Power Electronic Devices

1. Power Supply Systems Electrical Energy Conversion and Power Systems Universidad de Oviedo Power Electronic Devices Semester 1 Lecturer: Javier Sebastián

2. Outline • Review of the physical principles of operation of semiconductor devices. • Thermal management in power semiconductor devices. • Power diodes. • Power MOSFETs. • The IGBT. • High-power, low-frequency semiconductor devices (thyristors).

3. Electrical Energy Conversion and Power Systems Universidad de Oviedo Lesson 5 – The Insulated Gate Bipolar Transistor (IGBT). Semester 1 - Power Electronic Devices

4. Outline • The main topics to be addressed in this lesson are the following: • Introduction. • Review of the basic structure and operation of bipolar junction transistors (BJTs). • Internal structures of IGBTs. • Static characteristics of the IGBTs. • Dynamic characteristics of the IGBTs. • Losses in the IGBTs.

5. Introduction (I). • Power MOSFETs are excellent power devices to be used in power converters up to a few kWs. • They have good switching characteristics because they are unipolar devices. • This means that the current is due to majority carriers exclusively and that it does not pass through any PN junction. • Due to this, conductivity modulation does not take place. • This fact limits the use of these devices for high power applications, because high-voltage devices exhibit high RDS(ON) values. • The challenge is to have a device almost as fast as a MOSFET, as easy to control as a MOSFET, but with conductivity modulation. Source Channel N+ - + P N- N+ Gate Drain Drain Current

6. B E SiO2 Introduction (II). N+ P- • On the other hand, Bipolar Junction Transistors (BJTs) are devices in which the current passes through two PN junctions. • Although the current is due to the emitter majority carriers, these carriers are minority carriers in the base. Therefore, the switching process strongly depends on the minority base carriers. • Due to this, BJTs (bipolar devices) are slower than MOSFETs (unipolar devices). • Moreover, the control current (base current) is quite high (only 5 -20 times lower than the collector current) in power BJTs. • However, as the collector current in BJTs passes through two PN junctions, they can be designed to have conductivity modulation. • As a consequence, BJTs have superior characteristics in on-state than MOSFETs. N- N+ Base Current C Collector Current

7. Introduction (III). • Summary of a comparison between BJTs and MOSFETs • Could we have the advantages of both types of devices together in a different device? • The answer is that we can design a different device with almost all the advantages of both BJTs and MOSFETs for medium and high voltage (from several hundreds of volts to several thousand of volts). • This device is the IGBT (the Insulated Gate Bipolar Transistor). • To understand its operation, we must review the structure and operation of the BJT.

8. Collector (P) Collector(N) Base (N) Base(P) PNP Emitter (P) NPN Emitter (N) Review of the basics of BJTs (I). • PNP transistor:Two P-type regions and a N-type region • NPN transistor:Two N-type regions and a P-type region • Conditions for such device to be a transistor: • The emitter region must be much more doped than the base region. • The base region must be a narrow region (narrower than the diffusion length corresponding to the base minority carrier).

9. Review of the basics of BJTs (II). • Example: a PNP-type silicon low-power transistor (the actual geometry is quite different) N- P P+ Base Collector Emitter 1m • The emitter region must be much more doped than the base region. NDB=1013 atm/cm3 NAE=1015 atm/cm3 • The base region must be narrower than the diffusion length corresponding to the holes in the base region. WB = 1 mm << Lp = 10 mm

10. V2 V1 C B E - + N- P+ P - + WB x 0+ 0- WB- WB+ Review of the basics of BJTs (III). • Operation in active region: E-B junction is forward biased and B-C junction is reverse biased. • The concentration of minority carries when the junctions have been biased can be easily deduced form slide #80, Lesson 1. High gradient ÞHigh current due to holes in the B-C junction High gradient ÞHigh current due to holes in the E-B junction Low gradient Þlow reverse current due to electrons in the B-C junction Low gradient Þlow forward current due to electrons in the E-B junction Electrons in the emitter Electrons in the collector Holes in the base

11. Minority carrier concentration Linear scale iE -iC pB3 pB2 C E -iB pB1 nE nC VEB V2 B 0 Base contact Currents iE1 iE3 -iC3 iE2 -iC2 -iC1 0 Review of the basics of BJTs (IV). • Currents passing through the transistor in active region. VEB1 < VEB2 < VEB3 iE»IS·evEB/VT • -iC»iE·a(a » 0.98-0.995) • iC » b·iB (b » 20-200)

12. pB (active) nE (active) -iC iE V1 nC V2 Base contact Currents IE (active) -IC (active) IE (cut-off) -IC (cut-off) Review of the basics of BJTs (V). • Operation in cut-off region: E-B and B-C junctions are reverse biased. • Minority carrier concentration • Linear scale nE (cut-off) pB (cut-off) Active region 0 iE -iC V1 V2 Cut-off region 0

