10.3
Types of Semiconductors
Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all…
A pure silicon wafer used in integrated circuits is an intrinsic semiconductor that lacks impurities and exhibits low electrical conductivity.
At zero kelvin, its conduction band is empty. The probability of electron occupancy at different energy levels is given by the Fermi level. Here, it lies in the middle of the band gap.
When the temperature increases, several electrons move to the conduction band, resulting in an equal concentration of electrons and holes in their respective bands.
The conductivity of intrinsic silicon can be tailored by introducing impurities through doping, which converts it into an extrinsic semiconductor.
Silicon has four valence electrons. When doped with a pentavalent impurity, it replaces a silicon atom and donates an excess electron, which acts as the majority carrier. Such semiconductors are called N-type semiconductors. Here, the Fermi level is shifted near the conduction band.
Conversely, trivalent impurities create vacant sites called holes, forming the majority charge carriers. These semiconductors are called p-type semiconductors. Here, the Fermi level is shifted towards the valence band.
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Q1: What is an intrinsic semiconductor and how does it conduct electricity?
An intrinsic semiconductor is a highly pure material with no impurities, such as pure silicon. At absolute zero, it behaves as a perfect insulator because all valence electrons are bound and the conduction band is empty. When temperature increases, thermal energy excites electrons from the valence band to the conduction band, creating electron-hole pairs that enable electrical conduction.
Q2: How does doping convert an intrinsic semiconductor into an extrinsic semiconductor?
Doping introduces impurities into an intrinsic semiconductor to change its electrical properties. Pentavalent dopants donate excess electrons, creating N-type semiconductors with electrons as majority carriers. Trivalent dopants create holes as majority carriers, forming P-type semiconductors. This process fundamentally alters the material's conductivity and Fermi level position.
Q3: What role does the Fermi level play in semiconductors?
The Fermi level describes the probability of electron occupancy at different energy levels at thermal equilibrium. In intrinsic semiconductors, it is positioned at the midpoint of the band gap. In N-type semiconductors, the Fermi level shifts near the conduction band, while in P-type semiconductors, it shifts towards the valence band, reflecting the dominant charge carrier type.
Q4: What are electron-hole pairs and why are they important in semiconductors?
Electron-hole pairs (EHPs) form when thermal energy excites electrons from the valence band to the conduction band, leaving holes behind. Electrons move freely in the conduction band while holes act as positive charge carriers in the valence band. EHP creation enables electrical conduction in semiconductors and their generation and recombination rates must balance at thermal equilibrium.
Q5: How does temperature affect intrinsic carrier concentration in semiconductors?
Intrinsic carrier concentration (ni) is temperature-dependent and increases as temperature rises. At higher temperatures, more thermal energy excites electrons to the conduction band, increasing the number of free electrons and holes. This relationship is expressed mathematically using band gap energy, temperature, and material constants, directly impacting the semiconductor's conductivity.
Q6: What is the difference between N-type and P-type semiconductors?
N-type semiconductors are created by doping with pentavalent impurities, which donate excess electrons as majority carriers. P-type semiconductors result from trivalent dopants that create holes as majority carriers. The Fermi level in N-type materials shifts toward the conduction band, while in P-type materials it shifts toward the valence band, determining their electrical behavior.
Q7: Why must generation and recombination rates be equal in semiconductors at thermal equilibrium?
At thermal equilibrium, electron-hole pairs are generated at rate gi and recombine at rate ri. These rates must be equal to maintain a stable carrier concentration and prevent continuous changes in the semiconductor's electrical properties. The recombination rate depends on the product of electron and hole concentrations, ensuring the system remains in balance.