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 1/6

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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

1.1 Thermal 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 h
temperature. As the device draws more current its temperature rises, hence h
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.

1.2 Frequency 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.

1.3 Higher 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

due to the increase of

FE

rises and

FE

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