13. pB(saturation) nC (sat.) -iC -iC iE iE V1 V1 V2 V2 nE pB(active) -iC (active) iE (active) Review of the basics of BJTs (VI). • Operation in saturation region: E-B and B-C junctions forward biased. • Minority carrier concentration Linear scale nC (active) Active region Saturation region 0 • Currents • Base contact iE (saturation) -iC (saturation) • However, the operation in saturation usually takes place in other type of circuits. 0

14. -iC • Minority carrier concentration Linear scale pB (sat.) pB(boundary) R + vCB As the collector current is approximately constant, these concentration profiles have the same slope. nC nE - pB (active) 0 -iB V2 • Currents V1 iE (boundary) -iC (active) iE (active) -iC (boundary) iE 0 Review of the basics of BJTs (VII). • Usual circuit to study the saturation region. • We are going to increase the value of V1. • The transistor will be in active region while vCB< 0. When vCB > 0, it is in saturation. iE (saturation) -iC (saturation) V2/R

15. -iC pB (sat.) R + vCB nC nE - pB(bound.) -iB V2 V1 iE Review of the basics of BJTs (VIII). • Very important!!! • We can increase the height of point pB1 as much as we want, because we can increase V1 indefinitely. • However, the collector current (» emitter current) is limited to the maximum possible value of V2/R (otherwise, the transistor would behave as a power generator, which means that energy is generated from nothing). • As the current passing through the transistor (from emitter to collector) is limited, then the slope of pB is also limited. • As a consequence, pB2 must also increase to maintain the current constant, which implies that the base-collector junction becomes forward bias. The transistor becomes saturated. • Minority carrier concentration pB1 Linear scale Not possible pB1 pB2 pB2 0

16. iC Output curves Voltage and current references iC[mA] + iB iB=-400A vCE + -40 iB=-300A vBE - - iB=-200A -20 iB=-100A Saturation vCE [V] iB=0A -6 0 -4 -2 Active Cut-off Review of the basics of BJTs (IX). • Output characteristic curves.

17. -iC N P N iC R iB iE -iE V2 Review of the basics of BJTs (X). • The on-state of bipolar transistors is quite good, because the voltage drop between collector and emitter is quite low. • However, the turn-off is quite slow (next slide). R - + 0.5 V 0.5 V P + - - + 0.2 V 0.2 V -iB N + - P 0.7 V 0.7 V V2

18. Concentration pB (sat.) nE nC pBCut-off 0 Transistor in saturation N- P+ P These excesscarriers (holes in this case) must be eliminated to turn-off the transistor Transistor in cut-off Review of the basics of BJTs (XI). • The longest time in the switching process of a bipolar transistor is the one corresponding to eliminating the excess of minority carriers in the base region when the transistor turns-off.

19. -iC Concentration pB(boundary) nE nC pBCut-off 0 N- P+ P iE Review of the basics of BJTs (XII). • A good trade-off between switching speed and voltage drop in on-state can be reached using anti-saturation circuitry (circuits to maintain the transistor just in the boundary between active region and saturation). • Excess carriers tobe eliminated when the transistor turns-off (lower than in saturation). R + 0 V P - - 0.7 V 0.7 V -iB N + Voltages just in the boundary between active region and saturation P V2

20. Review of the basics of BJTs (XIII). • Hard-saturation circuits • (the voltage across the transistor terminals is the same). R R 0.7 V P + + + 0.7 V V2 0.2 V 0.2 V N 0.2 V P P - -iB -iB P N - - - N 0.5V + - -iB R V2 V2 P P 0.5V 0.5V +

21. Review of the basics of BJTs (XIV). • Soft-saturation circuit • (anti-saturation circuit). R R + 0.7 V + + P 0.7 V 0.7 V - -iB N + 0.7 V - - P P - S1 -iB N • In soft-saturation (boundary), when S1 closed. • In cut-off, when S1 open. V2 V2 P

22. SiO2 B E Review of the basics of BJTs (XV). P N+ N- • As a bipolar transistor is a “bipolar device”, conductivity modulation can take place if the transistor is properly designed. N+ C N+ P+ N- Drift region Structure needed to have conductivity modulation (from slide #100, Lesson 1)

23. Principle of operation and structure of the IGBT (I). • The IGBT (the Insulated Gate Bipolar Transistor) is based on a structure that allows: • Conductivity modulation (good behaviour for high voltage devices when they are in on-state). • Anti-saturation (not so slow switching process as in the case of complete saturation). • And control from an insulated gate (as in the case of a MOSFET). R R D P G P N S N P S1 V2 P V2

24. Collector (C) Gate (G) Emitter (E) Principle of operation and structure of the IGBT (II). Collector P E N B P C Gate • Schematic symbol for a N-channel IGBT. Emitter D • Simplified equivalent circuit for an IGBT. G S • Another schematic symbol also used.

