Understanding Doping in Germanium Crystals

Semiconductors like Germanium (Ge) are materials that have electrical conductivity between that of a conductor and an insulator. The electrical properties of semiconductors can be significantly altered by adding small amounts of other elements, a process called doping.

Doping involves adding impurities to the intrinsic semiconductor crystal. The type of semiconductor created depends on the type of impurity atom added.

Doping Germanium with Phosphorus

Germanium is a Group IV element, meaning it has 4 valence electrons that form covalent bonds with neighboring Germanium atoms in the crystal lattice. Phosphorus (P) is a Group V element, having 5 valence electrons.

When a Phosphorus atom is introduced into a Germanium crystal lattice during doping, it replaces a Germanium atom. The Phosphorus atom uses 4 of its 5 valence electrons to form covalent bonds with the four neighboring Germanium atoms. The fifth valence electron of the Phosphorus atom is not needed for bonding and is relatively free to move within the crystal lattice. This extra electron is loosely bound to the Phosphorus atom and can easily become a mobile charge carrier.

Impurities that donate extra electrons to the semiconductor crystal are called donor impurities. Phosphorus is a donor impurity when added to Germanium or Silicon.

Formation of N-type Semiconductor

The addition of donor impurities, like Phosphorus to Germanium, increases the number of free electrons in the crystal. Since electrons carry a negative charge, the semiconductor becomes an N-type semiconductor (N stands for Negative). In an N-type semiconductor, electrons are the majority charge carriers, and holes (absence of an electron) are the minority charge carriers.

Analyzing the Options

• P-type semiconductor: This type is formed when a semiconductor is doped with acceptor impurities (Group III elements) which create holes, not extra electrons. Doping Ge with Phosphorus (Group V) does not create a P-type semiconductor.
• N-type semiconductor: As explained above, doping Germanium with Phosphorus (a Group V element) introduces extra electrons, making it an N-type semiconductor.
• Photosensitive: While doping can affect photosensitivity, the primary outcome of doping Ge with P is the creation of an N-type semiconductor by changing its conductivity type, not necessarily making it photosensitive. Photosensitivity depends on the material's band gap and interaction with light.
• An insulator: Doping a semiconductor like Germanium increases its conductivity, it does not turn it into an insulator. Insulators have very low conductivity.

Therefore, doping a Germanium crystal with a Phosphorus atom results in the formation of an N-type semiconductor.