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BJTs:

The current conduction in bipolar transistor is because of the types of (majority and minoritycharge carriers), holes and electrons. Hence this is called Bipolar junction transistor, thereforreferred to as BJT.

The BJTs are of two types:

• n-p-n type

• P-n-p type

Construction

When a transistor is formed by sandwiching a single p-region between two n-regions, as shownin the Fig (a), it is an n-p-n type transistor. The p-n-p type transistor has a single n-regionbetween two p-regions, as shown in Fig (b).

The middle region of each transistor type is called the base of the transistor. This region is verythin and lightly doped. The remaining two regions are called emitter and collector. The emitterand collector are heavily doped. But the doping level in emitter is slightly greater than that ofcollector and the collector region-area is slightly more than that of emitter.

Fig (a) and (b) shows the symbols of n-p-n and p-n-p transistors. Standard transistor symbols.

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A transistor has two p-n junctions. One junction is between the emitter and the base, and iscalled the emitter base junction, or simply the emitter junction jE. The other junction isbetween the base and the collector, and is called collector-base junction, or simply collectorjunction Jc. Thus transistor is like two pn junction diodes connected back-to-back as shown inthe Fig. (a) and (b).

Working and Operation of n-p-n Transistor:

The base to emitter junction is forward biased by the dc source VEE. Thus, the depletion regionat this junction is reduced. The collector to base junction is reverse biased, increasing depletionregion at collector to base junction as shown in Fig 1.

The forward biased EB junction causes the electrons in the n-type emitter to flow towards thebase. This constitutes the emitter current lE. As these electrons flow through the p-type base,they tend to combine with holes in p-region (base). We know that, the base region is very thinand lightly doped. The light doping means that the free electrons have a long lifetime in the

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base region. The very thin base region means that the free electrons have only a short distanceto go to reach the collector. For these two reasons, very few of the electrons injected into thebase from the emitter recombine with holes to constitute base current, lB (Refer Fig 2) and theremaining large number of electrons cross the base region and move through the collectorregion to the positive terminal of the external d.c source as shown in Fig 3.

This constitutes collector current Ic. Thus the electron flow constitutes the dominant current inan n-p-n transistor. Since, the most of the electrons from emitter flow in the collector circuitand very few combine with holes in the base. Thus, the collector current is larger than the basecurrent.

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Characteristic Curve of BJT:

Fig shows the V-I characteristics of BJT. These characteristics are also called outputcharacteristics. The collector current (ic ) is plotted with respect to collector emitter voltage(VCE ) for different values of base current (iB )

There are four regions: Cut-off region, Active region, quasi-saturation and hard saturation.

Cut-off region: The cut-off region is the area where base current is almost zero. Hence nocollector current flows and transistor is 'off'.

Quasi-saturation: In the quasi-saturation and hard saturation, the base drive is applied andtransistor is said to be 'on'. Hence collector current flows depending upon the load. The BJT isnever operated in the active region (i.e. as an amplifier). It is operated in cut-off and saturation.Thus BJT acts as a switch. The 'BVCBO' is the maximum collector to emitter voltage that can besustained when BJT is carrying substantial collector current. BVCEO is the maximum collector toemitter breakdown voltage that can be sustained when base current is zero (i.e. base opencircuited). And BVCBO is the collector base breakdown voltage when the emitter is opencircuited.

Primary breakdown: The primary breakdown in BJT takes place because of avalanchebreakdown of the collector base junction. The large power dissipation normally leads toprimary breakdown.

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Second breakdown: It is clear front Fig that, at the large collector currents, the collectoremitter voltage drops. Due to this drop in voltage, the collector current increases. Here there issubstantial increase in power dissipation. This power dissipation is not evenly spread across theentire volume of the device. But it is concentrated in the highly localized regions. In theseregions the local temperature grows very rapidly and the BIT is damaged.

MOSFETs:

The metal oxide semiconductor field effecttransistors (MOSFET) are majority carrierdevices. Fig shows the symbols ofMOSFETs.

