In the field of electronics, understanding the behavior of a semiconductor is crucial. The properties of a semiconductor, especially its electrical conductivity, are heavily influenced by the position of its Fermi energy level.

Semiconductor Introduction and Fermi Energy

A semiconductor is a material that has electrical conductivity properties between those of a conductor (like copper) and an insulator (like glass). Its conductivity can be controlled through a process called doping, where impurities are intentionally added. Key energy levels in a semiconductor include the valence band (where electrons are normally bound to atoms) and the conduction band (where electrons are free to move and conduct electricity). Between these two bands lies the band gap, an energy range where no electron states exist.

The Fermi energy level ($\text{E}_{\text{F}}$) is a hypothetical energy level at which the probability of an electron occupying an energy state is exactly 0.5 at any temperature above absolute zero. Its position within the energy band structure of a semiconductor indicates the type of material and its dominant charge carriers (electrons or holes).

Fermi Energy Level in Conduction Band

The question specifically asks what type of semiconductor is formed when the Fermi energy level lies within the conduction band. When this occurs, it signifies a very high concentration of free electrons in the conduction band. This condition arises from extremely heavy doping of an intrinsic semiconductor with donor impurities (elements from Group V, like Phosphorus or Arsenic, for silicon).

• Heavy doping leads to a large number of excess electrons.
• These excess electrons fill the lower energy states of the conduction band.
• The Fermi energy level, which represents the highest occupied energy level at absolute zero (and the 50% probability level at higher temperatures), is pushed up into the conduction band due to the abundance of electrons.

Degenerate Semiconductors Explained

A semiconductor is considered degenerate when the doping concentration is so high that the Fermi energy level moves into either the conduction band (for n-type) or the valence band (for p-type). This makes the material behave more like a metal in terms of conductivity, even at low temperatures, because there are a large number of charge carriers readily available.

• Degenerate n-type semiconductor: This is characterized by the Fermi energy level lying within the conduction band. It implies a very high concentration of electrons as majority carriers.
• Degenerate p-type semiconductor: In this case, the Fermi energy level lies within the valence band, indicating a very high concentration of holes as majority carriers.

N-type Semiconductor Characteristics

An n-type semiconductor is formed by doping an intrinsic semiconductor with donor impurities. These impurities contribute extra electrons to the material, making electrons the majority charge carriers. In a lightly doped or "non-degenerate" n-type semiconductor, the Fermi energy level lies in the band gap, but closer to the conduction band edge.

However, when the doping level is exceptionally high, leading to the Fermi energy level entering the conduction band, the semiconductor transitions from a "non-degenerate n-type" to a "degenerate n-type". This distinction is critical in various electronic device applications.

Therefore, based on the definition and characteristics, if the Fermi energy level lies in the conduction band, the semiconductor is known as a degenerate n-type semiconductor.