier collection. Due to the change in the band gap, a discontinuity exists in the conduction and valence band edges at the junction.

The potential step will affect the effective fields for the two carrier types in different ways. Usually, one carrier type is assisted by the field change, while the other is opposed. In fig. 5.9(b), the field that drives electrons to the n side is increased, while the field driving holes towards the p side is decreased.

Fig. 5.9(a): Band profiles of p and n-type semiconductor of different band gap in isolation.

Fig. 5.9(b): Band profile of p-n heterojunction in equilibrium.