Observe that there are two types of powerMOSFETs: n-channel MOSFET and p-channel MOSFET. The MOSFET has threeterminals gate (G), drain (D) and source (S).When the MOSFET is turned 'on' thecurrent flows from drain to source. The voltage is applied between gate-source to turn 'on' theMOSFET.

Construction and operation of MOSFETS

MOSFETs can be n-channel or p-channel. Fig. 1 shows the structure of n-channel enhancementtype MOSFET. The source and drain are connected to n+ regions. These regions are heavilydoped with the intensity of 10^19 per cm^3. The p-type body region forms the channel

between drain and source.

The body region has the doping level of 10^16 per cm^3. The gate is not directly connected tothe p-type region. There is insulating oxide (Si02) layer between gate metal and p-type layer.

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When gate is made positive with respect to source, an accumulation layer is formed in thechannel as shown in Fig 2. This accumulation layer is formed because of VGs. The gate terminal(metal) is positive. The other side of oxide layer is p-type of body region. Accumulation layer ofelectrons is generated in the body region near oxide layer. This is also called induced channel ofregion. There is insulating oxide (Si02) layer between gate metal and p-type layer. When gate ismade positive with respect to source, an accumulation layer is formed in the channel as shownin Fig 2. This accumulation layer is formed because of VGs. The gate terminal (metal) is positive.The other side of oxide layer is p-type of body region. Accumulation layer of electrons isgenerated in the body region near oxide layer. This is also called induced channel of electrons.Therefore current (fps) starts flowing through this induced channel.

The current flows from drain to source. If VGS = 0, then induced channel is absent and nocurrent flows. Since channel is made of electrons, this is called n-channel MOSFET.

V-I characteristics of MOSFET

Fig shows the V-1 characteristics of n-channel power MOSFET. The drain current iD is plotted

with respect to drain to source voltage vDs. These characteristics are plotted for various valuesof gate source voltages (VGS). In Fig observe that there are three regions in the characteristics:Ohmic region, active region and cut-off region.

In the cut-off region, the drain current is negligible and the MOSFET is said to be in 'OFF' state.The MOSFET is driven in cut-off region by applying VGS < VGS(th) . VGS(th)is the threshold gatesource voltage. When gate to source voltage is less than threshold gate source voltage, MOSFETis off, i.e. in cut-off region. The MOSFET is driven into ohmic region when VGS >>VGS (th). In the

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ohmic region, the MOSFET conducts heavily. It is said to be 'on' in the ohmic region. Thus byapplying heavy gate to source voltage, MOSFET can be turned on. In the power electronicapplications, MOSFET is never operated in the active region. In active region it acts as anamplifier. For switching applications, MOSFET is operated only in ohmic and cut-off regions. TheBVDss is the drain to source breakdown voltage, when the gate is open circuited. The MOSFET isdamaged if drain to source voltage is increased above BVDss.

Insulated Gate Bipolar Transistor (IGBT)

The Insulated Gate Bipolar Transistor (IGBT) is the latest devicein power electronics. It is obtained by combining the propertiesof BJT and MOSFIT.

The IGBT has three terminals: Gate (G), collector (C) and emitter(E). Current flows from collector to emitter whenever a voltagebetween gate and emitter is applied. The IGBT is said to haveturned 'on'. When gate emitter voltage is removed IGBT tums-off. Thus gate has full control over the conduction of IGBT.

Construction and Working

The structure of IGBT is similar to that of MOSFET. Fig shows the vertical cross-section of IGBT.In this structure observe that there is additional r layer. This layer is collector (Drain) of IGBT.

This p+ injecting layer is heavily doped. It has the doping intensity of 10^19per cm3. The dopingof other layers is similar to that of MOSFET. n+ layers have 10^19 per cm3.The p-type body

region has doping level of 10^16 per cm 3 . The n- drift region is lightly doped (10^14 percm3).When VGs > VGs(th), then the channel of electrons is formed beneath the gate as shown

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in Fig. These electrons attract holes from p+ layer. Hence, holes are injected from p+ layer inton- drift region. Thus hole/electron current starts flowing from collector to emitter. When holesenter p-type body region, they attract more electrons from n+ layer. This action is exactlysimilar to MOSFET.

