Question:
I am looking for some undoped n-type gallium nitride (GaN) wafers with known values of charge carrier mobility and carrier lifetime. Does University Wafer have such wafers?
A diameter of 25 or 50 mm will work.
What Is The Importance of Carrier Mobility?
Carrier Mobility of GaN Substrates
A semiconductor researcher requested help with the following.
UniversityWafer, Inc. Answer:
Undoped n-type gallium nitride (GaN) wafers, 2'' dia. 50mm, P/E, Epi-ready polish
Carrier Concentration: < 5x1017 /cm^3
Mobility: </= 300cm2/V•s
Reference #238170 for specs and pricing.
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Why Is Carrier Mobility Important?
Carrier mobility is a fundamental parameter in semiconductor physics that quantifies how quickly charge carriers—electrons and holes—can move through a semiconductor material when subjected to an electric field. In the context of semiconductor substrates, carrier mobility plays a pivotal role in determining the electrical properties and overall performance of electronic devices. Understanding its importance is crucial for the design and optimization of semiconductors used in integrated circuits, sensors, and other electronic applications.
Definition of Carrier Mobility
Carrier mobility (μ\mu) is defined as the average drift velocity (vdv_d) of charge carriers per unit electric field (EE):
μ=vdE\mu = \frac{v_d}{E}
Higher mobility means that carriers can move more swiftly through the material under the influence of an electric field, leading to better electrical conductivity and faster device operation.
Impact on Electrical Conductivity
The electrical conductivity (σ\sigma) of a semiconductor is directly proportional to the carrier mobility:
σ=q(nμn+pμp)\sigma = q(n\mu_n + p\mu_p)
where:
- qq is the elementary charge,
- nn and pp are the concentrations of electrons and holes, respectively,
- μn\mu_n and μp\mu_p are the mobilities of electrons and holes.
Higher carrier mobility enhances conductivity, allowing for efficient charge transport even at lower carrier concentrations. This is particularly important for substrates in which doping levels are controlled to achieve specific electrical characteristics.
Influence on Device Performance
-
Switching Speed: In high-speed electronic devices such as microprocessors and radio-frequency (RF) transistors, the switching speed is limited by how quickly carriers can respond to changing electric fields. Higher mobility allows for faster carrier response, enabling higher operational frequencies.
-
Drive Current: In field-effect transistors (FETs), the drive current (IDI_D) is proportional to carrier mobility:
ID∝μI_D \propto \mu
Higher mobility leads to increased drive current capability, improving the transistor's ability to source or sink current, which is essential for driving loads and achieving desired voltage levels.
- Transconductance: The transconductance (gmg_m) of a transistor, which measures the change in the output current per unit change in the input voltage, is also proportional to mobility:
gm∝μg_m \propto \mu
Higher transconductance improves the amplification properties of the device, which is crucial in analog and RF applications.
Role in Device Scaling
As semiconductor devices continue to scale down in size according to Moore's Law, maintaining performance becomes challenging due to short-channel effects and increased resistance. High carrier mobility helps mitigate these issues by:
- Reducing Channel Resistance: Higher mobility decreases the channel resistance in transistors, which is critical for maintaining performance in smaller devices.
- Lowering Power Consumption: Devices with higher mobility can achieve the same performance at lower voltages, reducing power consumption and heat generation.
Material Selection and Engineering
The choice of substrate material greatly influences carrier mobility. For instance:
- Silicon (Si): The most commonly used semiconductor substrate, but has moderate mobility.
- Gallium Arsenide (GaAs): Offers higher electron mobility than silicon, making it suitable for high-speed and high-frequency applications.
- Silicon-on-Insulator (SOI) and Strained Silicon: Engineered substrates that enhance carrier mobility through structural modifications.
Temperature and Impurity Effects
Carrier mobility is affected by temperature and impurities:
- Temperature: Mobility generally decreases with increasing temperature due to increased phonon scattering.
- Impurities and Defects: Doping introduces impurities that can scatter carriers, reducing mobility. Therefore, there is a trade-off between increasing carrier concentration and maintaining high mobility.
Applications Requiring High Mobility
- High-Frequency Devices: Such as RF amplifiers and oscillators.
- High-Speed Digital Circuits: In processors and memory devices where rapid switching is essential.
- Power Electronics: Where efficient charge transport reduces losses.
Conclusion
Carrier mobility is a critical factor in the performance of semiconductor devices. It influences electrical conductivity, switching speed, power consumption, and overall efficiency. By optimizing carrier mobility through material selection, doping levels, and structural engineering, semiconductor substrates can be tailored to meet the demands of various electronic applications. Understanding and controlling carrier mobility is thus essential for advancing semiconductor technology and enabling the continued miniaturization and enhancement of electronic devices.