Problem 39 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

Which kind of defects is introduced by doping in silicon crystal? (a) Dislocation defect (b) Schottky defect (c) Frenkel defect (d) Electronic defect

Step-by-Step Solution

Answer

Doping introduces an electronic defect in the silicon crystal.

Step 1: Understanding Doping in Silicon Crystals

Doping in silicon crystals involves adding a small amount of impurity atoms to the intrinsic (pure) silicon to modify its electrical properties. The impurity atoms introduce additional charge carriers (electrons or holes) in the silicon lattice.

Step 2: Knowing the Types of Defects

Dislocation defect is a type of defect that occurs in the crystal lattice; Schottky and Frenkel defects are types of point defects caused by vacancies and interstitial atoms respectively; Electronic defect refers to the change in electronic structure due to the presence of extra electrons or holes.

Step 3: Identifying the Defect Caused by Doping

When an impurity atom is added to a silicon crystal, it donates or accepts electrons, thereby changing the number of free carriers without disturbing the overall charge neutrality. This type of defect is related to the electronic structure, and is known as an electronic defect.

Key Concepts

Doping in Semiconductors

Doping is a critical process in semiconductor manufacturing, where a carefully controlled amount of impurity atoms is introduced into an intrinsic, or pure, semiconductor material, such as silicon. This process alters the semiconductor's electrical properties to meet specific needs, such as improving its conductivity.

Doping can be done through various techniques, including diffusion and ion implantation. Diffusion involves exposing the semiconductor to impurity atoms at high temperatures so that the atoms can diffuse into the substrate. Ion implantation, on the other hand, involves directly shooting impurity atoms into the semiconductor using an ion beam.

There are two types of doping:

• 'N-type doping' adds atoms that have more electrons than silicon (like phosphorus), introducing extra negative charge carriers known as electrons.
• 'P-type doping' adds atoms with fewer electrons than silicon (like boron), thereby creating positive charge carriers called holes.

Doping must be controlled precisely because the number of impurity atoms relative to the silicon is typically on the order of one to a million, depending on the desired electrical characteristics.

Crystal Lattice Defects

Crystal lattice defects are disruptions in the regular, repeating pattern of a crystal's lattice. They play a crucial role in dictating many of the physical properties of materials, including mechanical strength and electrical conductivity.

There are several types of defects that may occur in a crystal lattice:

• 'Point defects' occur at a single point and include vacancies where a lattice point is unoccupied and interstitial defects where an atom is located in a space between lattice points.
• 'Line defects or dislocations' are lines around which atoms are misaligned.
• 'Plane defects' occur on the surface of a crystal or on the internal planes such as grain boundaries, where misalignment of atomic planes takes place.

Doping does not typically introduce dislocations or Frenkel defects but may sometimes result in localized strain fields due to size mismatch between the impurity and silicon atoms.

Electronic Defect

An electronic defect in a semiconductor arises when the addition of impurity atoms affects the electronic structure of the material. Unlike structural defects that alter the crystal lattice, electronic defects specifically refer to changes in the energy levels within the semiconductor.

In the context of silicon doping, the impurity atoms introduced can create new energy levels near the material's conduction or valence bands. This is because when an atom with more or fewer electrons than silicon is added to the lattice, it can either donate its extra electrons to the conduction band ('donor' impurities for N-type doping) or accept electrons from the valence band ('acceptor' impurities for P-type doping), thereby altering the electronic properties.

Impact on Electrical Conductivity

These new energy levels make it easier for electrons to be thermally excited into the conduction band or for holes to be created in the valence band, significantly increasing the material's electrical conductivity. Impressively, this modification to the electronic structure does not compromise the charge neutrality of the crystal, but it does lead to an increase in the number of free charge carriers—electrons in N-type and holes in P-type semiconductors.