Extrinsic Semiconductor

Different dopants have distinct effects on the semiconductor’s conductivity and behavior, giving rise to two primary types of extrinsic semiconductors: n-type and p-type.

1. N-type Semiconductors

N-type semiconductors are characterized by excess electrons, resulting in a negative charge carrier. This abundance of electrons is achieved by introducing donor dopants into the semiconductor crystal lattice. Group V elements such as phosphorus (P), arsenic (As), and antimony (Sb) have five valence electrons (pentavalent) and are commonly used as dopants.

When these pentavalent impurities replace some of the host semiconductor atoms, they introduce an extra electron into the crystal structure. These additional electrons contribute to the semiconductor’s conductivity, making it more efficient in carrying an electric current.

2. P-type Semiconductors

On the other hand, p-type semiconductors feature a surplus of “holes” or locations where an electron is missing. This positive charge carrier is achieved by incorporating acceptor dopants into the semiconductor lattice. Group III elements, such as boron (B), aluminum (Al), and gallium (Ga) with three valence electrons (trivalent), are typically used in this context.

Introducing trivalent impurities creates holes in the crystal structure, where electrons can move freely. These holes effectively act as positive charge carriers, enhancing the semiconductor’s overall conductivity.

The choice of dopant is a critical factor in semiconductor manufacturing, influencing the performance and applications of electronic devices. Engineers carefully select trivalent or pentavalent dopants based on the desired electrical characteristics and functionality of the semiconductor.

In an extrinsic semiconductor, variations in ambient temperature induce the generation of minority charge carriers. Additionally, dopant atoms contribute to the production of majority carriers. The recombination process destroys a significant portion of these minority carriers, reducing their concentration. As a result, this alteration impacts the energy band structure of the semiconductor. In such semiconductors, additional energy states come into play:

1. Energy state attributed to donor impurity
2. Energy state attributed to acceptor impurity

The energy band diagram below illustrates an n-type and a p-type semiconductor. 

The energy level of the donor is lower than that of the conduction band in the n-type. Consequently, electrons can transition into the conduction band with minimal energy requirements (approximately 0.01 eV for Si). Moreover, at room temperature, a majority of donor atoms, along with a small fraction of host atoms, become ionized. The conduction band predominantly accommodates electrons originating from the donor impurities.

In the p-type, the energy level of the acceptor surpasses that of the valence band. Consequently, electrons can move from the valence band to the acceptor level with minimal energy expenditure. At room temperature, many acceptor atoms undergo ionization, creating holes in the valence band. Therefore, the valence band primarily contains holes originating from the impurities.

Here is a table highlighting the differences between intrinsic and extrinsic semiconductors:

Characteristic	Intrinsic Semiconductor	Extrinsic Semiconductor
Origin	Pure semiconductor material	Semiconductor material with intentionally added impurities
Carrier Type	Electrons and holes (intrinsic carriers)	Electrons (n-type) or holes (p-type) as predominant charge carriers
Carrier concentration	An equal number of electrons and holes	Predominance of either electrons (n-type) or holes (p-type) depending on the dopant used
Dopants	No intentional dopants	Intentional addition of dopants to modify electrical properties
Conductivity	Relatively lower conductivity	Enhanced conductivity due to added charge carriers
Examples	Pure silicon or germanium	Silicon doped with phosphorus (n-type) or boron (p-type)
Applications	Used in essential electronic components like diodes and transistors	Widely employed in advanced electronic devices and integrated circuits