What Are Quantum Well Devices?

Quantum well devices are semiconductor structures in which charge carriers are spatially confined in one dimension to a layer so thin, typically between 1 and 20 nanometers, that quantum mechanical effects dominate the electronic and optical properties. Confinement quantizes the allowed energy states perpendicular to the well plane, producing discrete subbands rather than the continuous conduction and valence bands found in bulk semiconductors. By engineering the well width and the composition of the surrounding barrier material, designers can tune these energy levels with a precision unavailable in bulk materials, enabling devices with customized emission wavelengths, threshold currents, and carrier transport properties.

The concept emerged from theoretical work by Esaki and Tsu in the early 1970s and was rapidly confirmed by molecular beam epitaxy (MBE) techniques that could deposit atomic-scale semiconductor layers with the precision required to observe quantum size effects. The combination of predictive theory and precise growth methods transformed quantum well structures from a research curiosity into the basis for a wide class of practical devices.

Quantum Well Lasers

The quantum well laser is the most commercially significant quantum well device. Confining the active region to a thin quantum well reduces the volume of material that must be excited to achieve population inversion, dramatically lowering threshold current density compared with bulk double-heterostructure lasers. Multiple quantum well (MQW) designs stack several wells to increase optical gain while maintaining low threshold current. Strained-layer quantum well lasers, where the well material has a slightly different lattice constant than the barrier, further improve performance by lifting the valence band degeneracy and reducing hole effective mass. Research published in IEEE Journal of Quantum Electronics traces the development of strained quantum well structures and their impact on the performance of telecom-wavelength lasers.

Two-Dimensional Electron Gas

When a quantum well is formed at the interface between two dissimilar semiconductors, such as AlGaAs and GaAs, band bending at the interface creates a potential well that traps electrons in a thin sheet. The electrons in this two-dimensional electron gas (2DEG) are free to move parallel to the interface but are confined perpendicular to it, giving them quantized subbands and high mobility because they are spatially separated from the ionized donor atoms that supplied them. The 2DEG is the active element in high-electron-mobility transistors (HEMTs), and its properties are central to the physics of the quantum Hall effect, which provides the resistance standard used in metrology. NIST's documentation of the quantum Hall resistance standard describes how 2DEG structures underpin the SI definition of electrical resistance.

High-Electron-Mobility Transistors

High-electron-mobility transistors (HEMTs) exploit the 2DEG at a heterostructure interface to achieve transistor operation with extremely low noise and high gain at microwave and millimeter-wave frequencies. Because the channel electrons are separated from the dopant atoms, impurity scattering is minimized and carrier mobility is much higher than in conventionally doped transistors. Pseudomorphic HEMTs (pHEMTs) use a strained InGaAs channel within the GaAs material system to increase electron velocity further. A review in Nature Electronics examines GaN-based HEMTs, which combine high electron velocity with a wide bandgap to enable high-power amplification at frequencies needed for 5G base stations and radar.

Applications

Quantum well devices serve as the enabling technology for a wide range of photonic and electronic systems:

• Optical fiber communications: strained MQW lasers and electro-absorption modulators in transmitters operating at 1310 nm and 1550 nm wavelengths
• Consumer and industrial optical storage: red and near-infrared quantum well lasers in Blu-ray, CD, and DVD drive heads
• Wireless infrastructure: GaN HEMTs as power amplifiers in 4G and 5G base station transmitters
• Satellite communications: pHEMT low-noise amplifiers in satellite receivers and phased array terminals
• Military radar and electronic warfare: high-power, high-frequency HEMTs in active electronically scanned array radar systems
• Scientific instrumentation: quantum cascade lasers based on intersubband transitions for infrared spectroscopy and trace gas sensing