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Diodes and SCRsDr. Tahir Izhar

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Power semiconductor devices bear highcurrent densities while they are ‘on’ andwithstand high voltage across them when‘off’.

When ‘on’ the drop across the diode is 2-3volts and it is conducting large current,therefore the power dissipation of the powerdiode is large.

The power diode are much bigger in sizeand encapsulated in metal body to bemounted on metal heat sink for properthermal design.

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Reverse BiasedThe reverse bias voltage across a bipolar

diode is limited because of high leakagecurrent and avalanche breakdown.

The v-j relation of a diode is given as

---------(1)

Under reverse biased, VA <<0, J=Js

)1( / kTqVs

Aejj

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Js is called the leakage current density.At room temperature (25OC) and under

forward bias, Js is very small compared to J. If VA = 0.6 V, J is 2.6X1010 times greater than

Js.Such a small leakage current results in

negligible power dissipation.However, Js is proportional to ni

2.ni

2 is a strong function of temperature

kTEi

geTn /32 4

--------------- (2)

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7th order of magnitude increase gives aleakage current whose effect is no longernegligible.

The diode equation (1) given above doesnot account for all the current under reversebias,

As we know, under thermal equilibrium

CcmXn

CcmXno

i

oi

175@/102

25@/1023272

3202

2ioo npn

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However, under reverse bias, the device isno longer in thermal equilibrium.

The excess carrier concentration in SCL andadjacent regions is substantially below thanno and po.

Therefore the thermal generation rateexceeds the recombination rate.

The carrier concentrations do not built usbecause thermally generated carriers areswept out of SCL.

These carrier flow give a component ofmeasured leakage current that is notaccounted for by equation (1)

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Generation in wide SCL & adjacent regions of apower diode will yield substantially more leakagecurrent at 25OC than predicted by diode equation.

To keep the leakage current in acceptably small, theminority carriers life time in the SCL should be aslong as possible.

The component of leakage current resulting fromthermal generation in the SCL, grows as ni, whichapproximately doubles for every 11OC increase in Tbetween -50OC to 200OC. But Js grows as ni

2, so iteventually dominates at high temperature.

This increase in reverse leakage current limits themaximum operating junction temperature of a powerdiode.

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The transient performance of diodes tendsto deteriorate as the thickness of siliconwafer is increased in attaining higherreverse voltage.

The asymmetric doping concentration ispreferred in Power Diodes fabrication.

Under reverse bias, nearly all the voltage issupported by the SCL in the lightly dopedregion.

Power diode are vertical in structure andfabricated by simply diffusing P+ regioninto n- substrate.

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The substrate thickness is usuallyabout 500um to maintainmechanical strength.

A long n- region results in a largeresistive component of the diodedrop under forward bias.

The SCL width is approximatelybetween 10 to 200um and most ofit is in the n- region.

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The lower part of the substrate canbe heavily doped to lower theresistive component of the forwardvoltage drop as shown in thediagram.

Performance can be furtherimproved by doping the n- regionso lightly that is nearly intrinsic.

Resulting diode is called a PINdiode.

Almost all power diodes have PINtype of structure.

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Evolution of PIN structure

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Analysis of PN diode is based on theassumption that both sides are in low-levelinjection.

Low level injection means minority carrierconcentration remains small compared tomajority carrier concentration even if thediode may not be in thermal equilibrium.

This is not valid for the i-region of PINdiode. This is in high level injection.

Consequently n is nearly equal to p in theneutral region.

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High level injection is also called

“Conductivity Modulation”.

Under conductivity modulation, theconductivity of the material is no longerdetermined by the majority doping level;it is now a function of injection level.

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One dimensional PIN structure15

In PN diode, the minority current flows only bydiffusion.

There is also a drift component of current in theintrinsic region and drift field gives rise to avoltage drop Vi which adds to the on statevoltage drop of the diode.

All the current flowing through the diode resultsfrom recombination within the i-region therefore,Ei & Vi are dependent only on physicalparameters and length of i-region.

Because ni increases rapidly with increasingtemperature, J increases with temperature andthe forward drop decreases with temperature ifthe forward current is fixed.

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For simplicity, let us ignore the SCL capacitanceand focus on the excess stored charge in theneutral regions outside the SCL.

