Glossary entry

Heterojunction

An interface between two semiconductor materials with different bandgaps. The structural basis of modern diode lasers, LEDs, and high-speed transistors.

Lasers & gain Optics fundamentalsUpdated October 2026

A heterojunction is the interface between two crystalline semiconductors with different compositions and (typically) different bandgaps. The discontinuity in band alignment at the interface creates potential steps for electrons and holes that can be engineered for carrier confinement, transport, and recombination control.

Band alignment types. When materials with bandgaps $E_{g 1}$ and $E_{g 2}$ are joined, the bandgap difference $Δ E_{g}$ is split between conduction-band ( $Δ E_{C}$ ) and valence-band ( $Δ E_{V}$ ) discontinuities. Three alignment regimes exist:

Type	Description	Carrier confinement
Type I (straddling)	Both bands of narrow-gap material lie inside the wide-gap material's bands	Electrons and holes both confined in the narrow-gap region
Type II (staggered)	Bands offset in same direction	Electrons and holes confined in different regions
Type III (broken)	Conduction band of one material below valence band of the other	Strong band-to-band tunneling

Most III–V active-region heterojunctions are Type I. Common examples:

Heterojunction	$Δ E_{C}$	$Δ E_{V}$	Used in
GaAs / AlGaAs	$\sim 0.65 Δ E_{g}$	$\sim 0.35 Δ E_{g}$	850 nm lasers, HEMTs
InGaAs / InP	$\sim 0.40 Δ E_{g}$	$\sim 0.60 Δ E_{g}$	1300 / 1550 nm lasers
InGaAsP / InP	tunable composition	tunable	Telecom lasers, EAMs
InGaAlAs / InAlAs	$\sim 0.70 Δ E_{g}$	$\sim 0.30 Δ E_{g}$	Higher-temperature InP lasers
GaN / AlGaN	composition-dependent	composition-dependent	Blue lasers, UV LEDs, HEMTs

Double heterostructure (DH) lasers. A thin narrow-gap active layer sandwiched between two wider-gap cladding layers forms a double heterostructure. The heterojunctions:

Confine carriers in the active region (both electrons from the n-cladding and holes from the p-cladding accumulate in the narrow-gap layer because they cannot escape the band offsets)
Confine light through the index difference between active and cladding (narrow-gap material has higher refractive index, forming a slab waveguide)
Enable independent doping of the cladding regions for current injection without absorbing the emitted light

This separation of carrier confinement, optical confinement, and current pathways was the foundational architectural advance that made room-temperature continuous-wave semiconductor lasers practical (Alferov / Kroemer, 1970, Nobel Prize 2000).

Lattice matching. The two materials of a heterojunction should have nearly identical crystal lattice constants ( $Δ a / a < 0.1$ %) for defect-free epitaxial growth. Mismatched layers accumulate strain energy and form misfit dislocations beyond the critical thickness. Common lattice-matched systems:

GaAs / AlAs / Al $_{x}$ Ga $_{1 - x}$ As: nearly identical lattice constants for all $x$
InP / In $_{0.53}$ Ga $_{0.47}$ As / In $_{0.52}$ Al $_{0.48}$ As: lattice-matched at specific compositions
Si / Si $_{1 - x}$ Ge $_{x}$ : lattice mismatch up to 4 %; used in strained-silicon transistors

Strained heterojunctions intentionally use lattice-mismatched layers thinner than the critical thickness. Compressive strain reshapes the valence band structure to reduce hole effective mass, increasing differential gain and reducing Auger recombination. This was a key technique enabling efficient long-wavelength telecom diode lasers in the 1990s.

Quantum wells and superlattices are arrays of thin heterojunctions stacked at the de Broglie wavelength scale, producing quantum confinement effects in addition to the classical heterojunction band offsets.

References: L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012). S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, 2007). B. E. A. Saleh and M. C. Teich, Fundamentals of Photonics, 3rd ed. (Wiley, 2019).