BASIC KNOWLEDGE Wide bandgap in miniature format? Why new materials are becoming important for energy harvesting

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Three hardware trends — wide‑bandgap power devices, specialized low‑leakage silicon processes and all‑in‑one power‑management ICs — are reshaping what energy harvesting can achieve in practice. This article examines where GaN and SiC matter, why SOI and low‑leakage CMOS dominate the lowest‑power designs, and how integrated PMICs and buffer chemistries (EDLC vs. lithium‑ion capacitors) drive form factor, reliability and real‑world viability.

Three hardware shifts are converging on energy harvesting converters at once: wide-bandgap power devices, specialized low-leakage silicon processes, and power-management chips that fold a harvester's entire analog front end onto a single die.

STMicroelectronics launched an energy-harvesting PMIC with an integrated boost converter in January 2025, and Asahi Kasei Microdevices began mass production of its AP4413 charging-control ICs for harvesting applications in April 2025, parts built for unstable sources such as indoor light. These chips matter more than any single material because at the microwatt scale, the dominant loss is leakage rather than switching, and integration is what drives both leakage and board area down.

What an all-in-one PMIC integrates

A discrete harvesting power stage once meant a rectifier, a boost converter, an MPPT controller, charge-control logic, and protection circuitry as separate components. The current generation, however, collapses those functions into one package. Texas Instruments' BQ25570 carries a boost charger with maximum-power-point tracking and a nanopower buck converter in a 3.5mm x 3.5mm footprint, storing energy to a lithium cell, a thin-film battery, or a supercapacitor. Belgian designer e-peas built its AEM family around the same idea, pairing ambient capture from photovoltaic, thermal, kinetic, and RF sources with on-chip storage management aimed at battery-free sensor nodes.

Integration buys two things that discrete designs cannot match: Board area and bill-of-materials cost both fall, which decides whether a sensor node is cheap and small enough to deploy by the thousand. Leakage falls because on-die interconnect avoids the parasitic paths and component count of a discrete layout, and because a single vendor can tune every stage to nanoampere quiescent current together.

The same die also absorbs the housekeeping that a discrete design scatters across extra parts, such as undervoltage lockout to stop a depleted buffer from browning out the load, an overvoltage clamp to protect the storage element, and battery-status outputs the host can poll. A BQ25570 reaches a working node with little more than an inductor and a handful of passives, where the discrete equivalent ran to a dozen components. Mordor Intelligence attributes much of the category's momentum to exactly this squeeze, with regulation functions now fitting into sub-millimeter footprints.

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Where wide bandgap helps

GaN earned its reputation by shrinking power electronics: GaN transistors switch faster, withstand higher breakdown fields, and conduct with lower on-resistance than silicon, which is why they displaced silicon in compact high-power chargers and why Asus showed GaN desktop PSUs claiming up to 30% efficiency gains. The same physics scales up, and Nvidia's plan to move AI racks to 800V HVDC distribution leans on GaN and silicon carbide to reach the required power densities.

That advantage is real where switching loss dominates, which is to say at watts and kilowatts. At the microwatt to milliwatt scale of an ambient harvester, switching loss is a minor term, and leakage current is the constraint, so GaN's headline benefit largely doesn’t reach the conversion chip in a battery-free sensor. GaN's published efficiency also collapses outside its sweet spot: a DC boost converter that hits close to 95% at 500 kHz can fall to 35% above 600 MHz at the same low output power.

Where GaN does enter the harvesting conversation is at the higher-power edge of the field, in vibration and electromagnetic harvesters tied to industrial machinery, and in rectifying larger RF and inductive transfers. The material's supply also carries a geopolitical cost that silicon avoids, as China's grip on gallium and its export controls have shown, prompting the U.S. to fund diamond and aluminum nitride alternatives through DARPA.

Process choices for the low-leakage end

For the smallest harvesters, the relevant lever is the silicon process rather than an exotic material. Silicon-on-insulator and other specialized low-leakage CMOS flows reduce the substrate leakage paths that drain a node during its long idle periods, and they let designers hold quiescent current in the nanoampere range that meets harvesting demands.

The mechanism is the buried oxide layer that isolates each transistor from the bulk substrate, cutting the junction leakage and latch-up paths that bleed charge in a conventional bulk process, and in fully-depleted SOI, it also opens up body biasing to trim leakage and threshold voltage at runtime. The research converters that self-start from a fraction of a volt are typically fabricated in standard or specialized CMOS nodes rather than wide-bandgap processes: one 88% efficient thermoelectric boost converter was built in a 180nm CMOS process and self-starts from 128 mV. Microcontroller vendors are pushing the same envelope from the digital side, with STMicroelectronics rating its STM32U3 line at 117 CoreMark per milliwatt and positioning it for coin-cell and ambient-energy operation.

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Supercapacitors and the buffer question

The storage element that a harvesting PMIC feeds has its own materials race. Conventional electric double-layer capacitors (EDLCs) charge and discharge almost indefinitely but leak badly, which makes them poor at holding harvested energy across hours of darkness.

Lithium-ion capacitors, on the other hand — a hybrid of capacitor and battery electrodes — change that balance. Pre-doping the negative electrode with lithium raises the output voltage to around 3.8 V and cuts self-discharge sharply, giving several times the energy density of an EDLC at similar power density. The contrast is stark in practice: an EDLC charged to 3.8 V can fall to 80% of that within a month, while a comparable lithium-ion capacitor can hold above 3.7 V for 100 days, a difference documented in an EnOcean sensor-module study.

As for leakage, Abracon rates its lithium-ion supercapacitors with leakage as low as 1 µA and self-discharge under 5% after 72 hours, while CAP-XX builds organic-electrolyte parts down to 800mF in a 20mm x 18mm x 1.7mm package with about 1 µA leakage.

A buffer that leaks more than the harvester supplies defeats the system as surely as a converter that does. That single specification, how little current the design wastes while it waits, is what the integrated PMIC, the storage chemistry, and the process node are all converging on, and it is the reason the materials story in harvesting reads so differently from the one in laptop chargers or AI server racks.

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