Semiconductors power everything from smartphones to solar panels. At the most basic level, they are classified as intrinsic (pure) or extrinsic (intentionally doped). This post explains what each term means, how doping changes carrier concentrations and conductivity, and why the distinction matters in real devices.
What Is an Intrinsic Semiconductor?
An intrinsic semiconductor is a highly pure crystal of silicon (Si) or germanium (Ge) with no intentional impurity atoms. Charge carriers arise only from thermal excitation: the number of free electrons in the conduction band equals the number of holes in the valence band (n = p). Because carriers are scarce at room temperature, conductivity is relatively low. Conductivity depends primarily on temperature: warming the material creates more electron–hole pairs.
- Doping: None (pure crystal).
- Carrier balance: n = p (electrons equal holes).
- Conductivity: Low; increases with temperature.
- Fermi level: Near mid-gap between conduction and valence bands (shifts slightly with temperature).
- Examples: Ultra-pure Si and Ge wafers used as starting material for device fabrication.
What Is an Extrinsic Semiconductor?
An extrinsic semiconductor is created by doping a pure semiconductor with a tiny amount of impurity atoms (typically parts per million). Doping introduces energy levels near a band edge, dramatically increasing the number of mobile carriers and hence the conductivity. Two important types arise:
- n-type (donor dopants like P, As, Sb): electrons are majority carriers; Fermi level shifts toward the conduction band.
- p-type (acceptor dopants like B, Al, Ga, In): holes are majority carriers; Fermi level shifts toward the valence band.
In extrinsic materials n ≠ p (one carrier type dominates), and conductivity depends on both temperature and doping concentration/type.
Intrinsic vs Extrinsic: Comparison Table
| Basis of Difference | Intrinsic Semiconductor | Extrinsic Semiconductor |
|---|---|---|
| Doping of impurity | No intentional doping; pure Si/Ge crystal. | Small, controlled dopant added (donor or acceptor). |
| Carrier concentrations | Electrons equal holes (n = p). | Electrons and holes unequal (n ≠ p); one dominates. |
| Electrical conductivity | Low at room temperature. | High relative to intrinsic (can vary over many orders of magnitude). |
| What affects conductivity? | Mainly temperature (more e–h pairs with heat). | Temperature and dopant type/concentration. |
| Fermi level position | Near mid-gap between bands. | Shifts toward conduction band (n-type) or toward valence band (p-type). |
| Examples | High-purity Si, Ge. | Si doped with P/As/Sb (n-type) or B/Al/Ga/In (p-type). |
| Use in devices | Substrate or baseline material; educational references. | Practical devices: diodes, BJTs, MOSFETs, LEDs, solar cells. |
Why Doping Changes Conductivity
In a pure crystal, electrons must jump the bandgap to reach the conduction band, which requires energy. Dopants introduce donor (near conduction band) or acceptor (near valence band) levels that require very little energy to ionize. As a result:
- n-type materials contribute many free electrons with minimal thermal energy.
- p-type materials contribute many holes by accepting electrons into acceptor states.
- By tuning dopant concentration, engineers tailor resistivity, threshold voltages, leakage, and device speed.
- At high temperatures, intrinsic carriers increase and can dominate even doped material (“intrinsic takeover”).
Applications & Relevance
The intrinsic/extrinsic distinction underpins modern electronics:
- Integrated circuits (ICs): Selective doping creates adjacent p- and n-regions, forming p–n junctions, channels, and wells in CMOS chips.
- Power electronics: Controlled doping sets breakdown fields, on-resistance, and switching performance in devices like IGBTs and MOSFETs.
- Optoelectronics: LEDs and laser diodes rely on p–n junction recombination; solar cells exploit junctions to separate photo-generated carriers.
- Sensors: Changing carrier density with gas exposure, light, or temperature enables resistive, photoconductive, and thermistor-type sensors.
Key Takeaways
- Intrinsic: Pure material, n = p, low conductivity, temperature-dependent.
- Extrinsic: Doped material, n ≠ p, higher conductivity, depends on doping and temperature.
- Doping shifts the Fermi level toward the band that supports majority carriers.
- The ability to dope precisely is what makes practical electronic devices possible.
FAQs
1) Is intrinsic silicon useful on its own?
Mostly as a high-purity starting substrate. For devices, regions are doped to create the desired electrical behavior.
2) Which conducts better: intrinsic or extrinsic?
Extrinsic. Doped carriers raise conductivity by many orders of magnitude compared to intrinsic material at room temperature.
3) Does temperature always increase conductivity?
In intrinsic semiconductors, yes (more e–h pairs). In extrinsic ones, the effect depends on the dopant ionization and scattering; at very high temperatures intrinsic carriers can dominate again.
Bottom line: Intrinsic semiconductors show the baseline behavior of pure Si/Ge, while extrinsic semiconductors, engineered by precise doping, make practical electronics possible—from microprocessors to LEDs and solar panels.