25. Principle of operation and structure of the IGBT (III). • Internal structure (I). Emitter (E) Gate (G) Emitter Gate Emitter Gate Collector (C) N+ N+ N- P N+ Collector P+ Collector 25

26. Principle of operation and structure of the IGBT (IV). • Internal structure (II). Emitter Gate Emitter Gate Gate Rdrift Emitter • Simplest model for an IGBT. N+ N+ P N- Rdrift N+ • Model taking into account the drift region resistance. Collector Collector P+ Collector 26

27. Principle of operation and structure of the IGBT (V). • The IGBT blocking (withstanding) voltage. N+ N+ Emitter Depletion region Gate Emitter Gate Rdrift V2 V2 P N- N+ R R P+ Collector Collector 27

28. Principle of operation and structure of the IGBT (VI). • The IGBT conducting current (a first approach). N+ N+ Conductivity modulation Emitter Transistor effect V1 Gate V1 Emitter Gate Rdrift V2 V2 P N- N+ R R P+ Collector Collector Rdrift 28

29. Principle of operation and structure of the IGBT (VII). N+ N+ N+ • A more accurate model. • However, there is another parasitic transistor. Gate Emitter Rbody Emitter P Gate Rdrift Rbody N- P N- N+ N+ P+ P+ Collector • Model taking into account the MOSFET-body resistance. Collector Rdrift • Model taking into account the parasitic NPN transistor. 29

30. Principle of operation and structure of the IGBT (VIII). N+ Emitter Gate Emitter Gate Rbody P Rdrift Rbody N- Rdrift N+ P+ Collector Collector • Model taking into account the parasitic NPN transistor. • The final result is that there is a parasitic thyristor. 30

31. Principle of operation and structure of the IGBT (IX). • The basics of the thyristor: the PNPN structure (I). N+ Gate Emitter Rbody N+ E1 E1 N+ B1 P P B1 P P C2 C2 N- C1 N N N C1 B2 B2 N+ E2 P+ E2 P+ P+ Collector 31

32. Principle of operation and structure of the IGBT (X). • The basics of the thyristor: the PNPN structure (II). Forward bias N+ N+ Reverse bias - P E1 + P B1 VDC - C2 C1 Forward bias N + B2 N E2 - P+ P+ R • There are two junctions forward biased and one is reverse biased. • As a consequence, the PNPN device can block (withstand) voltage without conducting current. + • However, it will be able to conduct current as well, as it is going to be shown in the next slide. 32

33. Principle of operation and structure of the IGBT (XI). • The basics of the thyristor: the PNPN structure (III). Forward bias Forward bias VB Reverse bias - - N+ P N+ + + P iR N N VDC - P + VDC Forward bias!! - + P Forward bias + N - Forward bias + - N - • If VB is high enough (0.6-0.7 V in a silicon device), then the NPN transistor becomes saturated. • As a consequence, the base-collector junctions corresponding to both the NPN and the PNP transistor become forward biased. Both transistors are saturated. • Therefore, all the junctions are forward biased right now and the voltage across the device is quite low (e.g., 0.9-1.2 V). The current passing through R can be quite high (approximately VDC/R). + - - P+ P+ R R + + 33

34. Principle of operation and structure of the IGBT (XII). • The basics of the thyristor: the PNPN structure (IV). VB VB iB Q1 + Q1 + • iB_1 - - + + N+ N+ - - • iC_2 - - P P • iB_1=iC_2 • iC_1=iB_2 • iC_1=iB_2 + + Q2 Q2 N N VDC VDC iR iR P P N N • Initially, the current needed for transistor Q1 to start conducting (active region) comes from the voltage source VB. • When iC_1 increases, iC_2 strongly increases because iC_2 = b2·iB_2 = b2·iC_1. Therefore, current iB_1 will increase a lot due to the iC_2 component and the transistors become saturated very fast. • As iC_2 is the main current needed to maintain both transistors saturated, the situation does not change if we remove VB. - - P+ P+ R R + + 34

35. Principle of operation and structure of the IGBT (XIII). • The basics of the thyristor: the PNPN structure (V). • A PNPN structure has two different stable states (so, it works as a flip-flop): - Forward bias • As a open-circuit (IR = 0). • As a short-circuit (IR » VDC/R). + N+ N+ VDC P P Q1 Q1 - + Reverse bias - Forward bias - + N + N - P P + - • iB_1=iC_2 R + Q2 Q2 - N N VDC • iC_1=iB_2 Forward bias + P+ P+ iR = 0 iR»VDC/R - R + • The device state at a specific moment depends on whether Q1 emitter-base junction has been forward biased previously. • The only way to turn-off the device is by decreasing IR up to zero. 35

