Power MOSFET Basics By Vrej Barkhordarian, International Rectifier, El Segundo, Ca. Breakdown Voltage......................................... 5 On-resistance.................................................. 6 Transconductance............................................ 6 Threshold Voltage........................................... 7 Diode Forward Voltage.................................. 7 Power Dissipation........................................... 7 Dynamic Characteristics................................ 8 Gate Charge.................................................... 10 dV/dt Capability............................................... 11www.irf.com
Power MOSFET Basics Vrej Barkhordarian, International Rectifier, El Segundo, Ca.Discrete power MOSFETs Source Field Gate Gate Drainemploy semiconductor Contact Oxide Oxide Metallization Contactprocessing techniques that aresimilar to those of todays VLSIcircuits, although the devicegeometry, voltage and currentlevels are significantly different n* Drain n* Sourcefrom the design used in VLSI toxdevices. The metal oxidesemiconductor field effect p-Substratetransistor (MOSFET) is basedon the original field-effect Channel ltransistor introduced in the70s. Figure 1 shows thedevice schematic, transfer (a)characteristics and devicesymbol for a MOSFET. The IDinvention of the powerMOSFET was partly driven bythe limitations of bipolar powerjunction transistors (BJTs)which, until recently, was thedevice of choice in powerelectronics applications. 0 Although it is not possible to 0 VT VGSdefine absolutely the operating (b)boundaries of a power device,we will loosely refer to the IDpower device as any devicethat can switch at least 1A. DThe bipolar power transistor is SBa current controlled device. A (Channel or Substrate)large base drive current as Ghigh as one-fifth of thecollector current is required to Skeep the device in the ON (c)state. Figure 1. Power MOSFET (a) Schematic, (b) Transfer Characteristics, (c)Also, higher reverse base drive Device Symbol.currents are required to obtainfast turn-off. Despite the very advanced state of manufacturability and lower costs of BJTs, theselimitations have made the base drive circuit design more complicated and hence more expensive than thepower MOSFET.
Another BJT limitation is that both electrons and holes 2000contribute to conduction. Presence of holes with their higher Holdoff Voltage (V)carrier lifetime causes the switching speed to be several orders of 1500magnitude slower than for a power MOSFET of similar size and Bipolarvoltage rating. Also, BJTs suffer from thermal runaway. Their Transistors 1000forward voltage drop decreases with increasing temperaturecausing diversion of current to a single device when several 500 MOSdevices are paralleled. Power MOSFETs, on the other hand, aremajority carrier devices with no minority carrier injection. They 0are superior to the BJTs in high frequency applications where 1 10 100 1000switching power losses are important. Plus, they can withstand Maximum Current (A)simultaneous application of high current and voltage without Figure 2. Current-Voltageundergoing destructive failure due to second breakdown. Power Limitations of MOSFETs and BJTs.MOSFETs can also be paralleled easily because the forwardvoltage drop increases with increasing temperature, ensuring an even distribution of current among allcomponents.However, at high breakdown voltages (>200V) the on-state voltage drop of the power MOSFET becomeshigher than that of a similar size bipolar device with similar voltage rating. This makes it more attractiveto use the bipolar power transistor at the expense of worse high frequency performance. Figure 2 showsthe present current-voltage limitations of power MOSFETs and BJTs. Over time, new materials,structures and processing techniques are expected to raise these limits. Source Gate Polysilicon Oxide Gate Source Metallization p+ Body Region Channels p+ + p n+ n D Drift Region n- Epi Layer G n+ Substrate S (100) Drain Metallization Drain Figure 3. Schematic Diagram for an n-Channel Power MOSFET and the Device.
Figure 3 shows schematic diagram and Figure 4 shows the physical origin of the parasitic components inan n-channel power MOSFET. The parasitic JFET appearing between the two body implants restrictscurrent flow when the depletion widths of the two adjacent body diodes extend into the drift region withincreasing drain voltage. The parasitic BJT can make the device susceptible to unwanted device turn-onand premature breakdown. The base resistance RB must be minimized through careful design of thedoping and distance under the source region. There are several parasitic capacitances associated withthe power MOSFET as shown in Figure 3.CGS is the capacitance due to the overlap of the source and the channel regions by the polysilicon gateand is independent of applied voltage. CGD consists of two parts, the first is the capacitance associatedwith the overlap of the polysilicon gate and the silicon underneath in the JFET region. The second part isthe capacitance associated with the depletion region immediately under the gate. CGD is a nonlinearfunction of voltage. Finally, CDS, the capacitance associated with the body-drift diode, varies inverselywith the square root of the drain-source bias. There are currently two designs of power MOSFETs, usuallyreferred to as the planar and the trench designs. The planar design has already been introduced in theschematic of Figure 3. Two variations of the trench power MOSFET are shown Figure 5. The trenchtechnology has the advantage of higher cell density but is more difficult to manufacture than the planardevice. Metal Cgsm LTO CGS2 CGD n- CGS1 RCh n- JFET - RB p BJT CDS REPI n- Epi Layer n- Substrate Figure 4. Power MOSFET Parasitic Components.
