What is an n-type semiconductor?
An n-type semiconductor is a type of semiconductor where electrons serve as the majority charge carriers, leading to a negative charge transport characteristic. These electron-donating properties make n-type semiconductors suitable for electrical applications, particularly in transistors, LEDs, solar cells and electrodes. The two main types of n-type semiconducting materials are:
- Doped Inorganic n-type Semiconductors
- Organic n-type Semiconductors
Examples of n-type Semiconductors
There are many examples of both inorganic and organic n-type semiconductors. Please see some listed in the table below:
| n-type inorganic semiconductors | n-type organic semiconductors |
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Key Characteristics of n-type Inorganic Semiconductors
N-type inorganic semiconductors are typically crystals of inorganic elements that are modified using dopants to become electron donating. The key characteristics of n-type inorganic semiconductors:
- Doping: In an n-type inorganic semiconductor, dopant atoms from elements in Group V of the periodic table (such as phosphorus, arsenic, or antimony) are introduced into the semiconductor crystal lattice. These atoms have five valence electrons, one more than silicon or germanium, which have four valence electrons.
- Free Electrons: The extra valence electron from the donor atoms becomes loosely bound and easily released at room temperature. They are described as free electrons in the conduction band of the semiconductor and serve as the primary charge carriers.
- Majority and Minority Carriers:
- Majority carriers: Electrons are the majority carriers in an n-type semiconductor. They are responsible for most of the electrical conduction.
- Minority carriers: Holes (positive charge carriers) are the minority carriers, created when an electron leaves a covalent bond.
- Electrical Conductivity: Since electrons are more mobile than holes, n-type semiconductors typically have higher conductivity than their undoped or p-type counterparts.
- Fermi Level: The presence of extra electrons affects the band structure and Fermi level of the n-type inorganic semiconductor (the energy level at which the probability of an electron being present is 50%). The Fermi level is shifted closer to the conduction band. This is because the extra electrons populate the energy states near the bottom of the conduction band, reducing the energy gap.
Key Characteristics of n-type Organic Semiconductors
For n-type organic semiconducting molecules and polymers the electron donating properties are accessed in a different way. The semiconducting properties can come from doping as with inorganic semiconductors but typically it comes from the structure of the molecule or polymer. The key characteristics of n-type organic semiconductors are:
- Molecular Structure: Organic semiconductors consist of π-conjugated systems, where alternating single and double bonds along the carbon backbone allow for the delocalization of electrons. These π-electrons are also described as free electrons and are responsible for the semiconducting properties. The energy levels these electrons can occupy are typically described by the highest occupied molecular orbital (HOMO) the lowest unoccupied molecular orbital (LUMO) (similar to the valence and conduction bands in inorganic semiconductor).
- Doping: If the material isn't intrinsically designed to accept electrons it can be doped with electron-donating materials. Common dopants are molecules that readily lose electrons, making them electron-rich and shifting the balance of charge carriers to favor electron conduction. Unlike inorganic doping, which involves adding impurity atoms to a crystal, doping in organic semiconductors often involves blending different molecules.
- Electron Transport: Conductivity in n-type organic semiconductors occurs primarily as a result of electron transport. It is facilitated by molecular orbitals that can accept electrons. The LUMO in n-type materials is more accessible, allowing the material to more easily accept electrons, making them the majority carriers.
- Majority and Minority Carriers:
- Majority carriers: electrons are the majority carriers, responsible for electrical conduction.
- Minority carriers: Holes are the minority carriers, similar to n-type inorganic semiconductors, but their mobility in organic materials is typically lower.
- Energy Levels and Fermi Level: In organic n-type semiconductors, the Fermi level is positioned closer to the LUMO, indicating that electrons are the primary charge carriers. The ability of the material to conduct electrons depends on how well the LUMO aligns with the energy levels of external contacts or electrodes.
- Stability Issues: One challenge with n-type organic semiconductors is their air stability. Many n-type organic materials are prone to oxidation when exposed to air, which can reduce their performance and longevity. Stabilizing these materials often requires careful molecular design or encapsulation to protect them from environmental degradation. Materials may have to be handled and sealed off within a glove box.
Applications of n-type semiconductors
Both inorganic and organic n-type semiconductors are used in electronic applications. Inorganic semiconductors are more widely used commercially due to their higher stability, well-established manufacturing processes, superior electrical performance, and ability to function at high temperatures and power levels. Silicon, gallium nitride, and other inorganic materials have a long history of use in industries such as power electronics, integrated circuits, and optoelectronics, making them more mature and reliable for mass production. Organic semiconductors have mainly been explored for use in organic electronics and in particular organic solar cells. Here is a summary of the uses of n-type semiconductors:
- Power Electronics: high-power devices and RF amplifiers.
- Optoelectronics: LEDs, OLEDs, photodetectors, and solar cells.
- Transparent Electronics: touchscreens, transparent electrodes, and displays.
- Sensing: gas and UV sensors.
- Organic Electronics: flexible and printable electronics, organic solar cells, and transistors.
Non-fullerene acceptors
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