Test: P-Type Semiconductor - Electronics and Communication Engineering (ECE) MCQ

Semiconductor: 

• Semiconductors are materials that have a conductivity between conductors and insulators.
• Semiconductors are made of compounds such as gallium arsenide or pure elements, such as germanium or silicon.

There are two types of semiconductors:

• ​N-type Semiconductor
• P-type Semiconductor

In a P-type semiconductor material, holes are generated because a trivalent impurity atom has one less electron than the surrounding silicon atom. Thus, leaving a vacancy in a covalent bond that acts as a hole. The presence of a hole does not make the semiconductor material positively charge because in an impurity atom, no. of electron and proton is equal before and after the doping, and the same applies for the silicon atom.

Thus, p-type semiconductor crystal remains neutral.

Important Points:

• Extrinsic P-type Semiconductor is formed when a trivalent impurity is added to a pure semiconductor.
• Example of trivalent impurity is Boron, Gallium, and Indium.
• Trivalent impurity like boron has 3 valence electrons.
• Each atom of the impurity fits in the silicon crystal by forming covalent bonds with the surrounding silicon atoms.
• The dopant boron atom has one less electron than surrounding silicon and thus vacancy is generated that acts as a hole.

P-type semiconductor:

• Extrinsic P-type Semiconductor is formed when a trivalent impurity is added to a pure semiconductor
• Example of trivalent impurity are Boron, Gallium, and Indium
• Trivalent impurity like boron has 3 valence electrons.
• Each atom of the impurity fits in the silicon crystal by forming covalent bonds with the surrounding silicon atoms
• The dopant boron atom has one less electron than surrounding silicon and thus vacancy is generated that acts as a hole. 
• Therefore, holes are in majority in the p-type semiconductors. 

Important Point

Pentavalent impurities:

Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing an extra electron.

This is as explained in the figure:

Concept:

The conductivity for a doped semiconductor is obtained as:

σ = q n₀ μ_n = q p₀ μ_p

where,

n_i = Intrinsic carrier concentration.

μ_n = electron mobility

μ_p = Hole mobility

When a semiconductor crystal is optically excited, one hole is created for every electron, i.e. electron-hole pass are created.  

Application:

When the light is ON, excess electron and holes are generated in pairs, i.e.

Δp = Δn = 10¹⁶ cm³

Before Illumination:

N_A = 10¹⁷ cm^-3 and n_i = 2.2 × 10⁶ cm^-3

Since N_A ≫ n_i, the excess hole concentration at thermal equilibrium will be:

p₀ = N_A = 10¹⁷ cm^-3

The electron concentration using mass-action law is obtained as:

The electron concentration before illumination is negligible.

After illumination:

When an electron is generated optically, one hole is also created, i.e. they always occur in pairs.

∴ The excess hole concentration and excess electron concentration will be the same, i.e.

Δp = Δn = 10¹⁶ cm^-3

Now, the electron and hole concentration after illumination will be:

n = n₀ + Δn

n = 4.8 × 10^-5 + 10¹⁶

n = 10¹⁶

Similarly,

p = p₀ + Δp

p = 10¹⁷ + 10¹⁶ cm^-3

p = 11 × 10¹⁶ cm^-3

∴ the conductivity of the given sample when the light is ON will be:

σ = q(n₀ + Δ_n)μ_n + q(p₀ + Δ_p)μ_p

σ = q[(10¹⁶)μ_n + (11 × 10¹⁶)μ_p]

= (1.6 × 10^-19) (10¹⁶ × 5300 + (11 × 10¹⁶) (230))

= 1.6 × 10^-19 (5.3 × 10¹⁹ + 2.53 × 10¹⁹)

= 1.6 × (5.3 + 2.53)

σ = 12.528 (Ω-cm)^-1

• Extrinsic p-Type Semiconductor is formed when a trivalent impurity is added to a pure semiconductor.
• Example of trivalent impurity are Boron, Gallium, and Indium
• Trivalent impurity like boron, have 3 valence electrons.
• Each atom of the impurity fits in the silicon crystal by forming covalent bonds with the surrounding silicon atoms.
• The dopant boron atom has one less electron than surrounding silicon and thus vacancy is generated that acts as a hole.