Serge Juhel ABSTRACT power transistors consist type devices: Bipo

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Serge Juhel ABSTRACT power transistors consist type devices: Bipo

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POWER TRANSISTORS: COMPARATIVE STUDY LDMOS VERSUS BIPOLAR TECHNOLOGY
Serge Juhel
ABSTRACT power transistors consist type devices: Bipolar Junction (BJT) Field Effect (FET). differences technology, bipolar junction transistor will yield superior performance certain applications while field effect transistor will better employed others. This application note will discuss compare their parameters performances. LDMOS ADVANTAGES. LDMOS (and MOSFETs general) enjoy superior characteristics following points: Thermal stability Frequency stability Higher gain Increased ruggedness Lower noise Lower feedback capacitance Simpler bias circuitry Constant input impedance Better performances Lower Thermal Resistance Better Capability 2.1. Thermal stability. LDMOS, unlike bipolars, have negative temperature coefficient therefore protected against thermal runaway. This illustrated follows: device draws more current, temperature rises. rise temperature causes increase gate threshold voltage (VGth) which turns device resulting drop current. Bipolars, other hand, have positive temperature coefficient prone thermal runaway. main reason this increase with increase temperature. device draws more current temperature rises, hence rises even more current drawn resulting further temperature hike. This goes until device fails. Hence, bipolars need elaborate temperature compensation prevent such occurrence. MOSFETs, however, protected against thermal runaway compensation required.
July 2000
AN1223 APPLICATION NOTE 2.2. Frequency stability. Lack diode junctions higher ratio feedback capacitance versus input impedance make LDMOS more stable than bipolars. Moreover, bipolars suffer instability mode known half varactor effect base-emitter junction lower ratio feedback capacitance versus input impedance. 2.3. Higher gain. factors contribute LDMOS superior gain characteristics compared equivalent bipolar (see Figure First, wire-bonded connections, which normally connect source external circuitry (because vertical bipolar structure collector bottom), longer required thus greatly reducing negative feedback wires self-capacitance inductance. This leads higher gain high frequencies. Second, bipolar thermal stability achieved detriment gain. attempt lessen likelihood bipolar thermal runaway, ballast resistors placed emitters device. This helps prevent current hogging. Current hogging occurs when many emitters within transistor draws more current than others. single emitter draws more current temperature hikes increases, leading thermal runaway situation. Placing resistors emitters device helps share current more equally, therefore decreasing likelihood thermal runaway. However, gain pays price this increased thermal stability. Consequently, bipolar gain lesser than that LDMOS. fact that LDMOS higher gain means that less amplifying stages needed, which turn, means higher reliability lower costs. Figure Gain Output Power
Gain (dB) Output Power LDMOS Bipolar
2.4. Increased Ruggedness. LDMOS, source channel shorted, hence there more breakdown voltage (BVCEO). Consequently, device ruggedness significantly improved. MOSFETs indeed more rugged than their bipolar counter part. Ruggedness important factor applications such commercial radios where output devices generally protected isolator (circulator) often experience large load mismatches.
AN1223 APPLICATION NOTE 2.5. Lower noise. Another drawback ballast emitter resistors bipolars increased noise since current flowing through these resistors will generate noise. MOSFETs unaffected since they fitted with ballast resistors. most amplifiers this unimportant applications, such transceivers, which have transmitter biased linear operation receiver located nearby, noise critical factor. 2.6. Lower feedback capacitance. Many broadband amplifiers negative feedback achieve good gain flatness across wide frequency range. These applications request feedback capacitance (the between output lead input lead device). MOSFET feedback capacitance (LDMOS) typically times lower than feedback capacitance comparable bipolar. 2.7. Simpler bias circuitry. MOSFETs voltage controlled devices, therefore current drawn from bias circuitry. Furthermore, MOSFETs have negative temperature coefficient, hence temperature compensating component bias circuitry required. Consequently, bias circuit remains very simple biasing done with plain voltage divider. 2.8. Constant input impedance. MOSFETs input impedance varies only slightly with gate voltage fluctuation. This makes very suitable amplitude modulation applications where constant load driver stage necessary order prevent parasitic amplitude modulation. Also, MOSFETs constant gate impedance permits identical input matching network class operation (class class Figure Third Order Intermodulation Distortion Output Power
IMD3 (dB)
Output Power (WPEP) LDMOS Bipolar
2.9. Better performances. stated above, MOSFETs constant input impedance, function input power level, allows better Intermodulation Distortion (IMD) performances power levels (see Figure Bipolars input impedance varies with input power level, hence transistor becomes unmatched from matching network generates higher level IMD.
AN1223 APPLICATION NOTE 2.10. Lower thermal resistance. Since LDMOS have lower power density (LDMOS' dice larger than bipolars'), dissipated heat occurs through larger area. Moreover, LDMOS require electrical isolator (BeO). Consequently, LDMOS thermal resistance considerably better than comparable bipolar. 2.11. Better Capability. relationship between drain current (Id) gate voltage (Vg) makes LDMOS (MOSFET) ideal Automatic Gain Control (AGC) applications. This relationship linear almost from turn-on saturation. This means that LDMOS gain controlled throughout wider range power levels. Typically LDMOS have range better, while range comparable bipolar around LDMOS MAINS DRAWBACKS. Lower power density Damage electrostatic discharge 3.1. Lower power density. comparable power level more area required LDMOS than bipolar. This results less dice wafer therefore higher MOSFET (LDMOS) cost. larger area will also restrict maximum available power given package. 3.2. Damage electrostatic discharge. Electrostatic discharge, which reach several hundreds volts, deteriorate gate source channel LDMOS anti-static protection handling mandatory. CONCLUSION LDMOS products best suited applications such CDMA, W-CDMA, TETRA, Digital Terrestrial etc., requiring wide frequency range, high linearity good ruggedness performances. above all, LDMOS flawless high linearity requisite. They used class reducing output power until wanted linearity achieved), where comparable bipolar could only attain same linearity class occasioning high current consumption.
AN1223 APPLICATION NOTE
Information furnished believed accurate reliable. However, STMicroelectronics assumes responsibility consequences such information infringement patents other rights third parties which result from use. license granted implication otherwise under patent patent rights STMicroelectronics. Specification mentioned this publication subject change without notice. This publication supersedes replaces information previously supplied. STMicroelectronics products authorized critical components life support devices systems without express written approval STMicroelectronics. logo trademark STMicroelectronics 2000 STMicroelectronics Printed Italy rights reserved
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Serge Juhel ABSTRACT power transistors consist type devices: Bipo

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