BREAKDOWN VOLTAGE S G SBreakdown voltage,BVDSS, is the voltage atwhich the reverse-biasedbody-drift diode breaksdown and significantcurrent starts to flowbetween the source anddrain by the avalanchemultiplication process,while the gate andsource are shortedtogether. Current-voltagecharacteristics of a Electron Flowpower MOSFET areshown in Figure 6.BVDSS is normallymeasured at 250µA draincurrent. For drainvoltages below BVDSSand with no bias on thegate, no channel is Dformed under the gate at (a)the surface and the drainvoltage is entirely Source Sourcesupported by the Gatereverse-biased body-driftp-n junction. Two related Oxidephenomena can occur in Gate Oxidepoorly designed andprocessed devices:punch-through andreach-through. Punch-through is observed Channel n- Epi Layerwhen the depletionregion on the source sideof the body-drift p-njunction reaches the n+ Substratesource region at drain (100)voltages below the ratedavalanche voltage of thedevice. This provides acurrent path between Drainsource and drain and (b)causes a soft breakdowncharacteristics as shown Figure 5. Trench MOSFET (a) Current Crowding in V-Groove Trench MOSFET,in Figure 7. The leakage (b) Truncated V-Groove MOSFETcurrent flowing betweensource and drain is denoted by IDSS. There are tradeoffs to be made between RDS(on) that requires shorterchannel lengths and punch-through avoidance that requires longer channel lengths.The reach-through phenomenon occurs when the depletion region on the drift side of the body-drift p-njunction reaches the epilayer-substrate interface before avalanching takes place in the epi. Once thedepletion edge enters the high carrier concentration substrate, a further increase in drain voltage willcause the electric field to quickly reach the critical value of 2x105 V/cm where avalanching begins.
ON-RESISTANCEThe on-state resistance of a power MOSFET is made up of several components as shown in Figure 8:R DS(on) = R source + R ch + R A + R J + R D + R sub + R wcml (1)where: 25Rsource = Source diffusion resistanceRch = Channel resistance 7RA = Accumulation resistanceRJ = "JFET" component-resistance of the Gateregion between the two body regions VoltageRD = Drift region resistance 20Rsub = Substrate resistance 6 Linear RegionWafers with substrate resistivities of up to (Saturation20mΩ-cm are used for high voltage Region)devices and less than 5mΩ-cm for low 15voltage devices.Rwcml = Sum of Bond Wire resistance, the 5 Normalized Drain CurrentContact resistance between the sourceand drain Metallization and the silicon,metallization and Leadframe 10 IDS VS VDS LOCUScontributions. These are normallynegligible in high voltage devices but can 4become significant in low voltage devices.Figure 9 shows the relative importance ofeach of the components to RDS(on) over the 5 3voltage spectrum. As can be seen, at highvoltages the RDS(on) is dominated by epiresistance and JFET component. This 2component is higher in high voltagedevices due to the higher resistivity or 1lower background carrier concentration in 0 0 5 10 15the epi. At lower voltages, the RDS(on) is Drain Voltage (Volts)dominated by the channel resistance andthe contributions from the metal to Figure 6. Current-Voltage Characteristics of Power MOSFETsemiconductor contact, metallization,bond wires and leadframe. The substrate contribution becomes more significant for lower breakdownvoltage devices.TRANSCONDUCTANCETransconductance, gfs, is a measure of the sensitivity of drain current to changes in gate-source bias.This parameter is normally quoted for a Vgs that gives a drain current equal to about one half of themaximum current rating value and for a VDS that ensures operation in the constant current region.Transconductance is influenced by gate width, which increases in proportion to the active area as celldensity increases. Cell density has increased over the years from around half a million per square inch in1980 to around eight million for planar MOSFETs and around 12 million for the trench technology. Thelimiting factor for even higher cell densities is the photolithography process control and resolution thatallows contacts to be made to the source metallization in the center of the cells.
