ST AN1223 Application note

AN1223

Application note

RF power transistors: comparative study of

LDMOS versus bipolar technology

Introduction

RF power transistors consist of two type of devices: Bipolar Junction (BJT) and Field Effect (FET). Due to differences in technology, the bipolar junction transistor yields superior performance for certain applications while the field effect transistor is better employed for others. This application note discusses and compares their parameters and performances.

October 2007

Rev 4

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LDMOS advantages	AN1223

1 LDMOS advantages

LDMOS (and MOSFETs in general) have superior characteristics on the following points:

1.Thermal stability

2.Frequency stability

3.Higher gain

4.Increased ruggedness

5.Lower noise

6.Lower feedback capacitance

7.Simpler bias circuitry

8.Constant input impedance

9.Better IMD performances

10.Lower Thermal Resistance

11.Better AGC Capability

1.1Thermal stability

LDMOS, unlike bipolars, have a negative temperature coefficient and therefore are protected against thermal runaway. This is illustrated as follows: as the device draws more current, its temperature rises. A rise in temperature causes an increase in the gate threshold voltage (VGth) which turns the device off resulting in a drop in current.

Bipolars, on the other hand, have a positive temperature coefficient and are prone to thermal runaway. The main reason for this is the increase of hFE due to the increase of temperature. As the device draws more current its temperature rises, hence hFE rises and even more current is drawn resulting in a further temperature hike. This goes on until the device fails. Hence, bipolars need elaborate temperature compensation to prevent such occurrence. MOSFETs, however, are protected against thermal runaway and no compensation is required.

1.2Frequency stability

Lack of diode junctions and a higher ratio of feedback capacitance versus input impedance make LDMOS more stable than bipolars. Moreover, bipolars suffer an instability mode known as half f0 due to the varactor effect in base-emitter junction and a lower ratio of feedback capacitance versus input impedance.

1.3Higher gain

Two factors contribute to LDMOS superior gain characteristics compared to an equivalent bipolar (see Figure 1). First, wire-bonded connections, which normally connect the source and the external circuitry (because of the vertical bipolar structure - collector on the bottom), are no longer required. The negative feedback due to the wires’ self-capacitance and inductance is greatly reduced. This leads to higher gain at high frequencies. Second, in a bipolar, thermal stability is achieved at the detriment of gain. In an attempt to lessen the likelihood of bipolar thermal runaway, ballast resistors are placed in the emitters of the

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