What Are DH-HEMTs?

DH-HEMTs (Double Heterojunction High Electron Mobility Transistors) are a class of field-effect transistors that use two distinct semiconductor heterojunction interfaces to confine and enhance a two-dimensional electron gas (2DEG) in the transistor channel, achieving higher current density and improved electron confinement compared to conventional single-heterojunction HEMTs. A conventional HEMT creates one heterojunction, typically between a wide-bandgap barrier layer and a narrower-bandgap channel material, generating a 2DEG at that single interface through piezoelectric and spontaneous polarization effects. A DH-HEMT adds a second heterojunction beneath the channel, introducing a back-barrier layer that suppresses electron leakage into the substrate and allows a second 2DEG to form, increasing total charge density available for current conduction.

The double-heterojunction architecture emerged as compound semiconductor device scaling ran into limitations in carrier confinement. Electrons escaping the channel into the underlying buffer reduce transconductance and increase off-state leakage; the back barrier counteracts this by presenting an additional potential barrier to confine carriers within the defined channel region. GaN-based DH-HEMTs, typically using AlGaN/GaN/AlGaN or AlN/GaN/AlN layer sequences, have received particular attention because the large polarization discontinuities in these III-nitride material systems generate high 2DEG densities without intentional doping.

Double-Heterojunction Structure and 2DEG Formation

In a DH-HEMT, the channel layer is sandwiched between two wider-bandgap semiconductor layers, forming heterojunctions at both the top and bottom interfaces. The top barrier, directly below the gate metal, generates the primary 2DEG whose density is controlled by gate voltage. The bottom back-barrier layer creates an additional potential well that both confines the primary 2DEG and can contribute a secondary electron channel under appropriate bias. The net result is a higher total sheet charge density than a single-heterojunction device of comparable gate-to-channel distance can provide. Research on 2DEG enhancement through epilayer stress engineering in AlN/GaN/AlN DH-HEMTs demonstrates how the back-barrier composition is tuned to balance confinement against strain-induced defect formation.

RF and Low-Noise Performance

The primary motivation for DH-HEMT development is improved high-frequency performance for microwave and millimeter-wave applications. Higher transconductance, arising from the increased charge density, raises the transit frequency (fT) and the maximum oscillation frequency (fmax), which are the two key figures of merit for RF transistors. The back-barrier design also reduces the output conductance, which improves gain flatness and power-added efficiency in amplifier circuits. Minimum noise figure, a critical parameter for low-noise amplifiers (LNAs) used in receiver front ends, is improved because reduced short-channel effects lower the channel resistance fluctuations that generate noise. The ETH Zurich Millimeter-Wave Electronics Laboratory documents research on HEMT variants including double-heterojunction designs targeted at millimeter-wave communications and imaging.

GaN-based DH-HEMTs

GaN-based DH-HEMTs have attracted particular interest for power amplifiers in wireless base stations and radar systems because gallium nitride supports high breakdown voltage and high-temperature operation in addition to high electron mobility. Device structures such as AlGaN/GaN/InAlGaN use an indium-containing back barrier to achieve lattice matching while maintaining the polarization discontinuity needed for carrier confinement. Performance characteristics of GaN DH-HEMT variants, including analysis of polarization-graded back-barrier layers, are reported in Applied Physics B research on GaN DH-HEMTs.

Applications

DH-HEMTs have applications in a wide range of disciplines, including:

• Microwave power amplifiers for cellular base station transmitters
• Low-noise amplifier front ends in satellite and radar receivers
• Millimeter-wave transceivers for 5G New Radio and imaging systems
• High-efficiency power switching in aerospace and defense electronics