Understanding n-type Semiconductors

Semiconductors are materials that exhibit electrical conductivity between that of a conductor and an insulator. Their conductivity can be precisely controlled by adding impurities, a process known as doping. Based on the dominant charge carriers (electrons or holes), semiconductors are classified into two main types: n-type and p-type.

What Defines an n-type Semiconductor?

An n-type semiconductor is defined by having an abundance of free electrons as the majority charge carriers. This condition is typically achieved by doping an intrinsic semiconductor material, such as Silicon ($Si$) or Germanium ($Ge$) (both Group IV elements), with impurity atoms that possess five valence electrons (pentavalent impurities). Examples of pentavalent impurities include Phosphorus ($P$), Arsenic ($As$), and Antimony ($Sb$). When these impurity atoms integrate into the semiconductor's crystal lattice, they form covalent bonds with the surrounding semiconductor atoms. Since the impurity atom has five valence electrons and only four are needed for bonding, one electron remains loosely bound. This extra electron requires very little energy to break free and become a mobile charge carrier in the conduction band, thus creating an n-type semiconductor.

Analyzing the Semiconductor Options

Let's evaluate each of the provided options:

Option 1: Silicon doped with Aluminium

• Silicon ($Si$) belongs to Group IV and has 4 valence electrons.
• Aluminium ($Al$) is a Group III element with 3 valence electrons.
• Doping $Si$ with $Al$ introduces acceptor atoms. An $Al$ atom, substituting for a $Si$ atom, cannot provide enough electrons to complete the four covalent bonds with its neighbors. This deficit creates a vacancy, known as a 'hole'.
• Holes act as positive charge carriers. Therefore, Silicon doped with Aluminium is a p-type semiconductor.

Option 2: Silicon doped with Boron

• Silicon ($Si$) is a Group IV element.
• Boron ($B$) is also a Group III element, possessing 3 valence electrons.
• Similar to doping with Aluminium, introducing Boron into the Silicon crystal lattice creates a deficiency of electrons, resulting in the formation of holes.
• Consequently, Silicon doped with Boron is also classified as a p-type semiconductor.

Option 3: ZnO with Zinc atoms as impurities

• Zinc Oxide ($ZnO$) is a compound semiconductor material.
• $ZnO$ often exhibits non-stoichiometric crystal structures, frequently containing native defects like oxygen vacancies ($V_O$) or interstitial zinc atoms ($Zn_i$).
• Interstitial zinc atoms ($Zn_i$), where extra $Zn$ atoms reside in the spaces between lattice sites, often act as donor impurities. Each interstitial $Zn$ atom can readily release one or two electrons into the conduction band.
• Due to these donor defects, Zinc Oxide ($ZnO$) typically behaves as an n-type semiconductor.

Option 4: $Cu_2O$ with $Cu^{2+}$ ions as impurities

• Copper(I) Oxide ($Cu_2O$) is another oxide semiconductor.
• Pure $Cu_2O$ commonly exhibits p-type conductivity. This is primarily due to the presence of copper vacancies ($V_{Cu}$) in its crystal structure. A missing $Cu^{+}$ ion creates a deficit of positive charge, which is equivalent to the presence of a hole.
• The presence of $Cu^{2+}$ ions as impurities does not typically lead to n-type behavior; the inherent defect structure of $Cu_2O$ strongly favors p-type characteristics.

Final Determination

Based on the properties of semiconductor doping and intrinsic defects:

• Options 1 and 2 result in p-type semiconductors.
• Option 4 ($Cu_2O$) is typically p-type.
• Option 3 ($ZnO$ with Zinc impurities/defects) results in an n-type semiconductor.

Therefore, ZnO with Zinc atoms as impurities is the correct example of an n-type semiconductor among the given choices.