A simple model describing the symmetric I-V characteristics of p polycrystalline Si/n monocrystalline Si, and n polycrystalline Si/p monocrystalline Si junctions

We present an analytical model for the current transport in polycrystalline (poly)Si/interfacial oxide/monocrystalline (c)-Si base junctions, which consistently describes the symmetrical behavior of an n⁺ poly-Si emitter/p c -Si base and p⁺ poly-Si emitter/ n c-Si base configuration. Our model is focused on a regime within which the current transport is possibly dominated by a flow through oxide pinholes rather than by tunneling. For an emitter region assumed to form underneath the interfacial oxide by diffusion of dopants from the poly-Si into the c-Si, we calculate the minority charge carrier distribution and the resistance implied for majority charge carriers. With reasonable parameters, our model simultaneously reproduces the experimentally observed low emitter saturation current densities and low junction resistances values. Our model provides a plausible explanation for the high current gain observed in p-n-p and n-p-n bipolar transistors featuring a poly-Si emitter. In principle, the obtained correlation between recombination current and series resistance is analogous to the situation in a base region of a solar cell with local rear contacts. Thus, a poly-Si/c-Si junction can be explained within the framework of a classical p-n junction picture for a passivated, locally contacted emitter.