This article discusses different power electronics devices that are in use like power diodes, power thyristors, power transistors, IGBT, GTO, IGCT and others. This article will give a basic view of these devices and their operations.

1. Power Semi – Conductor
Devices
ER. FARUK BIN POYEN
ASST. PROFESSOR
DEPT. OF APPLIED ELECTRONICS AND INSTRUMENTATION ENGINEERING
FARUK.POYEN@GMAIL.COM

2. Contents:
 Power Diodes
 Power Transistors
 Snubber Circuit
 Comparison between Ideal and Practical Switch
 Bipolar Junction Transistor (BJT)
 BJT Darlington Pair
 Metal Oxide Silicon Field Effect Transistor MOSFET
 Insulated Gate Bipolar Transistor (IGBT)
 GTO
2

3. Power Semi – Conductor Devices
 These devices act as switches without any mechanical movement.
 Few of the Power Devices are
1. Power Diodes
2. Power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)
3. Power Bipolar -Junction Transistor (BJT)
4. Insulated-Gate Bipolar Transistor (IGBT)
5. Thyristors (SCR, GTO, MCT, IGCT)
3

4. Power Semi – Conductor Devices Ratings 4

5. Semiconductor Symbols 5

6. The Shockley Equation – Diode Law
• The trans-conductance curve is characterized by the following equation:
𝑰 𝑫 = 𝑰 𝑺 𝒆
𝑽 𝑫
ɳ𝑽 𝑻 − 𝟏
• ID is the current through the diode, IS is the saturation current and VD is the applied
biasing voltage.
• VT is the thermal equivalent voltage and is approximately 26 mV at room temperature.
The equation to find VT at various temperatures is:
• 𝑽 𝑻 =
𝒌𝑻
𝒒
• k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C
•  is the emission coefficient for the diode. It is determined by the way the diode is
constructed. It somewhat varies with diode current. For a silicon diode  is around 2 for
low currents and goes down to about 1 at higher currents
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7. Comparison between Ideal and Practical Switch
Ideal Switch Practical Switch
Block arbitrarily large forward and reverse
voltage with zero current flow when off.
Finite blocking voltage with small current
flow during turn-off.
Conduct arbitrarily large currents with zero
voltage drop when on.
Finite current flow and appreciable voltage
drop during turn-on (e.g. 2-3V for IGBT).
Switch from on to off or vice versa
instantaneously when triggered.
Requires finite time to reach maximum
voltage and current. Requires time to turn on
and off.
Very small power required from control source
to trigger the switch.
In general voltage driven devices (IGBT,
MOSFET) require small power for triggering.
GTO requires substantial amount of current to
turn off.
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8. Ideal Characteristics of a Power Device
 Arbitrarily large forward and reverse voltages are blocked with zero current flow
during OFF condition.
 No conduction losses i.e. large current flows with no voltage drop during ON
condition.
 ON – OFF switching is instantaneous without any delay.
 For triggering action, negligible power is required.
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9. Types of Diodes
 PN Junction Diodes: Are used to allow current to flow in one direction while
blocking current flow in the opposite direction. The pn junction diode is the typical
diode that has been used in the previous circuits.
9

10. Types of Diodes 10

11. Types of Diodes - Nomenclature 11
Diode Types
Signal Diode Shottkey Diode Zener Diode PIN Diode
Avalanche Diode Shockley Diode Laser Diode Photo Diode
Vacuum Diode Peltier Diode Super Barrier Diode Constant Current
Diode
Tunnel Diode Gunn Diode Varactor Diode Crystal Diode
Light Emitting
Diode
IR Light Emitting
Diode
Step Recovery
Diode
Transient Voltage
Suppression Diode

12. Types of Diodes
 Zener Diodes: Are specifically designed to operate under reverse breakdown
conditions. These diodes have a very accurate and specific reverse breakdown voltage.
 Schottky Diodes: These diodes are designed to have a very fast switching time which
makes them a great diode for digital circuit applications. They are very common in
computers because of their ability to be switched on and off so quickly.
 Very low forward voltage drop (typical 0.3V)
 Limited blocking voltage (50-100V)
12

13. Types of Diodes
 Shockley Diodes: The Shockley diode is a four-layer diode while other diodes are
normally made with only two layers. These types of diodes are generally used to
control the average power delivered to a load.
 Fast Recovery Diode:
 – Very low t rr (<1us).
 – Power levels at several hundred volts and several hundred amps
 – Normally used in high frequency circuits
13

