Question

Which of the following substances, when added in trace amounts to germanium, would produce an n-type semiconductor: (a) sulfur, (b) aluminum, (c) tin, (d) cadmium sulfide, (e) arsenic, (f) gallium arsenide? Explain.

Short answer

Hence, when added to germanium, only arsenic (e) would produce an n-type semiconductor.

Step-by-step solution

Number of Valance Electrons

As a first step, get the number of valence electrons in each atom or molecule. Germanium (Ge) has 4, Sulfur (S) has 6, Aluminum (Al) has 3, Tin (Sn) has 4, Cadmium Sulfide (CdS) Cadmium (Cd) has 2 and Sulfur (S) has 6, Arsenic (As) has 5, Gallium Arsenide (GaAs) Gallium (Ga) has 3 and Arsenic (As) has 5.

Identify Dopant Types

Impurities which possess 5 valence electrons are called Pentavalent impurities. When Pentavalent impurities such as Phosphorus, Arsenic, Antimony are added to Ge or Si, they substitute the Ge or Si atoms in the crystal. Thus Pentavalent impurity creates n-type Semiconductors by donating free electrons. On the other hand, Trivalent impurities are those impurities which possess three valence electrons. When trivalent impurities such as Boron, Aluminium, Indium are added to Ge or Si, they fit into the crystal lattice of Ge or Si by substituting the Ge or Si atoms in the lattice. Trivalent impurity creates p-type semiconductors by creating holes.

Determine Doping Effect

From step 1, we know that only As and GaAs have five valence electrons which make them Pentavalent impurities and capable of providing a free electron to Ge (n-type). Besides, Al is a Trivalent impurity, making it capable of creating the p-type of Semiconductors. Hence, sulfur, tin, and cadmium sulfide are unfit in this instance as they do not satisfy the needs to be either pentavalent for n type or trivalent for p type. So, only arsenic (e) would produce n-type semiconductor when added in trace amounts to germanium. Gallium arsenide (f) could also behave like a donor (n-type) in theory, but in practical use, it would be difficult to separate the effects of the gallium and arsenic.

Key concepts

Pentavalent impurities

Pentavalent impurities are elements that contain five valence electrons in their outer shell. These impurities are pivotal in creating n-type semiconductors as they contribute additional electrons to the semiconductor's crystal structure.
Such impurities are often referred to as donor atoms because they donate free electrons. These free electrons significantly increase the material's conductivity by allowing for easier movement of electric charge.
Examples of pentavalent impurities include:

• Phosphorus (P)
• Arsenic (As)
• Antimony (Sb)

When these elements are introduced into a semiconductor like silicon or germanium, they replace a host atom in the lattice structure, thereby increasing the number of free electrons available for conduction.

Doping in semiconductors

Doping is a critical process in the field of semiconductor technology. It involves adding a small amount of impurity to a pure semiconductor to alter its electrical properties. The goal of doping is to increase the conductivity of the semiconductor by introducing free charge carriers.
In n-type doping, pentavalent impurities are introduced into the semiconductor. These impurities donate free electrons, thus increasing the electron concentration and enhancing conductivity.
In contrast, p-type doping involves introducing trivalent impurities, which create "holes" in the structure. These holes act as positive charge carriers.

Germanium doping

Germanium is a well-known semiconductor material, and doping it can significantly alter its electrical characteristics. When you add pentavalent impurities to germanium, you are effectively creating n-type germanium.
Germanium's atomic structure naturally includes four valence electrons. By introducing pentavalent dopants, like arsenic, which have five valence electrons, germanium becomes infused with additional free electrons.
These extra electrons are what give n-type germanium its improved conduction properties, making it very effective for use in electronic devices. However, if trivalent impurities such as boron or aluminum are added instead, p-type germanium is created due to the formation of positive charge carriers or holes.

Valence electrons and semiconductor types

Understanding valence electrons is crucial for grasping the concepts of semiconductor types. Valence electrons are the electrons in the outer shell of an atom that are involved in bonding and electrical conduction.
The number of valence electrons determines how an element behaves when interacting with other materials, especially in the context of semiconductors.

• Atoms with five valence electrons, like arsenic, might act as donors in semiconductors, creating n-type materials.
• Atoms with three valence electrons, like aluminum, typically behave as acceptors, forming p-type semiconductors by creating holes.

When elements are strategically added to semiconductors, they either increase the free electron count or create holes for more effective conduction. This method significantly affects the electrical properties of materials like germanium and silicon.