LDMOS

Updated July 19, 2009

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This page was contributed by Andrei during June 2006, a true friend of Microwaves101! We have only done some minor edits, Andrei is the subject matter expert, not us. If anyone wants to contribute material on other semiconductor technologues, by all means contact us.

The story behind LDMOS is quite sad for everyone outside silicon (you GaAs people know what we're talking about), because LDMOS is a real winner. A corollary of Moore's law is, "everything that can be made of silicon, should be made of silicon!"

LDMOS summary

LDMOS stands for lateral double-diffused MOSFET, the lateral version of power MOSFET, DMOS. Although some vendors offer RF versions of DMOS, its vertical structure has serious problems with excessive parasitic capacitance starting at around 500 MHz. LDMOS fares much better at higher frequencies, not least due to extensive technology development recent years. The first cellular base stations used silicon BJTs, and many GaAs developers expected an easy win as cell frequencies were going up, and linearity and efficiency requirements were getting tighter. This never happened, it was LDMOS that invaded the territory. LDMOS components with output power over 100 Watts at 2.7 GHz are available at the moment of writing this. The frequency range is likely to extend further as Freescale has recently announced 3.5 GHz high power LDMOS for coming WiMAX applications. If or when this happens, the hopes AlGaN developers had for exciting new markets might not come true, just as those of GaAs folks a decade ago.

High power LDMOS devices typically provide internal impedance matching for intended frequency band. The practical power limit for LDMOS without internal matching is around 10 Watts at 2.5 GHz. This might leave a window of opportunity for other materials in broadband power designs. Wide bandgap materials offer higher output power per 1 pF of gate capacitance, which is an advantage for broadband applications.

Comments on LDMOS device technology

What is so special with LDMOS? As a device of MOSFET variety, LDMOS uses an inversion channel at the silicon-oxide interface. The inversion channel is induced under the gate by positive gate potential. Under practically relevant conditions the inversion layer only exists over the laterally diffused P-well, which is sometimes called depletion stopper. As the electrons leave the region over the stopper they are picked up by the electric field due to positive drain bias and abandon the inversion channel going deeper into the bulk. The effective gate length defines the lateral extension of the stopper layer. It may be therefore shorter than the physical length of the gate electrode.

The cross-section below was borrowed from a report by A. Litwin at a small workshop at Chalmers. Hope he doesn't mind!

GaAs MESFETs and HEMTs with very short gates suffer from high output conductance due to short-channel effects. In a standard MESFET, high electric field from the drain side penetrates underneath the channel. A parasitic channel may be formed at a high drain bias, which penetrates through the buffer and/or substrate, i.e. underneath the nominal physical channel layer. The problem becomes more and more pronounced as one decreases the gate length and increases the drain bias. In silicon LDMOS the short-channel effect is taken care of by the P-type depletion stopper. In addition to the depletion stopper underneath the channel, most of recent LDMOS designs also use a field plate on the top, which overlaps thick dielectric over gate and provides additional shielding of the gate from the drain potential. Combined action of stopper and
field plate minimizes the feedback (drain-to-gate) capacitance, which means further improvement of RF signal gain.

High breakdown voltage of LDMOS is one of its most important advantages. For a given output impedance the power output is the square of voltage swing, therefore you get over 7 dB more power going from 12 to 28 Vdd. Even if you are able to match that low-voltage component you will lose some bandwidth and you will certainly need a lot of current out of your power supply. So why does LDMOS have higher operation voltage than silicon than GaAs MESFET if the breakdown field in GaAs is higher than in silicon? Examination of the cross-sections will give the answer to this question. Electric field crowding at the gate edge of the MESFET takes up all the advantages of the higher breakdown field of GaAs. In addition, the current pathway through the buffer layer does
not offer the designer much choice apart from increasing the gate length to make a tradeoff between gain and output power. In LDMOS the depletion stopper and the field plate form a fairly uniform electric field between gate and drain. Electric field crowding at the apparently sharp edge of the field plate has little effect on the breakdown in LDMOS, since the breakdown field of the oxide is 30X that of silicon.

LDMOS utilizes epitaxial silicon, low-doped P-type layers grown on low-resistivity (i.e. highly doped) silicon wafers. Either a diffused sinker or a trench etched through the epitaxial layer is used to ground the source to the substrate. Each source is comfortably grounded to the baseplate, as you get it in a high-end GaAs HEMT with individual source vias. Gate and drain impedance is pre-matched using lumped element technique. Bondwires are used as inductors and extra capacitor chips are soldered next to the LDMOS chip into the same package. 50-Ohm components are available, although those with the highest output power are push-pull and are often matched to lower impedance, anywhere between a few Ohms and 20 Ohms.

One cannot go in too much detail about the power capabilities of LDMOS, as we don't have any discussion/demystification of AB class operation on the site. That's why we cannot discuss efficiency and linearity for CDMA-type signal transmission here!.