Semiconductor Types — Intrinsic, N-type, P-type Doping and Properties

Question

What are intrinsic, n-type, and p-type semiconductors, and how does doping change their electrical properties?

Solution — Step by Step

Pure silicon or germanium is an intrinsic semiconductor. Each atom forms 4 covalent bonds. At room temperature, some bonds break thermally, creating equal numbers of:

Free electrons (negative charge carriers)
Holes (positive charge carriers — missing electrons in bonds)

In intrinsic semiconductors: $n_e = n_h = n_i$ (equal electron and hole concentrations).

Conductivity is low because $n_i$ is small at room temperature (~ $10^{10}$ per cm $^3$ for Si, compared to ~ $10^{22}$ for copper).

Add a Group 15 element (P, As, Sb — 5 valence electrons) to Si. Four electrons bond with Si neighbours; the fifth electron is free.

Majority carriers: Electrons
Minority carriers: Holes
Dopant is called: Donor (donates an electron)
$n_e \gg n_h$

The material is still electrically neutral — the extra electron came with its proton in the dopant nucleus.

Add a Group 13 element (B, Al, Ga — 3 valence electrons) to Si. Only three bonds are complete; the fourth bond has a hole.

Majority carriers: Holes
Minority carriers: Electrons
Dopant is called: Acceptor (accepts an electron to fill the hole)
$n_h \gg n_e$

Again, the material is electrically neutral overall.

graph TD
    A[Semiconductor] --> B{Pure or Doped?}
    B -->|Pure| C[Intrinsic: ne = nh]
    B -->|Doped| D{Dopant valence?}
    D -->|5 valence electrons| E[N-type]
    E --> E1[Majority: electrons]
    E --> E2[Dopant: Donor - P, As, Sb]
    D -->|3 valence electrons| F[P-type]
    F --> F1[Majority: holes]
    F --> F2[Dopant: Acceptor - B, Al, Ga]

Why This Works

Doping works because it introduces energy levels close to the conduction band (donors) or valence band (acceptors). A donor level is just 0.01-0.05 eV below the conduction band — at room temperature, this small gap is easily crossed, freeing the extra electron. Similarly, acceptor levels just above the valence band easily capture electrons, creating holes.

Even tiny doping concentrations (1 dopant per million Si atoms) increase conductivity by orders of magnitude because the intrinsic carrier concentration is so low.

For remembering: N-type has extra electroNs (donors give Negative carriers). P-type has extra holes (Positive carriers). Donors are Group 15 (penta), Acceptors are Group 13 (tri).

Alternative Method

The mass action law provides a quantitative check: for any semiconductor at a given temperature,

n_e \times n_h = n_i^2

In n-type: $n_e$ increases due to doping, so $n_h$ must decrease (below intrinsic value). This is why n-type has very few holes — the product is fixed.

Common Mistake

Students often say “n-type semiconductor is negatively charged.” This is wrong. N-type is electrically NEUTRAL. The dopant atom brings 5 protons along with 5 electrons. After donating one electron, the dopant becomes a positive ion, balanced by the free electron. The “n” refers to the majority carrier type (electrons), not the overall charge.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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