2.8 Electronic Configuration of Modern Electronic Materials | Atomic Structure | Chemistry 11

This section explores how the electronic configuration of elements, particularly silicon, is fundamental to their properties as semiconductors — the cornerstone of modern electronics.

Rules of Electronic Configuration→

What is a Semiconductor?

Semiconductors are materials with electrical conductivity between that of a conductor (e.g., copper) and an insulator (e.g., glass). Their conductivity can be precisely controlled, which is crucial for electronic devices.

Electronic Configuration of Silicon ( $S i$ )

Silicon is the most widely used semiconductor material. Its unique properties arise directly from its electronic structure.

Atomic Number: 14 (14 protons, 14 electrons)
Electronic Configuration: $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{2}$

Electron distribution:

Shell	Electrons	Subshells
$n = 1$	2	$1 s^{2}$
$n = 2$	8	$2 s^{2} 2 p^{6}$
$n = 3$	4	$3 s^{2} 3 p^{2}$

The 4 electrons in the outermost (third) shell are valence electrons, responsible for covalent bonding and electrical properties.

Role of Electronic Configuration in Semiconductor Behavior

Band Structure

The arrangement of electrons in a silicon crystal lattice creates energy bands:

Valence Band: Highest occupied energy band; electrons are tightly bound to atoms and cannot move freely.
Conduction Band: Higher-energy empty band; electrons here are free to move and conduct electricity.
Band Gap ( $E_{g}$ ): The energy difference between the valence and conduction bands. In semiconductors, $E_{g}$ is small enough that electrons can be excited across it by heat or light, creating free electrons and holes (vacancies) — both of which carry charge.

Intrinsic vs. Extrinsic Semiconductors

Intrinsic Semiconductor: A pure semiconductor (e.g., Si, Ge) with low inherent conductivity.
Extrinsic Semiconductor: Conductivity enhanced by intentionally adding impurities — a process called doping.

Doping: Creating n-type and p-type Semiconductors

Doping substitutes a few silicon atoms with atoms of a dopant element. The dopant's electronic configuration determines the semiconductor type:

n-type (Negative-type) Semiconductor:
- Dopant: pentavalent element (5 valence electrons), e.g., Phosphorus ( $[N e] 3 s^{2} 3 p^{3}$ )
- Four of P's valence electrons bond with Si neighbours; the fifth electron is free in the conduction band.
- Majority charge carriers: electrons
p-type (Positive-type) Semiconductor:
- Dopant: trivalent element (3 valence electrons), e.g., Boron ( $[H e] 2 s^{2} 2 p^{1}$ )
- B's three valence electrons bond with Si neighbours, leaving a hole (electron vacancy).
- Holes migrate through the crystal as neighbouring electrons fill them.
- Majority charge carriers: holes

Feature	n-type	p-type
Dopant	Pentavalent (e.g., P, As)	Trivalent (e.g., B, Ga)
Majority Carrier	Electrons (negative)	Holes (positive)
Mechanism	Dopant donates an extra electron	Dopant creates a hole

Gallium Arsenide (GaAs) — A III-V Semiconductor

Gallium ( $G a$ , $Z = 31$ ): $[A r] 3 d^{10} 4 s^{2} 4 p^{1}$ — Group 13, 3 valence electrons.
Arsenic ( $A s$ , $Z = 33$ ): $[A r] 3 d^{10} 4 s^{2} 4 p^{3}$ — Group 15, 5 valence electrons.

In GaAs, Ga and As together provide an average of 4 valence electrons per atom (like Si), but GaAs has a direct band gap, making it highly efficient at emitting light — ideal for LEDs and laser diodes.

Transition Metals in Electronics

While silicon dominates semiconductor substrates, transition metals (d-block elements) play vital roles in specialised electronic materials:

Their partially filled $(n - 1) d$ orbitals contain unpaired electrons, generating magnetic moments.
This enables ferromagnetism (e.g., Fe, Co, Ni in permanent magnets) and properties exploited in superconductors.

Electronic Configuration of Silicon (

S i

)

Silicon is the most widely used semiconductor material. Its unique properties arise directly from its electronic structure.

Atomic Number: 14 (14 protons, 14 electrons)

Electronic Configuration:

1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{2}

Electron distribution:

Shell	Electrons	Subshells
$n = 1$	2	$1 s^{2}$
$n = 2$	8	$2 s^{2} 2 p^{6}$
$n = 3$	4	$3 s^{2} 3 p^{2}$

The 4 electrons in the outermost (third) shell are valence electrons, responsible for covalent bonding and electrical properties.

Role of Electronic Configuration in Semiconductor Behavior

The arrangement of electrons in a silicon crystal lattice creates energy bands:

Valence Band: Highest occupied energy band; electrons are tightly bound to atoms and cannot move freely.

Conduction Band: Higher-energy empty band; electrons here are free to move and conduct electricity.

Band Gap ( $E_{g}$ ): The energy difference between the valence and conduction bands. In semiconductors,

E_{g}

is small enough that electrons can be excited across it by heat or light, creating free electrons and holes (vacancies) — both of which carry charge.

Intrinsic Semiconductor: A pure semiconductor (e.g., Si, Ge) with low inherent conductivity.

Extrinsic Semiconductor: Conductivity enhanced by intentionally adding impurities — a process called doping.

Doping substitutes a few silicon atoms with atoms of a dopant element. The dopant's electronic configuration determines the semiconductor type:

n-type (Negative-type) Semiconductor:

Dopant: pentavalent element (5 valence electrons), e.g., Phosphorus ( $[N e] 3 s^{2} 3 p^{3}$ )
Four of P's valence electrons bond with Si neighbours; the fifth electron is free in the conduction band.
Majority charge carriers: electrons

p-type (Positive-type) Semiconductor:

Dopant: trivalent element (3 valence electrons), e.g., Boron ( $[H e] 2 s^{2} 2 p^{1}$ )
B's three valence electrons bond with Si neighbours, leaving a hole (electron vacancy).
Holes migrate through the crystal as neighbouring electrons fill them.
Majority charge carriers: holes

Feature	n-type	p-type
Dopant	Pentavalent (e.g., P, As)	Trivalent (e.g., B, Ga)
Majority Carrier	Electrons (negative)	Holes (positive)
Mechanism	Dopant donates an extra electron	Dopant creates a hole

Gallium (

G a

Z = 31

[A r] 3 d^{10} 4 s^{2} 4 p^{1}

— Group 13, 3 valence electrons.
Arsenic (

A s

Z = 33

[A r] 3 d^{10} 4 s^{2} 4 p^{3}

— Group 15, 5 valence electrons.

While silicon dominates semiconductor substrates, transition metals (d-block elements) play vital roles in specialised electronic materials:

Their partially filled

(n - 1) d

orbitals contain unpaired electrons, generating magnetic moments.

This enables ferromagnetism (e.g., Fe, Co, Ni in permanent magnets) and properties exploited in superconductors.