Semiconductors conduct electricity only partly; while silicon is famous, compound semiconductors such as gallium arsenide allow electrons to travel much faster. Alferov and Kroemer proposed and demonstrated the “heterostructure,” in which ultra-thin layers with different band gaps are stacked so that electrons and photons are confined at the interface. This design enabled heterojunction bipolar transistors with nearly one hundred times the frequency performance of ordinary transistors and semiconductor lasers that oscillate with very low threshold currents. As a result, sensitive amplifiers in cellular base stations and laser diodes for optical-fiber links became possible, accelerating worldwide communications. The same technology also made high-brightness LEDs for signals and displays more efficient. These applications greatly support the information society and help save energy.

A semiconductor heterostructure is a multilayer assembly of two or more semiconductors with different lattice constants or band gaps, joined at the atomic level by epitaxial growth. The GaAs/AlGaAs system, having minimal lattice mismatch, suppresses carrier scattering at the interface and forms a very high-mobility two-dimensional electron gas. Alferov’s team demonstrated room-temperature continuous-wave heterostructure lasers in the early 1970s, enabling practical optical-fiber communication. Kroemer, on the other hand, theoretically designed the heterojunction bipolar transistor (HBT), showing that lowering the emitter barrier drastically improves the collector–base high-frequency response. This paved the way for microwave amplifiers and low-noise receivers operating into the hundreds of gigahertz, now used in satellite links and cellular base stations. Quantum wells and quantum dots confined at the heterointerface play essential roles in both fundamental physics, such as the quantum Hall effect, and applied technology, such as reducing laser threshold currents. Today, molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) allow atomic-layer precision control of heterointerfaces. The techniques have been extended to InP- and GaN-based systems for 5G communications, blue lasers, and even quantum-information devices. Thus, semiconductor heterostructures act as a hub linking device physics, materials science, and communication engineering.

The AlGaAs/GaAs heterostructure exhibits excellent lattice matching, suppressing interface defect densities below 10^2 cm^-2 and effectively eliminating interface-roughness scattering. A conduction-band offset of about 0.3 eV enables aggressive base-width scaling in HBTs, shortening τ_f=L_b^2⁄2D_nb and pushing f_T beyond 600 GHz. Alferov’s double-heterostructure lasers simultaneously optimized optical confinement Γ and carrier confinement η_c, achieving room-temperature CW gain. Co-location of the optical mode and recombination region reduces threshold carrier density to less than one-fifth of single-junction devices. The 2DEG formed at the interface reaches μ>1×10^6 cm^2/Vs (4 K), facilitating precision studies of integer and fractional quantum Hall effects. Modulation-doped InGaAs channel HEMTs now provide millimeter-wave reception with NF<0.5 dB. V/III flow-rate optimization in MOCVD limits in-plane compositional fluctuation to under 1 %. Distributed-feedback lasers with periodic multiple quantum wells show narrow linewidth and high-temperature stability for DWDM systems. In GaN/AlGaN structures, polarization-induced 2DEGs of high sheet density are promising for 5G/6G power amplifiers. Thus, heterojunction physics underpins high-frequency, high-power, and low-threshold photonic devices.