Power Semiconductor Devices UNIT I Power Semiconductor Devices 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Introduction What are Power Semiconductor Devices (PSD)? They are devices used as switches or rectifiers in power electronic circuits What is the difference of PSD and low-power semiconductor device? Large voltage in the off state High current capability in the on state 4/17/2017 Copyright by www.noteshit.com
Classification Fig. 1. The power semiconductor devices family 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Important Parameters Breakdown voltage. On-resistance. Trade-off between breakdown voltage and on-resistance. Rise and fall times for switching between on and off states. Safe-operating area. 4/17/2017 Copyright by www.noteshit.com
Power MOSFET: Structure Power MOSFET has much higher current handling capability in ampere range and drain to source blocking voltage(50-100V) than other MOSFETs. Fig.2.Repetitive pattern of the cells structure in power MOSFET 4/17/2017 Copyright by www.noteshit.com
Power MOSFET: R-V Characteristics An important parameter of a power MOSFET is on resistance: , where Fig. 3. Typical RDS versus ID characteristics of a MOSFET. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Thyristor: Structure Thyristor is a general class of a four-layer pnpn semiconducting device. Fig.4 (a) The basic four-layer pnpn structure. (b) Two two-transistor equivalent circuit. 4/17/2017 Copyright by www.noteshit.com
Thyristor: I-V Characteristics Three States: Reverse Blocking Forward Blocking Forward Conducting Fig.5 The current-voltage characteristics of the pnpn device. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Applications Power semiconductor devices have widespread applications: Automotive Alternator, Regulator, Ignition, stereo tape Entertainment Power supplies, stereo, radio and television Appliance Drill motors, Blenders, Mixers, Air conditioners and Heaters 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Thyristors Most important type of power semiconductor device. Have the highest power handling capability.they have a rating of 1200V / 1500A with switching frequencies ranging from 1KHz to 20KHz. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Is inherently a slow switching device compared to BJT or MOSFET. Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate. 4/17/2017 Copyright by www.noteshit.com
Different types of Thyristors Silicon Controlled Rectifier (SCR). TRIAC. DIAC. Gate Turn-Off Thyristor (GTO). 4/17/2017 Copyright by www.noteshit.com
SCR Symbol of Silicon Controlled Rectifier 4/17/2017 Copyright by www.noteshit.com
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Device Operation Simplified model of a thyristor 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com V-I Characteristics 4/17/2017 Copyright by www.noteshit.com
Effects of gate current 4/17/2017 Copyright by www.noteshit.com
Two Transistor Model of SCR 4/17/2017 Copyright by www.noteshit.com
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Turn-on Characteristics 4/17/2017 Copyright by www.noteshit.com
Turn-off Characteristic 4/17/2017 Copyright by www.noteshit.com
Methods of Thyristor Turn-on Thermal Turn-on. Light. High Voltage. Gate Current. dv/dt. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Thyristor Types Phase-control Thyristors (SCR’s). Fast-switching Thyristors (SCR’s). Gate-turn-off Thyristors (GTOs). Bidirectional triode Thyristors (TRIACs). Reverse-conducting Thyristors (RCTs). 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Static induction Thyristors (SITHs). Light-activated silicon-controlled rectifiers (LASCRs). FET controlled Thyristors (FET-CTHs). MOS controlled Thyristors (MCTs). 4/17/2017 Copyright by www.noteshit.com
Phase Control Thyristor These are converter thyristors. The turn-off time tq is in the order of 50 to 100sec. Used for low switching frequency. Commutation is natural commutation On state voltage drop is 1.15V for a 600V device. 4/17/2017 Copyright by www.noteshit.com
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Fast Switching Thyristors Also called inverter thyristors. Used for high speed switching applications. Turn-off time tq in the range of 5 to 50sec. On-state voltage drop of typically 1.7V for 2200A, 1800V thyristor. High dv/dt and high di/dt rating. 4/17/2017 Copyright by www.noteshit.com
Bidirectional Triode Thyristors (TRIAC) 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Mode-I Operation MT2 Positive, Gate Positive 4/17/2017 Copyright by www.noteshit.com
