The Explanation And History of LDMOS

 

MOSFETs or metal oxide semiconductor field-effect transistors or metal oxide silicon transistors are a type of insulated-gate field-effect transistors (FET) fabricated by controlled oxidation of silicon ( or some other semiconductor).

What Is LDMOS

LDMOS is a planar double diffused MOSFET used in audio power amplifiers, RF power amplifiers, and microwave amplifiers. LDMOS are fabricated on p/p+ silicon epitaxial layers; their fabrication mostly includes subsequent cycles and various ion implantations.

Ref (https://www.microwaves101.com/encyclopedias/ldmos)

Background of LDMOS Invention

Electronic appliances that are battery operated, such as laptops, computers, wireless communication devices, and personal digital assistants, often use power MOS as low on-resistance switches for power distribution from the battery.

The low on-resistance is very important for battery-operated applications as it ensures very little power consumption for the battery and prolongs battery life. DMOS are a double diffused MOS device which means that the DMOS has a back gate region and a source region diffused simultaneously. The back gate region is referred to as a double diffused well or dwell region. The channel is formed not by separate implantations but by the difference in the two diffusions.

The DMOS devices provide low power dissipation and high-speed capability due to them being able to decrease the length of the channels.

There are two types of DMOS devices

·         Lateral DMOS

·         Vertical DMOS

A lateral DMOS device or LDMOS has its drain and source at the surface of the semiconductor wafer. Hence the current is lateral. The characteristics of LDMOS that make it desirable are low on-resistance and high breakdown voltage (BV).

History of LDMOS

DMOS was first reported in the 1960s, while LDMOS was first reported in 1969 by “Tarui et al” from the electrotechnical laboratory. Between the years 1977 and 1983, Hitachi was the only LDMOS manufacturer. It was used as an audio power amplifier in applications like music, public address systems, HH electronics, and Ashley audio during that period.

For RF applications, LDMOS was introduced in the 1970s by “Caught et al.” However, it wasn’t until early 1990s RF LDMOS for wireless cellular network replaced RF bipolar transistors because RF LDMOS provide better efficiency, gain, and linearity all with lower costs.

After the introduction of 2G mobile networks, LDMOS was the most widely used power amplifier. They are further used in 3G mobile networks. RF LDMOS were the dominant RF power amplifier in broadcasting, radar, cellular base stations, and Scientific, Medical band, and industrial applications. Since then, LDMOS has greatly influenced the majority of the world’s data traffic and cellular voice.

When used in 3G and 4G (LTE) networks, RF power amplifiers based on single LDMOS produced low efficiencies in the mid-2000s due to higher peaks to the average power of the modulation schemes and OFDMA and CDMA access techniques. So in 2006, the the efficiency of LDMOS was increased using techniques like envelope tracking and Doherty topologies.

And since 2011, RF LDMOS is the dominant technology used in high-power RF power amplifiers for frequencies from 1MHz to more than 3.5GHz and for cellular infrastructure. As of 2018, LDMOS is the standard for power amplifiers in 4G and 5G mobile networks.