Semiconductor Junctions - 8.1 p-n homojunctions - 8.1.1 Formation of a space-charge region in the p-n junction

Oct 29, 2026

Semiconductor junctions are fundamental components of solar cells, where junction physics governs carrier transport and device operation. The text explains p-n homojunctions and compares them with p-n heterojunctions, p-i-n junctions, and metal-semiconductor (MS) junctions such as Schottky barriers. It then derives how bringing p-type and n-type materials together creates a space-charge (depletion) region, establishes an internal electric field, and leads to diffusive equilibrium where concentration gradients and field forces cancel, yielding no net current.

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8 Semiconductor Junctions

8.1 p-n homojunctions

8.1.1 Formation of a space-charge region in the p-n junction

8.1.2 The p-n junction under equilibrium

8 Semiconductor Junctions Almost all solar cells contain junctions between (different) materials of different doping. Since these junctions are crucial to the operation of the solar cell, we will discuss their physics in this chapter. A p-n junction fabricated in the same semiconductor material, such as c-Si, is an example of an p-n homojunction. There are also other types of junctions: A p-n junction that is formed by two chemically different semiconductors is called a p-n heterojunction. In a p-i-n junctions, the region of the internal electric ﬁeld is extended by inserting an intrinsic,i, layer between the p-type and the n-type layers. The i-layer behaves like a capacitor; it stretches the electric ﬁeld formed by the p-n junction across itself. Another type of the junction is between a metal and a semiconductor; this is called a MS junction. The Schottky barrier formed at the metal-semiconductor interface is a typical example of the MS junction. 8.1 p-n homojunctions 8.1.1 Formation of a space-charge region in the p-n junction Figure 8.1 shows schematically isolated pieces of a p-type and an n-type semiconductor and their corresponding band diagrams. In both isolated pieces the charge neutrality is maintained. In the n-type semiconductor the large concentration of negatively-charged free electrons is compensated by positively-charged ionised donor atoms. In the p-type semiconductor holes are the majority carriers and the positive charge of holes is compensated by negatively-charged ionised acceptor atoms. For the isolated n-type semicon- n-type region p-type region Figure 8.1: Schematic representation of an isolated n-type and p-type semiconductor and corresponding band diagrams. ductor we can write n = nn0 􀀙 ND, (8.1a) p = pn0 􀀙 n2i.ND . (8.1b) For the isolated p-type semiconductor we have p = pp0 􀀙 NA, (8.2a) n = np0 􀀙 n2i.NA . (8.2b) When a p-type and an n-type semiconductor are brought together, a very large difference in electron concentration between n-and p-type regions causes a diffusion current of electrons from the n-type material across the metallurgical junction into the p-type material. The term “metallurgical junction” denotes the interface between the n-and p-type regions. Similarly, the difference in hole concentration causes a diffusion current of holes from the p-to the n-type material. Due to this diffusion process the region close to the metallurgical junction becomes almost completely depleted of mobile charge carriers. The gradual depletion of the charge carriers gives rise to a space charge created by the charge of the ionised donor and acceptor atoms that is not compensated by the mobile charges any more. This region of the space charge is called the space-charge region or depleted region and is schematically illustrated in Fig. 8.2. Regions outside the depletion region, in which the charge neutrality is conserved, are denoted as the quasi-neutral regions. space charge E-!eld Figure 8.2: Formation of a space-charge region, when n-type and p-type semiconductors are brought together to form a junction. The coloured part represents the space-charge region. The space charge around the metallurgical junction results in the formation of an internal electric ﬁeld which forces the charge carriers to move in the opposite direction than the concentration gradient. The diffusion currents continue to ﬂow until the forces acting on the charge carriers, namely the concentration gradient and the internal electrical ﬁeld, compensate each other. The driving force for the charge transport does not exist any more and no net current ﬂows through the p-n junction. 8.1.2 The p-n junction under equilibrium The p-n junction represents a system of charged particles in diffusive equilibrium in which the electrochemical potential is constant and independent of position. The electro-chemical potential describes an average energy of electrons and is represented by the Fermi energy. Figur

FAQs

Why are semiconductor junctions crucial in solar cells?

Solar cells rely on junctions between differently doped materials, and the junction physics determines how charge carriers move and how the device operates.

What defines a space-charge (depletion) region in a p-n junction?

When p-type and n-type semiconductors are joined, diffusion causes mobile carriers near the metallurgical junction to be depleted. The remaining ionized donor and acceptor atoms create the space charge in that region.

What happens to current through a p-n junction at equilibrium?

Diffusion continues until the internal electric field balances the concentration gradient. The driving forces cancel, so no net current flows through the p-n junction.

Oct 29, 2026

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