Channel length also affects transconductance. Reducedchannel length is beneficial to both gfs and on-resistance,with punch-through as a tradeoff. The lower limit of this IDlength is set by the ability to control the double-diffusionprocess and is around 1-2mm today. Finally the lower thegate oxide thickness the higher gfs. SoftTHRESHOLD VOLTAGE SharpThreshold voltage, Vth, is defined as the minimum gateelectrode bias required to strongly invert the surfaceunder the poly and form a conducting channel between BVDSS VDSthe source and the drain regions. Vth is usually measuredat a drain-source current of 250µA. Common values are Figure 7. Power MOSFET Breakdown2-4V for high voltage devices with thicker gate oxides, and Characteristics1-2V for lower voltage, logic-compatible devices withthinner gate oxides. With power MOSFETs finding increasing use in portable electronics and wirelesscommunications where battery power is at a premium, the trend is toward lower values of RDS(on) andVth. GATEDIODE FORWARD VOLTAGE N+The diode forward voltage, VF, is the SOURCEguaranteed maximum forward drop ofthe body-drain diode at a specifiedvalue of source current. Figure 10 RCH RAshows a typical I-V characteristics for RSOURCE P-BASE RJthis diode at two temperatures. P-channel devices have a higher VF dueto the higher contact resistancebetween metal and p-siliconcompared with n-type silicon.Maximum values of 1.6V for high RDvoltage devices (>100V) and 1.0V forlow voltage devices (<100V) arecommon.POWER DISSIPATION N+ SUBSTRATE RSUBThe maximum allowable powerdissipation that will raise the dietemperature to the maximumallowable when the case temperature DRAINis held at 250C is important. It is give Figure 8. Origin of Internal Resistance in a Power MOSFET.by Pd where:Pd = T j m ax- 25 (2) R thJCTjmax = Maximum allowable temperature of the p-n junction in the device (normally 1500C or 1750C) RthJC= Junction-to-case thermal impedance of the device.DYNAMIC CHARACTERISTICS
When the MOSFET is used as a switch, its basic function is to control the drain current by the gatevoltage. Figure 11(a) shows the transfer characteristics and Figure 11(b) is an equivalent circuit modeloften used for the analysis of MOSFET switching performance. Voltage Rating: 50V 100V 500V Packaging Rwcml Metallization Source RCH Channel JFET Region REPI Expitaxial Layer Substrate Figure 9. Relative Contributions to RDS(on) With Different Voltage Ratings.The switching performance of a device is determined by the time required to establish voltage changesacross capacitances. RG is the distributed resistance of the gate and is approximately inverselyproportional to active area. LS and LD are source and drain lead inductances and are around a few tens ofnH. Typical values of input (Ciss), output (Coss) and reverse transfer (Crss) capacitances given in the datasheets are used by circuit designers as a starting point in determining circuit component values. The datasheet capacitances are defined in terms of the equivalent circuit capacitances as:
Ciss = CGS + CGD, CDS shorted 100Crss = CGDCoss = CDS + CGD ISD, Reverse Drain Current (A)Gate-to-drain capacitance, CGD, is a 10nonlinear function of voltage and is the mostimportant parameter because it provides a TJ = 1500Cfeedback loop between the output and theinput of the circuit. CGD is also called theMiller capacitance because it causes the total TJ = 250Cdynamic input capacitance to become greater 1than the sum of the static capacitances.Figure 12 shows a typical switching time testcircuit. Also shown are the components ofthe rise and fall times with reference to the VGS = 0VVGS and VDS waveforms. 0.1 0.0 0.5 1.0 1.5 2.0 2.5Turn-on delay, td(on), is the time taken to VSD, Source-to-Drain Voltage (V)charge the input capacitance of the devicebefore drain current conduction can start. Figure 10. Typical Source-Drain (Body) Diode Forward Voltage Characteristics.Similarly, turn-off delay, td(off), is the timetaken to discharge the capacitance after the after is switched off. D ID LD CGD D G RG C ID Body-drain Slope = gfs CDS Diode S CGS LS VGS S (a) (b) Figure 11. Power MOSFET (a) Transfer characteristics, (b) Equivalent Circuit Showing Components That Have Greatest Effect on Switching
GATE CHARGE RDAlthough input capacitance VDSvalues are useful, they do notprovide accurate results whencomparing the switching D.U.T. VGSperformances of two devicesfrom different manufacturers. -Effects of device size and RG VDDtransconductance make such +comparisons more difficult. Amore useful parameter from thecircuit design point of view is -10Vthe gate charge rather than µ Pulse Width < 1µscapacitance. Most Duty Factor < 0.1%manufacturers include bothparameters on their data sheets. (a)Figure 13 shows a typical gatecharge waveform and the test td(on) tr td(off) tfcircuit. When the gate isconnected to the supply voltage, VGSVGS starts to increase until it 100%reaches Vth, at which point thedrain current starts to flow andthe CGS starts to charge. Duringthe period t1 to t2, CGScontinues to charge, the gatevoltage continues to rise anddrain current rises 90%proportionally. At time t2, CGSis completely charged and the VDSdrain current reaches thepredetermined current ID and (b)stays constant while the drain Figure 12. Switching Time Test (a) Circuit, (b) VGS and VDSvoltage starts to fall. With Waveformsreference to the equivalentcircuit model of the MOSFET shown in Figure 13, it can be seen that with CGS fully charged at t2, VGSbecomes constant and the drive current starts to charge the Miller capacitance, CDG. This continuesuntil time t3.