14. Power Diode:
 Power Diode is a two terminal P – N junction semiconductor device.
 The two terminals are anode (A) and cathode (K/C).
 If terminal A experiences a higher potential compared to terminal K, the device is said
to be forward biased and a forward current will flow from anode to cathode.
 This causes a small voltage drop across the device (<1V) called as forward voltage
drop(V f), which under ideal conditions is usually ignored.
 By contrast, when a diode is reverse biased, it does not conduct and the diode then
experiences a small current flowing in the reverse direction called the leakage current.
It is shown below in the V-I characteristics of the diode.
 The structure of a power diode is different from a signal diode.
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15. Power Diode – Structure :
 The complexity in power diode is to accommodate for the high power applications.
 Power diodes find applications in rectifier circuits, freewheeling or fly back diodes.
 There is heavily doped n+ substrate (cathode) with doping level of 1019/cm3
 Lightly doped n – epitaxial layer is grown, also known as drift region.
 Anode is formed by the heavily doped p+ region.
 The breakdown voltage depends on the thickness of the n – layer.
 The n – layer is absent in signal diodes.
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16. Power Diode:
 Diodes block voltage in reverse direction and allow current in forward direction.
 They start conduction once the voltage in the forward direction goes beyond a certain
value.
 I – V Characteristics and Reverse Recovery Characteristics schematic
16

17. Power Diode
 When a diode is forward biased, it conducts current with a small forward voltage of 0.2
– 3 V across it.
 When reversed, a negligible small leakage current (µA to mA) flows until the reverse
breakdown occurs.
 When a diode is switched quickly from forward to reverse bias, it continues to conduct
due to the minority carriers which remains in the p – n junction.
 The minority carriers require finite time i.e. t rr (reverse recovery time) to recombine
with opposite charge and neutralize.
 𝑺𝒐𝒇𝒕𝒏𝒆𝒔𝒔 𝒇𝒂𝒄𝒕𝒐𝒓 𝑺 𝒓 = 𝒕 𝟐 − 𝒕 𝟏 𝒕 𝟏 − 𝒕 𝟎
17

18. Power Transistor Types
 Bipolar Junction Transistor (BJT)
 Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
 Insulated Gate Bipolar Transistor (IGBT)
18

19. Transistors 19

20. Bipolar Junction Transistor 20
 Three terminal, three layer, two junction semiconductor device.
 Emitter, Base and collector form the three terminals.

21. Structure - BJT 21
 The n – layer is added in power BJT which is known as drift region.
 There are alternating P – N – P – N layers.
 The characteristics is determined by the doping level in each layer and the thickness of
the layers.
 The thickness of the drift region determines the breakdown voltage of the Power
transistor.

22. V – I Curve - BJT 22
 The major differences between signal and power BJT are Quasi saturation region &
secondary breakdown region.
 The Quasi saturation region is available only in Power transistor characteristic not in
signal transistors.
 It is because of the lightly doped collector drift region present in Power BJT.
 The primary breakdown is similar to the signal transistor's avalanche breakdown.
 Operation of device at primary and secondary breakdown regions should be avoided as
it will lead to the catastrophic failure of the device.

23. V – I Curve - BJT 23

24. Rating - BJT 24
 Ratings:
 Voltage: VCE <1000,
 Current: IC < 400A.
 Switching frequency up to 5kHz.
 Low on-state voltage: V CE(sat): 2-3 V
 Low current gain (𝜷 <10). Need high base current to obtain reasonable IC .

25. BJT: NPN & PNP 25

26. BJT Darlington pair
 𝜷 =
𝑰 𝑪
𝑰 𝑩𝟏
= 𝑰 𝑪𝟏+𝑰 𝑪𝟐
𝑰 𝑩𝟏
=
𝑰 𝑪𝟏
𝑰 𝑩𝟏
+
𝑰 𝑪𝟐
𝑰 𝑩𝟏
= 𝜷 𝟏 +
𝑰 𝑪𝟐
𝑰 𝑩𝟐
𝑰 𝑩𝟐
𝑰 𝑩𝟏
 = 𝜷 𝟏 + 𝜷 𝟐
𝑰 𝑩𝟏+𝑰 𝑪𝟏
𝑰 𝑩𝟏
= 𝜷 𝟏 + 𝜷 𝟐. 𝟏 + 𝜷 𝟏 = 𝜷 𝟏 + 𝜷 𝟐 + 𝜷 𝟏 𝜷 𝟐
 Normally used when higher current gain is required.
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27. Metal Oxide Silicon Field Effect
Transistor (MOSFET)
 Three terminal (Gate, Drain, Source), four layer unipolar device.
 Majority carrier device.
 With no recombination delay in majority carriers, extremely high bandwidths and
switching times.
 Gate is electrically isolated from source giving MOSFET good input impedance and
good capacitance.
 No secondary breakdown area.
 Drain to source resistance has positive temperature coefficient hence self protective.
 Very low On resistance and no junction voltage drop when forward boased.
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28. MOSFET – Types 28