Forward Recovery Let us consider turn-on transient, The diode

current steps from zero to If, the terminal voltagefirst rises to Vfp and then decay to steady statevalue of Vf.

This process is called forward recovery of thediode. Vfp is known as the Forward recoveryvoltage

The duration of forward recovery is called Forwardrecovery time tfr.

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Process As forward current IF flows, holes drift through p+

region and injected into i-region. Similarlyelectrons drift through n+ region and injectedinto i-region.

The carrier concentration builds up with time asindicated below

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At t1 just after the forward currentstarts, the carrier builds up near thejunctions but not in the middle of thei-region.

In most of the i-region at t1, there is nogradient to the carrier profile.

Therefore, the injected carriers flowsacross the i-region not by diffusionbut by drift.

The voltage that results from this driftis very large.

i-region is not in high level injectionat this time so that conductivity is verylow and the region is carrying the fullcurrent IF.

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The high resistance is the source ofthe peak transient voltage Vfp.

As time passes, the carrierconcentration grows in the middle ofthe i-region modulating theconductivity and reducing theresistance.

As a result, the voltage across themiddle region drops.

The change in the middle region atsteady state is proportional to theforward current If.

The duration of forward recoverytransient is called “Forward RecoveryTime”, tfr. 20

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Reverse RecoveryConsider that that PIN diode is

carrying a forward current of IF. At t =0, the diode is connected to reversevoltage VR.

Because the excess charge in the i-region and diffusion regions of thediode can not changeinstantaneously, the p+-i and i-n+

junctions remain forward biased forsome time after t = 0.

As the diode voltage is zero duringthis time, the diode current isnegative.

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Reverse RecoveryThis reverse current adds

the removal of excesscharge, until theconcentrations at the SCLedges become negative andthe junction can begin tosupport a reverse voltage.

This process is calledreverse recovery.

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Following figure shows the carrier profile in the i-region as the reverse recovery current flows.

Just after t = 0, the excess carrier concentrationsat the junction edges are still positive, thereforethe junction voltages are also positive.

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To support the negative current,the excess carrier distributiondevelop a negative slope nearthe junction edges.

Continuing recover current floweliminates the excess carrierconcentrations in the i-region.

When the excess carrierconcentrations at the junctionedges reach zero, theconcentration gradientsdecreases and the diode currentcan no longer be maintained.

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The dynamics of this processproduce an exponential rise of iD tozero and fall of VD to -VR as shownin the Figure.

During the initial phase of reverserecovery, the diode voltage changesonly slightly this is due to thechange in sign of vi.

The duration of reverse current flowin the diode is trr, the reverserecovery time.

Based on the above discussion, trr isdirectly proportional to the reversecurrent IR and initial stored chargein i-region.

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The basic structure of an SCR withdoping profile is given below

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The SCR is a four layer (p+ n- p n+) device.Low rating (10-100A), device is built on a

small die of silicon wafer.High rating (100-4000A), the SCR is built

on an entire wafer.The Cathode is the heavily doped n-

region on the top of the device.The Anode is heavily doped p-region on

the bottom of the device.

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The P-region (p2)under the cathodeis the gate.

The gate of thedevice isconnected to themetal contact onthe top of the die.

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Two gate patterns are shown in the following Figure. This is the top view of the gate geometry. The center-gate geometry is used extensively for

low frequency SCRs. The involute-gate geometry is used for high

frequency SCRs.

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The SCR junctions are labeled as J1, J2, J3, and each ofthe four layers as p1, n1, p2, n2.

The Off State When the SCR is off, it can block a reverse voltage or

a forward voltage. Following diagram shows SCLs when SCR is blocking

Reverse voltage.VAK is negative, J1& J3 are reversebiased.

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The Off StateThe doping on each side of J3 is very heavy,

so breakdown voltage of J3 is relatively low.n1 region is long and lightly doped,

therefore, J1 can block large reverse voltage.The SCL at J1 grows mostly into n1 region.

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The Off StateWhen VAK is positive, J1& J3 are forward

biased and J2 is reverse biased. J2 withstands all the applied voltage.As the n1 region is more lightly doped than

p2 region, the SCL again grows into the n1region.

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The Off StateThe n1 region is used to block both

polarities of voltage when SCR is off.The doping level & length of n1 region must

be chosen to give the desired breakdownvoltage.