36. Principle of operation and structure of the IGBT (XIV). • The IGBT conducting current (actual paths). N+ - Gate - Emitter N+ P Q1 Channel + + N+ Rbody N Rbody P P P N Q2 Rbody N- N P+ N+ P+ P+ BJT current BJT current MOSFET current BJT current Collector • The voltage across Rbody must not be high enough to turn-on the PNPN structure, which is called latch-up. • Else, the total device cannot be turned-off by the gate voltage any more. 36

37. Principle of operation and structure of the IGBT (XV). • To avoid the IGBT latch-up, Rbody must be as low as possible. N+ N+ Gate Emitter Emitter Gate Channel Channel P P+ P N- Rbody N- N+ N+ P+ P+ Collector BJT current BJT current Collector • The new P+ region decreases Rbody, thus increasing the value of the current needed to reach the voltage drop on Rbody corresponding to latch-up. 37

38. D G S Principle of operation and structure of the IGBT (XVI). • The IGBT cannot conduct reverse current when vGE = 0 (it is not as the MOSFET). P P N N E E Parasitic diode P P G G External diode Reverse current Reverse current Reverse current • This means that it is able to block reverse voltage. • Symmetrical IGBTs are especially designed for blocking reverse voltage. However, they have worse forward voltage drop than asymmetrical (standard) IGBTs. • To conduct reverse current when vGE = 0, an external diode must be added. C C 38

39. Principle of operation and structure of the IGBT (XVII). • Asymmetrical versus symmetrical IGBT structures. N+ N+ Emitter Emitter Gate Gate P P P+ P+ N- N- N+ P+ P+ Collector Collector • Asymmetrical IGBT • (also called punch-through IGBT). • Symmetrical IGBT • (non-punch-through IGBT). 39

40. iD [A] 6 vGS= 6V vGS= 10V 4 vGS= 8V vGS= 5V 2 vGS= 4V 0 vDS [V] 4 2 Static output characteristic curves of a IGBT. + vEB_BJT - C iC [A] vGE= 10V 6 vGE= 8V vGE= 6V 4 vGS< VGS(TO) = 3V vGE= 5V G E 2 vEB_BJT vGE= 4V • Static output characteristic curve of a IGBT. • It can be easily obtained from the MOSFET characteristic curve by adding the voltage drop vEB_BJTcorresponding to the emitter-to-base junction of the BJT part of the IGBT. vGE< VGE(th) = 3V 0 vCE [V] 4 2 • Static output characteristic curve of a MOSFET. • It is also the one corresponding to the MOSFET part of a IGBT. 40

41. General characteristics of the IGBTs (I). • We will use a specific IGBT to address the general IGBT characteristics. 41

42. General characteristics of the IGBTs (II). • General information regarding the IRG4PC50W. 42

43. Static characteristics of the IGBTs (I). 43

44. Static characteristics of the IGBTs (II). IC_max @ T = 50 oC: 55 A IC_max @ T = 75 oC: 48 A 44

45. Static characteristics of the IGBTs (III). • Asymmetrical IGBT 45

46. Static characteristics of the IGBTs (IV). • Static output characteristic curve for a given vGE voltage. iC [A] vGE= 15V 6 4 2 vEB_BJT 0 vCE [V] 4 2 • As in slide #40 of this lesson. vEB_BJT» 1V 46

47. Static characteristics of the IGBTs (V). Thermal behaviour like a BJT Thermal behaviour like a MOSFET 47

48. iC IL A B + vCE + vGE - - Dynamic characteristics of the IGBTs (I). • Turn-off in a IGBT with inductive load and ideal diode • (see slide #32, lesson 4). C vGE vGE(th) G E • iC MOSFET-part turn-off BJT-part turn-off IGBT tail RG VDC vCE C VG G V’G E 48

49. Dynamic characteristics of the IGBTs (II). • Comparing IGBT and MOSFET Turn-off. C • IGBT turn-off vGE • MOSFET turn-off vGE(th) G E vGS • iC vDS(TO) MOSFET-part turn-off • iD BJT-part turn-off D IGBT tail G vCE vDS S Period with switching losses Switching losses 49

50. iC IL A B + vCE + vGE - - Dynamic characteristics of the IGBTs (III). • Turn-on in a IGBT with inductive load and ideal diode • (see slides #32-39, lesson 4, for comparison). C vGE vGE(th) G E • iC MOSFET-part turn-on Period with switching losses BJT-part turn-on vCE RG VDC C VG G V’G E 50