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Triac Characteristics 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com BJT structure heavily doped ~ 10^15 provides the carriers lightly doped ~ 10^8 lightly doped ~ 10^6 note: this is a current of electrons (npn case) and so the conventional current flows from collector to emitter. Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current. Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so the electron proceed to the collector where they are again the majority carrier. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com BJT characteristics Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current. Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so the electron proceed to the collector where they are again the majority carrier. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com BJT characteristics Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current. Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so the electron proceed to the collector where they are again the majority carrier. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com BJT modes of operation Mode EBJ CBJ Cutoff Reverse Forward active Forward Reverse active Saturation Remember that the base region is deliberately made very thin and lightly doped, while the emitter is made much more heavily doped. Because of that, applying a forward bias to the emitter-base junction causes vast majority carriers to be injected into the base, and straight into the reverse-biased collector-base junction. Those carriers are actually minority carriers in the base region, because that region is of opposite semiconductor type to the emitter. To those majority-turned-minority carriers, the collector-base junction depletion region is not a barrier at all but an inviting, accelerating filed; so as soon as they reach the depletion layer, they are immediately swept into the collector region. Forward biasing the emitter-base junction causes two things to happen that might seem surprising at first: Only a relatively small current actually flows between the emitter and the base. much smaller than would flow in a normal PN diode despite the forward bias applied to the junction between them. A much larger current instead flows directly between the emitter and the collector regions, in this case, despite the fact that the collector-base junction is reversed biased. From a practical point of view, the behavior of bipolar transistors means that, unlike the simple PN-junction diode, it is capable of amplification. In effect, a small input current made to flow between the emitter and collector. Only a small voltage--around 0.6 volts for a typical silicon transistor--is needed to produce the small input current required. 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com BJT modes of operation Cutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch. Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations. Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the βf in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region. Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. 4/17/2017 Copyright by www.noteshit.com
BJT structure (active) current of electrons for npn transistor – conventional current flows from collector to emitter. B C E IE IC IB - + VBE VCB VCE 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com MOSFET NMOS: N-channel Metal Oxide Semiconductor W W = channel width L L = channel length GATE oxide insulator n “Metal” (heavily doped poly-Si) n p-type silicon DRAIN SOURCE A GATE electrode is placed above (electrically insulated from) the silicon surface, and is used to control the resistance between the SOURCE and DRAIN regions 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com N-channel MOSFET Gate IG Drain Source IS oxide insulator gate ID n n p Without a gate-to-source voltage applied, no current can flow between the source and drain regions. Above a certain gate-to-source voltage (threshold voltage VT), a conducting layer of mobile electrons is formed at the Si surface beneath the oxide. These electrons can carry current between the source and drain. 4/17/2017 Copyright by www.noteshit.com
N-channel vs. P-channel MOSFETs NMOS PMOS p-type Si n+ poly-Si n-type Si p+ poly-Si n+ n+ p+ p+ For current to flow, VGS > VT Enhancement mode: VT > 0 Depletion mode: VT < 0 Transistor is ON when VG=0V For current to flow, VGS < VT Enhancement mode: VT < 0 Depletion mode: VT > 0 Transistor is ON when VG=0V (“n+” denotes very heavily doped n-type material; “p+” denotes very heavily doped p-type material) 4/17/2017 Copyright by www.noteshit.com
MOSFET Circuit Symbols G G NMOS p-type Si n+ poly-Si n+ n+ S S Body G G PMOS n-type Si p+ poly-Si p+ p+ S S Body 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com MOSFET Terminals The voltage applied to the GATE terminal determines whether current can flow between the SOURCE & DRAIN terminals. For an n-channel MOSFET, the SOURCE is biased at a lower potential (often 0 V) than the DRAIN (Electrons flow from SOURCE to DRAIN when VG > VT) For a p-channel MOSFET, the SOURCE is biased at a higher potential (often the supply voltage VDD) than the DRAIN (Holes flow from SOURCE to DRAIN when VG < VT ) The BODY terminal is usually connected to a fixed potential. For an n-channel MOSFET, the BODY is connected to 0 V For a p-channel MOSFET, the BODY is connected to VDD 4/17/2017 Copyright by www.noteshit.com
