RF POWER Explaining LDMOS: Better than MOSFET and GaN?

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LDMOS are MOSFETs but are manufactured with a different layout. LDMOS transistors are applicable in power electronics, audio systems, and radio frequency amplification. The article describes LDMOS technologies and their use in power electronics. It also draws a comparison between LDMOS and GaN-based devices.

LDMOS technology employs a lateral diffusion structure that optimizes current flow and thermal management in high-frequency applications, distinguishing it from traditional MOSFETs.

What are LDMOS and how are they different from traditional MOSFETs?

An LDMOS transistor, an acronym for “Laterally Diffused MOSFET”, is a type of MOSFET used in high-power and radio frequency (RF) applications. Just as the name suggests, LDMOS features a laterally diffused structure. It facilitates lateral current flow, which moves side to side, rather than vertically (from top to bottom or bottom to top), in conventional transistors. Lateral diffusion allows LDMOS transistors to become smaller, integrate effectively with other devices, and perform well in high-frequency operations.

Comparing LDMOS with MOSFETs

Typically, MOSFETs are vertically doped. The P-type layer is diffused above the N-type layer, followed by another P-type layer placed above it, and so on. In LDMOS transistors, doped layers are placed alongside each other (horizontally). The lateral layout alters conventional MOSFET operation. Images below show the difference between LDMOS and MOSFET.

Gallery

LDMOS features

LDMOS transistors are fabricated on p/p+ silicon epitaxial layers and are planar double-diffused structures. The lateral layout makes them a different type. The LDMOS package is compact. It is suitable for integrating into high-power electronics and radio devices, whereas a discrete MOSFET is not. The section lists LDMOS transistor features.

Lateral current flow

As mentioned above, the lateral layout gives rise to a lateral electric field. Electrons move from the drain to the source, or conventional current flows from source to drain, very quickly. In high-power applications, heat spreads alongside the transistor surface. Unlike MOSFETs, heat does not spread deep into the substrate. As a result, lateral current flow improves thermal efficiency.

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Shared plane

In LDMOS transistors, source and drain terminals are placed on the top of the chip. They are on the same plane, placed horizontally. Instead of wire bonds, the gate and drain are connected through metal layers. Surface connections and lateral structure lower parasitic inductance.

Shorter channel length

LDMOS houses a shorter conductive channel. In MOSFETs, channels are broader. MOSFETs here refer to discrete devices, not VLSI MOSFETs. As the source and drain are in the same plane, electrons move quickly from the drain to the source, reducing turn-on and turn-off times.

Smaller gate capacitance

In conventional MOSFETs, the gate region and conductive channel overlap. Due to the gate dielectric, a heavy capacitance, known as gate capacitance, exists between the two. Higher gate capacitance facilitates larger charge accumulation near the gate region, resulting in slower switching times.

In LDMOS, smaller gate capacitance leads to faster response and reduced delay in high-frequency applications. Additionally, LDMOS contains drain extension regions. It helps the device to exhibit a higher breakdown voltage.

LDMOS Applications

Instead of being standalone transistors, LDMOS transistors are used as LDMOS amplifiers. LDMOS amplifiers have been applicable in power, radio frequency power (RF power), and microwave applications for decades. Some of them are listed below.

RF power amplifier

LDMOS is a popular power RF amplifier. A power RF amplifier is an integration of power electronics and RF. LDMOS is used mostly in class AB and pulsed power amplifier types. Class AB LDMOS RF power amplifiers achieve an efficiency of up to 70 %.

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Doherty power amplifiers

A Doherty power amplifier (DPA) is a class B RF power amplifier, invented in 1936. LDMOS as DPA improves peak efficiency and power density.

Pulsed electronics

LDMOS power amplifiers can handle a few watts of power in motor drivers and up to thousands of watts in pulsed systems.

Audio power amplifiers

LDMOS power amplifiers are applicable in audio technologies, including loudspeakers, public announcement systems, and HiFi (High Fidelity) sound systems.

RF applications

They are used in microwave systems, broadcasting, base stations, battery-operated transceivers, and many other RF applications like radar, scientific, medical, microwave cooking, and RF lighting.

