Why are buffer layers critical for GaN HEMTs?

GaN is typically grown on SiC or Si substrates with significant lattice mismatch (3.5 percent for SiC, 17 percent for Si). Without a buffer, this mismatch creates billions of threading dislocations per square centimeter that act as electron traps, causing current collapse and degrading RF power performance. The buffer layer system (typically AlN nucleation plus graded AlGaN transition layers, totaling 1 to 3 micrometers) reduces dislocation density from over 10 to the 10th per square centimeter at the substrate interface to under 10 to the 8th at the channel. The buffer also provides semi-insulating isolation, preventing RF signal leakage through the conductive substrate.

What materials are used for RF buffer layers?

For GaN-on-SiC: AlN nucleation layer (20 to 50 nm) plus graded AlGaN or superlattice buffer (1 to 2 micrometers), often doped with carbon or iron for semi-insulating behavior. For GaN-on-Si: AlN nucleation plus AlN/GaN superlattice strain-relief layers (3 to 5 micrometers) to manage the large thermal expansion mismatch. For GaAs pHEMTs: undoped GaAs buffer (0.5 to 1 micrometer) plus AlGaAs barrier. For InP HBTs: InP buffer plus InGaAs graded layer for lattice-matched growth of the collector and base layers.

How does the buffer layer affect RF performance?

A well-designed buffer provides three RF benefits: low dislocation density reduces trap-related current collapse (also called knee walkout), which degrades large-signal gain and PAE by 2 to 5 dB in power amplifiers. High resistivity isolation (greater than 10 to the 8th ohm-centimeters) minimizes substrate parasitic capacitance and RF signal leakage, improving fmax and gain at millimeter-wave frequencies. Smooth surface morphology (RMS roughness under 0.5 nm) ensures uniform 2DEG formation at the AlGaN/GaN interface, giving consistent threshold voltage and transconductance across the wafer.

Semiconductor & Epitaxy

Buffer Layer

/buf-ur lay-ur/

An intermediate epitaxial layer grown between the substrate and active device layers in III-V RF semiconductors. The buffer accommodates lattice mismatch, reduces threading dislocations from >10¹⁰/cm² to <10⁸/cm², provides semi-insulating electrical isolation, and establishes smooth crystal surfaces for GaN HEMT, GaAs pHEMT, and InP HBT active layer growth.

Category: Semiconductor & Epitaxy

Thickness: 0.5 to 5 µm

Dislocation Target: < 10⁸/cm²

Understanding Buffer Layers

High-performance RF transistors require single-crystal active layers with extremely low defect densities. When the active material (GaN, GaAs, InP) has a different lattice constant than the substrate (SiC, Si, sapphire), the mismatch generates dislocations that propagate into the active layers, degrading mobility, causing current collapse, and reducing breakdown voltage. The buffer layer system absorbs this strain gradually.

For GaN-on-SiC (3.5% mismatch), a typical buffer consists of a thin AlN nucleation layer (20 to 50 nm) followed by a graded AlGaN transition (1 to 2 µm) doped with carbon or iron for semi-insulating behavior (>10⁸ Ω-cm). For GaN-on-Si (17% mismatch plus thermal expansion difference), more complex superlattice strain-relief layers (3 to 5 µm total) manage both lattice and thermal strain to prevent wafer cracking.

Buffer Layer Parameters

Lattice Mismatch:
Δa/a = (a_film − a_sub) / a_sub
GaN/SiC: 3.5% | GaN/Si: 17% | GaN/sapphire: 16%

Critical Thickness (Matthews-Blakeslee):
h_c ≈ b / (8πf(1+ν)) × ln(h_c/b)
Beyond h_c, misfit dislocations form

Dislocation Reduction:
Surface: < 10⁸/cm² (target for RF devices)
Interface: > 10¹⁰/cm² (at substrate)

Buffer Layer Systems by Technology

Technology	Substrate	Mismatch	Buffer Structure	Thickness	Isolation
GaN-on-SiC	4H-SiC	3.5%	AlN + graded AlGaN	1 to 2 µm	C/Fe doped SI
GaN-on-Si	Si(111)	17%	AlN + superlattice	3 to 5 µm	C doped SI
GaAs pHEMT	GaAs	<0.1%	Undoped GaAs	0.5 to 1 µm	SI substrate
InP HBT	InP	<0.1%	InP + InGaAs graded	0.3 to 1 µm	SI substrate

Common Questions

Frequently Asked Questions

Why critical for GaN HEMTs?

GaN on SiC/Si has 3.5 to 17% lattice mismatch. Without buffer, billions of dislocations act as traps causing current collapse and degrading RF power by 2 to 5 dB. Buffer reduces density from >10¹⁰ to <10⁸/cm² and provides semi-insulating isolation.

What materials are used?

GaN-on-SiC: AlN nucleation + graded AlGaN (C/Fe doped). GaN-on-Si: AlN + superlattice strain-relief (3 to 5 µm). GaAs pHEMT: undoped GaAs (0.5 to 1 µm). InP HBT: InP + InGaAs graded collector.

How does it affect RF performance?

Low dislocations reduce trap-related current collapse. High resistivity (>10⁸ Ω-cm) minimizes substrate parasitics improving fmax. Smooth surface (RMS < 0.5 nm) ensures uniform 2DEG for consistent Vth and gm across the wafer.

Related Terms

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