NMOSFET IG vs. VGS Characteristic Consider the current IG (flowing into G) versus VGS : IG G + S D VDS oxide semiconductor + VGS IG The gate is insulated from the semiconductor, so there is no significant steady gate current. always zero! VGS 4/17/2017 Copyright by www.noteshit.com
NMOSFET ID vs. VDS Characteristics Next consider ID (flowing into D) versus VDS, as VGS is varied: G ID + S D VDS oxide semiconductor + VGS ID Above threshold (VGS > VT): “inversion layer” of electrons appears, so conduction between S and D is possible VGS > VT zero if VGS < VT VDS Below “threshold” (VGS < VT): no charge no conduction 4/17/2017 Copyright by www.noteshit.com
The MOSFET as a Controlled Resistor The MOSFET behaves as a resistor when VDS is low: Drain current ID increases linearly with VDS Resistance RDS between SOURCE & DRAIN depends on VGS RDS is lowered as VGS increases above VT NMOSFET Example: oxide thickness tox ID VGS = 2 V VGS = 1 V > VT VDS Inversion charge density Qi(x) = -Cox[VGS-VT-V(x)] where Cox eox / tox IDS = 0 if VGS < VT 4/17/2017 Copyright by www.noteshit.com
ID vs. VDS Characteristics The MOSFET ID-VDS curve consists of two regions: 1) Resistive or “Triode” Region: 0 < VDS < VGS VT 2) Saturation Region: VDS > VGS VT process transconductance parameter “CUTOFF” region: VG < VT 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com The Evolution Of IGBT Part I: Bipolar Power Transistors Bipolar Power Transistor Uses Vertical Structure For Maximizing Cross Sectional Area Rather Than Using Planar Structure 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com The Evolution Of IGBT Part II: Power MOSFET Power MOSFET Uses Vertical Channel Structure Versus The Lateral Channel Devices Used In IC Technology 4/17/2017 Copyright by www.noteshit.com
Lateral MOSFET structure 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com The Evolution Of IGBT Part III: BJT(discrete) + Power MOSFET(discrete) Discrete BJT + Discrete Power MOSFET In Darlington Configuration 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com The Evolution Of IGBT Part IV: BJT(physics) + Power MOSFET(physics) = IGBT More Powerful And Innovative Approach Is To Combine Physics Of BJT With The Physics Of MOSFET Within Same Semiconductor Region This Approach Is Also Termed Functional Integration Of MOS And Bipolar Physics Using This Concept, The Insulated Gate Bipolar Transistor (IGBT) Emerged Superior On-State Characteristics, Reasonable Switching Speed And Excellent Safe Operating Area 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com The Evolution Of IGBT Part IV: BJT(physics) + Power MOSFET(physics) = IGBT IGBT Fabricated Using Vertical Channels (Similar To Both The Power BJT And MOSFET) 4/17/2017 Copyright by www.noteshit.com
Copyright by www.noteshit.com Device Operation Operation Of IGBT Can Be Considered Like A PNP Transistor With Base Drive Current Supplied By The MOSFET 4/17/2017 Copyright by www.noteshit.com
DRIVER CIRCUIT (BASE / GATE) Interface between control (low power electronics) and (high power) switch. Functions: – amplifies control signal to a level required to drive power switch – provides electrical isolation between power switch and logic level Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive. 4/17/2017 Copyright by www.noteshit.com
ELECTRICAL ISOLATION FOR DRIVERS Isolation is required to prevent damages on the high power switch to propagate back to low power electronics. Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used. 4/17/2017 Copyright by www.noteshit.com
ELECTRICAL ISOLATION FOR DRIVERS Power semiconductor devices can be categorized into 3 types based on their control input requirements: Current-driven devices – BJTs, MDs, GTOs Voltage-driven devices – MOSFETs, IGBTs, MCTs Pulse-driven devices – SCRs, TRIACs 4/17/2017 Copyright by www.noteshit.com
CURRENT DRIVEN DEVICES (BJT) Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current. The main problem with this circuit is the slow turn-off time. Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from Hewlett-Packard uses opto-coupling isolation. 4/17/2017 Copyright by www.noteshit.com
ELECTRICALLY ISOLATED DRIVE CIRCUITS 4/17/2017 Copyright by www.noteshit.com
EXAMPLE: SIMPLE MOSFET GATE DRIVER Note: MOSFET requires VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1. When B1 is high, Q1 conducts. VGS is pulled to ground. MOSFET is off. When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on. 4/17/2017 Copyright by www.noteshit.com