NATIONAL UNIVERSITY OF SCIENCES AND TECHNOLOGY PAKISTAN

LDMOS Introduction of LDMOS

SAAD AFTAB HASEEB AHMAD

Ali Khalid

This report was submitted as a part of Final Year Project Report, at School of Electrical Engineering and Computer Sciences, National University of Sciences and Technology, Pakistan.

DOUBLE DIFFUSED MOSFET (DMOS):

The power MOSFETs require higher current capability which can’t be provided by

simple MOSFETs. In order to increase current (ID), Width (W) should be increased

and Length (L) should be decreased according to the relation:

However, when the channel length is decreased, it also results in reduction of

breakdown voltage which means that the transistor then is not capable of

handling the high voltages of power transistor applications.

To cater this problem, the transistors with shorter channel lengths and high

breakdown voltages are used. The device used is called DMOS (Double Diffused

Mosfet) or VDMOS (Vertical Double Diffused Mosfet) transistor.

STRUCTURE:

The device is fabricated on a lightly doped n-type substrate with a heavily doped

region at the bottom for the drain contact. Two diffusions are employed, one to

form the p-type body region and another to form the n-type source region.

WORKING:

When VGS greater than threshold voltage (Vt) is applied, it induces a lateral n-

channel in the p-type body region under the gate oxide indicated by L in diagram.

The current is conducted by the electrons from the source moving through the

resulting short channel to the substrate and then vertically down the substrate to

the drain.

Thus, DMOS although has short channel length but its breakdown voltage is as

high as 600V and has a current capability of 50A.

(Ref: microelectronics by sedra and smith)

LDMOS

Laterally double Diffused MOSFET (LDMOS) transistor is widely used in high RF

power amplifiers below a few GHz. The LDMOS transistor is a modified type of the

MOSFET to enhance the high power capability. It is a high voltage transistor

design where the drain is designed to have a lower doped diffusion that allows it

to handle higher voltages without damage to the gate. There is a much larger

"Drift" region to support the higher voltages.

The main modifications to basic MOSFET are:

It has a low doped and long n type drift region, which enhances the

depletion region and increases the breakdown voltage. However the on-

resistance is high which increases the losses and degrade the RF

performance. Thus, there is always a trade-off between RF output power

and on-resistance.

It has a short channel length created by laterally diffused P-type

implantation, which increases the operating frequency.

The sinker principle is used to connect the source to the substrate backside,

which reduces the source inductance, hence, the gain increases.

(Ref: A study of LDMOS switched mode Power Amplifiers by Ahmed Al Tanany of

UNIVERSITY OF GAVLE)

Ldmos has a shield between gate and drain to reduce feedback capacitance

(Cgd) which increases the linearity.

ADVANTAGES OF LDMOS:

Thermal stability:

LDMOS have a negative temperature coefficient and therefore are

protected against thermal runaway. As the device draws more current,

its temperature rises. A rise in temperature causes an increase in the

threshold voltage (Vt) which turns the device off resulting in a drop in

current.

In LDMOS, carrier speed is larger than BJTs.

High gain

Reliable due to negative temperature coefficient (thermal

conductivity rises with temperature)

Simple bias circuitry: MOSFETs are voltage controlled devices,

therefore no current is drawn from the bias circuitry.

Cgd is lesser in LDMOS as compared to VDMOS

Lesser package cost as compared to VDMOS

High efficiency and power as compared to VDMOS

(Ref: AN1223 Application note RF power transistors: comparative study of LDMOS versus

bipolar technology)

LDMOS IN TRANSMITTER ARCHITECTURE:

Since the transmitters have more value of voltage supplies, the LDMOS is most

suitable to be used in transmitters as it can support higher voltages. The increase

in the breakdown voltage and also the decrease in the channel length have made

it possible to get more current to drive the loads as well as has made the

MOSFETs to be used at greater supply voltages.

According to a paper, the LDMOS is mostly used in following frequencies.

• 1 - 30 MHz HF Band - VDMOS

• 30 - 500 MHz VDMOS - LDMOS

• 500 - 1500 MHz LDMOS - VDMOS

• > 1500 MHz LDMOS

(Ref: MTT 2001 “Vdmos vs. LDMOS How to Choose” by Polyfet RF Devices S. K. Leong)

CONCLUSION

LDMOS products are best suited for applications such as CDMA, W-CDMA, Digital

Terrestrial TV, polar transmitter architecture etc., requiring wide frequency range,

high linearity and good ruggedness performances.