V-I characteristics of n-channel IGBT

The V-I characteristics of n-channel IGBT. Sometimethe collector is also called drain and emitter is alsocalled source. The characteristics are plotted for drain(collector) current id with respect to drain source(collector emitter) voltage VDs. The characteristics areplotted for different values of gate to source (VGs)voltages. When the gate to source voltage is greaterthan the threshold voltage VGS(th) then IGBT turns-on. The IGBT is off when VGs is less than VGs(th). Figshows the 'on' and 'off' regions of IGBT. The BVDsss isthe breakdown drain to source voltage when gate isopen circuited. The IGBT is the popular device now-a-days. IGBT has simplest drive circuit and it has low on-state losses.

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Power losses:

BJT MOSFET IGBTLosses are low. Losses are hight than BJTs. On state losses are reduced.

Testing, Rating, Frequency Response and Power handling capabilities:

Parameters BJTs MOSFETs IGBTsTriggered i.e. latch-ing or linear

Linear trigger Linear trigger Linear trigger

Type of carriers indevice

Bipolar device Majority CarrierDevice

Majority CarrierDevice

Control of gate Orbase

Base has full control Full Full

On-state drop < 2 volts 4-6v 3.3vSwitching frequency 10 kHz Current

Polarized NegativeUpto 100khz 20khz

Gate drive Current Voltage Voltage

Snubber Polarized+ Not essential Not essential

Temperaturecoefficient

-ve +ve +ve at hightemperature

Voltage and Currentratings

2 kV/1 kA 1 kV/50 1.5 kV/400 kA

Voltage blockingcapability

Asymmetric Asymmetric Asymmetric

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Protection:

Driver circuits for BJTs protection

The drive from control circuit is given to driver circuit through optocoupler.

• Overcurrent Protection: The signal is amplified, buffered by the comparator and then given toovercurrent protection circuit. The collector voltage of main transistor is sensed through diodeD. If the collector current increases, then collector voltage will rise. This rise is sensed byovercurrent protection circuit to disable the drive of BJT.

The output from overcurrent protection circuit is given to pair of pnp-npn transistors T1and T2. These transistors provide the required base current to power BJT to drive it insaturation.

Transistor T1 provides large positive voltage and current to drive BIT in saturation ofquasi-saturation. and transistor T2 provides large negative voltage and current forfaster turn-off of BJT.

The base-emitter wires of power BJT are twisted to minimize stray inductance.

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Driver circuits for MOSFET protection

The gate drive is applied across the terminals a-b.Initially the resistance R1 is bypassed by C1 and fulldrive voltage is applied to the gate. This charges thegate-source capacitance quickly. As the capacitor C,charges, the gate current reduces. Once the MOSFETis turned on required gate current is very small.

When MOSFET is to be turned off, the voltage vab ismade zero. This applies capacitor voltage acrossgate-source in negative direction. Therefore chargeon the gate-source capacitance is removed quickly.C1 then discharges through R1. The resistance R2provides additional discharge path for gate-source capacitance.

Driver Circuit for IBGT

Driver circuit for IGBT which uses IR 2125 Ic. IR 2125 is the high voltage, fast switchingMOS gate driver with single floating gate driver channel. This IC can be used to drive N-channel power MOSFET or IGBT. Overcurrent flowing through the IGBT is detectedthrough Rs and Cs terminal of the IC.

The error pin of the IC indicates fault conditions.

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References:

1. Construction Working and Operation of n-p-n Transistor, Characteristic Curve of BJT,CH06(BasicElectronics Engineering By U.A.Bakshi, A.P.Godse)

2. Characteristic Curve of BJT, Construction and Operation, Characteristic Curve of MOSFETS and IGBTS,Power losses, Testing, Rating, Frequency Response and Power handling capabilities CH01 (PowerDevices And Machines By Dr.J.S.Chitode, U.A.Bakshi)

By Ashakoor

Ciit Atd

BEE