power transistors consist type devices: Bipolar Junction (BJT) Field E

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power transistors consist type devices: Bipolar Junction (BJT) Field E

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power transistors: comparative study LDMOS versus bipolar technology
power transistors consist type devices: Bipolar Junction (BJT) Field Effect (FET). differences technology, bipolar junction transistor yields superior performance certain applications while field effect transistor better employed others. This application note discusses compares their parameters performances.
October 2007
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LDMOS advantages
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LDMOS advantages
LDMOS (and MOSFETs general) have 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
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 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.
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.
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. negative feedback wires' self-capacitance inductance greatly reduced. This leads higher gain high frequencies. Second, bipolar, thermal stability achieved detriment gain. attempt lessen likelihood bipolar thermal runaway, ballast resistors placed emitters
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LDMOS advantages 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
Increased ruggedness
LDMOS, source channel shorted, hence there more breakdown voltage (BVCEO). Consequently, device ruggedness significantly improved. MOSFETs indeed more rugged than their bipolar counterpart. Ruggedness important factor applications such commercial radios where output devices generally protected isolator (circulator) often experience large load mismatches.
Lower noise
Another drawback ballast emitter resistors bipolars increased noise since current flowing through these resistors generates 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.
Lower feedback capacitance
Many broadband amplifiers negative feedback achieve good gain flatness across wide frequency range. These applications request feedback capacitance (the
LDMOS advantages
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between output lead input lead device). MOSFET feedback capacitance (LDMOS) typically times lower than feedback capacitance comparable bipolar.
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.
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
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.
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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.
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LDMOS main drawbacks
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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 main drawbacks
Lower power density Damage electrostatic discharge
Lower power density
comparable power level more area required LDMOS than bipolar. This results less dice wafer therefore higher MOSFET (LDMOS) cost. larger area also restricts maximum available power given package.
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 desired linearity achieved), where comparable bipolar could only attain same linearity class occasioning high current consumption.
Revision history
Table
Date 21-Jun-2004 04-Oct-2007
Document revision history
Revision Minor text changes content changes, document reformatted Changes
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