Charge time for the Miller capacitance is VDDlarger than that for the gate to sourcecapacitance CGS due to the rapidly changingdrain voltage between t2 and t3 (current = Cdv/dt). Once both of the capacitances CGS ID Dand CGD are fully charged, gate voltage (VGS)starts increasing again until it reaches thesupply voltage at time t4. The gate charge CDG D(QGS + QGD) corresponding to time t3 is thebare minimum charge required to switch Gthe device on. Good circuit design practicedictates the use of a higher gate voltagethan the bare minimum required for S Sswitching and therefore the gate charge ID CGSused in the calculations is QGcorresponding to t4. TEST CIRCUIT (a)The advantage of using gate charge is thatthe designer can easily calculate theamount of current required from the drivecircuit to switch the device on in a desired OGS OGDlength of time because Q = CV and I = Cdv/dt, the Q = Time x current. For VGexample, a device with a gate charge of20nC can be turned on in 20µsec if 1ma is GATEsupplied to the gate or it can turn on in VG(TH) VOLTAGE20nsec if the gate current is increased to1A. These simple calculations would not t0 t1 t2 t3 t4have been possible with input capacitance t DRAINvalues. VOLTAGE DRAIN CURRENTdv/dt CAPABILITY VDD IDPeak diode recovery is defined as themaximum rate of rise of drain-sourcevoltage allowed, i.e., dv/dt capability. If this WAVEFORMrate is exceeded then the voltage across the (b)gate-source terminals may become higherthan the threshold voltage of the device, Figure 13. Gate Charge Test (a) Circuit, (b) Resulting Gateforcing the device into current conduction and Drain Waveforms.mode, and under certain conditions acatastrophic failure may occur. There are two possible mechanisms by which a dv/dt induced turn-onmay take place. Figure 14 shows the equivalent circuit model of a power MOSFET, including theparasitic BJT. The first mechanism of dv/dt induced turn-on becomes active through the feedback actionof the gate-drain capacitance, CGD. When a voltage ramp appears across the drain and source terminalof the device a current I1 flows through the gate resistance, RG, by means of the gate-drain capacitance,CGD. RG is the total gate resistance in the circuit and the voltage drop across it is given by: dvVGS = I1 R G = R G C GD (3) dtWhen the gate voltage VGS exceeds the threshold voltage of the device Vth, the device is forced intoconduction. The dv/dt capability for this mechanism is thus set by:
dv V = thdt R G C GD (4)It is clear that low Vthdevices are more prone to DRAINdv/dt turn-on. Thenegative temperaturecoefficient of Vth is of Dspecial importance in I1applications where high CDBtemperature environments CGD NPNare present. Also gate BIPOLAR APPLIEDcircuit impedance has to be G I2 TRANSISTOR RAMPchooses carefully to avoid VOLTAGEthis effect. RG RBThe second mechanism for CGSthe dv/dt turn-on in SMOSFETs is through theparasitic BJT as shown inFigure 15. The capacitanceassociated with the SOURCEdepletion region of the bodydiode extending into the Figure 14. Equivalent Circuit of Power MOSFET Showing Two Possibledrift region is denoted as Mechanisms for dv/dt Induced Turn-on.CDB and appears betweenthe base of the BJT and the drain of the MOSFET. This capacitance gives rise to a current I 2 to flowthrough the base resistance RB when a voltage ramp appears across the drain-source terminals. Withanalogy to the first mechanism, the dv/dt capability of this mechanism is:dv VBE = (5)dt R BC DB SOURCE GATEIf the voltage that develops across RB is greater N+ Athan about 0.7V, then the base-emitter junctionis forward-biased and the parasitic BJT is LN+turned on. Under the conditions of high (dv/dt) RSand large values of RB, the breakdown voltage ofthe MOSFET will be limited to that of the open-base breakdown voltage of the BJT. If theapplied drain voltage is greater than the open- CDSbase breakdown voltage, then the MOSFET willenter avalanche and may be destroyed if thecurrent is not limited externally.Increasing (dv/dt) capability therefore requiresreducing the base resistance RB by increasingthe body region doping and reducing thedistance current I2 has to flow laterally before itis collected by the source metallization. As in DRAINthe first mode, the BJT related dv/dt capabilitybecomes worse at higher temperatures because Figure 15. Physical Origin of the Parasitic BJTRB increases and VBE decreases with increasing Components That May Cause dv/dt Induced Turn-ontemperature.
References:"HEXFET Power MOSFET Designers Manual - Application Notes and Reliability Data," InternationalRectifier"Modern Power Devices," B. Jayant Baliga"Physics of Semiconductor Devices," S. M. Sze"Power FETs and Their Applications," Edwin S. Oxner"Power MOSFETs - Theory and Applications," Duncan A. Grant and John Gower