The breakdown of n1 can be either due topunch through or due to avalanche.

Most SCRs are designed with n1 longenough to cause the avalanche to be thebreakdown mechanism.

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If VAK is positive, the SCR will block thevoltage when the gate is open.

A momentary gate current can turn ‘on’the SCR and it will remain ‘on’ even if thegate current is made zero.

This latching of the SCR can beunderstood from the two transistormodel.

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Cross-section of the SCR showing its two transistor model derivation.

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The application of a positive voltage atAnode can not turn on the SCR, because thejunction J2 is reverse biased and Blocking.

Base-Collector junctions of both thetransistors are reverse biased and bothtransistors are off.

The collector current of T2 provides the basecurrent for T1.

The collector current of T1 along with gatecurrent supplies the base drive for T2.

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As the two transistor drive each other intosaturation, the excess carrier concentrationsin their base regions reach high levelinjection.

At this point doping concentrations in thebase regions are no longer relevant, and theSCR behaves as a three layer PIN diode, theto two middle layers corresponds to the i-region.

The forward voltage across the i-region isinversely proportional to the recombinationrate.

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Base current of each transistor is β timesits collector current.

The regenerative turn on process can beinitiated, if a short pulse of current isapplied at the gate terminal

As long as the product β1 β2 >1, the twotransistors will drive each other harderand harder until they saturate.

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The SCR does not breakdown in theforward direction, instead it turns on.

This process is known as Breaking overand the voltage at which it occurs iscalled the break over voltage VBO.

The breaking over process starts due toforward leakage current, IA of the SCRwhich must be kept small to save it fromBeak over.

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It can be shown that the SCR leakagecurrent is

------------------------- (1)

To keep the IA small, the loop gain,(α1+α2)<<1

If (α1+α2) = 1, the equation (1) shows thatSCR will enter into sustainedbreakdown.

)(1 21

21

COCOA

III

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The leakage current of the SCR increaseswith temperature, therefore at elevatedtemperature, the thermally generatedleakage current can be sufficient toincrease the SCR loop gain such that turnon occur.

If α2 is made smaller than α1, the reverseand forward breakdown voltages arenearly same.

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The SCR can also turn on by means of highdv/dt across anode and cathode.

The increasing voltage is supported by J2.The associated SCL width increases and a

charging current flows across the anode andcathode junctions, causing hole andelectron injection respectively.

The same mechanism occurs at the cathodewhen gate current is applied; hence if theterminal dv/dt is large enough, SCR turnson.

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A structural modification isused to reduce temperaturesensitivity of the device andto increase the rating byintroducing gate cathodeshorts.

The effect of this short is likeplacing a resistor across thebase-emitter junction of T2.

The cathode electroninjection efficiency iseffectively reduced therebydecreasing α2 which resultsin VBO and dv/dt rating.

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In the forward blocking state, J2leakage current will forward biasbase-emitter of T2.

As this junction voltage rises, thecathode short diverts some of theleakage current of p2 basereducing the current that ismultiplied by the transistor action,in effect the gain of T2 is reduced.

The designer will make 0.7/Rgklarger than the maximum leakagecurrent expected when SCR is inforward blocking state.

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To turn on the SCR, we need T2 to contributeto the regenerative process.

This contribution will not occur until thecurrent flowing through the SCR is 0.7/Rgk.

Because this value of current is usuallyexceeded by Ig, the SCR will turn on by thegate drive.

But if the gate drive is removed, theregenerative process stops and the SCRreturns to its off state.

The Anode current level required for theSCR to remain on when the gate drive isremoved is called the latching current IL.

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Similarly, if the SCR is on and the gate drive has beenremoved, the anode current must fall below than acritical level to turn off the SCR because of the failureof the regenerative process.

The anode current at which this occurs is called theholding current, IH.

Our simple description here suggests that

IL=IH=0.7/Rgk.

However, Rgk is slightly different for the turn-on andturn-off processes owing to the differences in excesscharge concentrations in the p2 region.

For a 100A device, IL and IH are typically in the rangeof 100 to 300mA , with IH < IL.

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An important property of the SCR is thatonce latched on, the gate control is lost.

The SCR can not turned off through gate.SCR turn off can only be achieved by

reducing the anode current externally toa level below which the loop gain issignificantly less than unity.

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