Mobile network infrastructures

LDMOS amplifiers are used in mobile networks. They are gaining popularity for their use in the S-band (2-4 GHz frequency band) and applications including WiMAX, LTE, and radar engineering.

Cell towers

Historically, bipolar amplifiers were used in base stations such as small cells and telecom towers. LDMOS replaced them years ago. The LDMOS market is mainly driven by its base station application.

LDMOS in comparison with GaN

Gallium Nitride (GaN) is a newer technology that has performed well in power, RF, and power RF applications. Due to excellent properties like high power density, breakdown voltage, and electron mobility, GaN captured the high-frequency market. However, LDMOS amplifiers still occupy a significant portion of the power and RF market. The section compares LDMOS and GaN technologies.

Current market

Until 2005, LDMOS amplifiers dominated the power and RF markets. GaAs (Gallium Arsenide) and LDMOS were initial competitors. The LDMOS market stood at USD1.31 billion latest. The entire GaN device market is valued at more than USD21 billion, with HEMTs dominating the GaN market. The LDMOS market is based on LDMOS amplifiers, a single device type!

Cost factor

Thanks to silicon, LDMOS devices are cheaper and abundant in the market compared to GaN devices. They are sold in plastic packaging, reducing overall costs. Legacy and simpler projects use LDMOS devices due to their ease of operation and price. However, advanced industrial applications do not consider costs.

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GaN amplifiers are either fabricated on SiC or Si substrates. Again, silicon being cheaper reduces the device cost. However, such devices are not efficient enough for use. GaN on SiC and even diamond substrates are gaining traction in the market. GaN on SiC technology is reliable and offers the utmost efficiency.

Efficiency

GaN exhibits a high power density due to its wide band gap energy. Between 22 to 30 GHz, GaN performs five times better than LDMOS. GaN amplifiers show smoother transitions into the saturation region. They can operate well and offer higher efficiency in saturation.

GaN shows lower input capacitance and output signal distortion. It leads to lower losses. On the other hand, LDMOS shows higher parasitic capacitance than GaN, which is undesirable. The input and output impedances of a GaN device become nearly the same, contributing to high-quality impedance matching.

P1dB

In RF electronics, P1dB is an important metric. P1dB or 1 dB compression point defines the output power level at which the gain of a power RF amplifier drops by 1 dB from its ideal linear amplifier. At P1dB, GaN shows an efficiency of up to 70 %. On the other hand, 28 V LDMOS shows an efficiency of 60 % and 50 V LDMOS shows an efficiency of 50 %.

Who is better?

While choosing a device, a healthy trade-off balance should be maintained between performance and costs. The choice of device is strictly dependent upon rating requirements, application, and budget. The article does not clearly state whether LDMOS technology should replace GaN devices.

In terms of performance, efficiency, and reliability, GaN devices are superior. GaN technology has been dominating the power and RF industries for years. LDMOS is a mature technology, a billion-dollar business. The performance of GaN-on-Si is comparable to LDMOS. GaN-on-SiC and GaN-on-diamond offer optimal results. However, cost is a challenging factor. Due to plastic packaging, LDMOS is cheaper than GaN. GaN is produced on 4” and 6” wafers. If mass-produced on 8”, GaN on Si can be sold at similar prices.

References

• https://www.researchgate.net/figure/Schematic-drawing-of-the-LDMOS-transistor-with-the-dual-layer-RESURF-in-the-drift-region_fig1_3254282

• https://www.st.com/resource/en/application_note/cd00004124-understanding-ldmos-device-fundamentals-stmicroelectronics.pdf

• https://www.mdpi.com/2079-9292/8/11/1260

• https://nuwaves.com/wp-content/uploads/2020/08/AN-007-A-Comparative-Review-of-GaN-LDMOS-and-GaAs-for-RF-and-Microwave-Applications.pdf

• https://www.rfwireless-world.com/terminology/gan-vs-ldmos

• https://www.rfwireless-world.com/terminology/gan-vs-ldmos-advantages-and-differences

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