29. MOSFET – Structure
 Vertically oriented four layer structure of alternate P and N type layers. (n+pn-n+)
 P type middle layer is the body where channel is formed between Source and Drain.
 N – layer is the drift region which determines the breakdown voltage.
 Gate terminal is isolated by Silicon Dioxide layer.
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30. MOSFET – Ratings
 Ratings:
 Voltage VDS < 500 V,
 Current IDS < 300 A.
 Devices (few hundred watts) may go up to MHz range.
 Frequency f >100 KHz for some low power.
 Turning on and off is very simple.
– To turn on: VGS = +15V
– To turn off: VGS = 0 V and 0V to turn off.
 Gate drive circuit is simple.
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31. MOSFET Features
 Basically low voltage device.
 High voltage device are available up to 600V but with limited current.
 Can be paralleled quite easily for higher current capability.
 Internal (dynamic) resistance between drain and source during on state, R DS(ON), ,
 Limits the power handling capability of MOSFET.
 High losses especially for high voltage device due to R DS(ON) .
 Dominant in high frequency application (>100kHz).
 Biggest application is in switched-mode power supplies.
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32. MOSFET Characteristics 32
 The current equation for MOSFET is given as
𝐼 𝐷 = 𝑢 𝑛 𝐶 𝑜𝑥
𝑊
2
𝑉 𝐺𝑆 − 𝑉 𝑇𝐻 𝑉 𝐷𝑆 −
1
2
𝑉 𝐷𝑆
2
un = Mobility of the electrons; Cox = Capacitance of the oxide layer; W = Width of the gate area;
L = Length of the channel; VGS = Gate to Source voltage; VTH = Threshold voltage; VDS = Drain to Source
voltage.

33. MOSFET Selection Factors 33
 To select a MOSFET for a particular application, following parameters have to be
considered from the device datasheet
1. Maximum Drain to Source voltage (VDSS)
2. On-state drain to source resistance RDS(ON)
3. Drain Current ID
4. Gate to source Voltage VGS
5. Reverse recovery time Trr
6. Gate charge QG
7. Power Dissipation PD

34. Comparison – BJT vs MOSFET
BJT MOSFET
It is a Bipolar Device It is majority carrier device
Current control Device Voltage control Device.
Output is controlled by controlling base current Output is controlled by controlling gate voltage
Negative temperature coefficient Positive temperature coefficient
So paralleling of BJT is difficult. So paralleling of this device is easy.
Dive circuit is complex. It should provide
constant current(Base current)
Dive circuit is simple. It should provide constant
voltage(gate voltage)
Losses are low. Losses are higher than BJTs.
So used in high power applications. Used in low power applications.
BJTs have high voltage and current ratings. They have less voltage and current ratings.
Switching frequency is lower than MOSFET. Switching frequency is high.
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35. Insulated Gate Bipolar Transistor – IGBT
 IGBTs are preferred devices for voltages above 300V and below 5kV.
 Three terminal device: Gate, Source, Drain.
 They are turned on and off by applying low voltage pulses to their gate.
 Combination of BJT and MOSFET characteristics.
 Other names: Gain Enhanced MOSFET (GEMFET); COMFET (Conductivity
Modulated FET); Insulated Gate Transistor (IGT).
 Superior on – state characteristics, good switching speed and excellent safe operating
area (SOA).
 Advantage: High current capability of BJTs and easy control of MOSFET.
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36. Insulated Gate Bipolar Transistor
 IGBTs are preferred devices for voltages above 300V and below 5kV.
 They are turned on and off by applying low voltage pulses to their gate.
 Combination of BJT and MOSFET characteristics.
 Gate behaviour similar to MOSFET - easy to turn on and off.
 Low losses like BJT due to low on-state Collector - Emitter voltage (2-3V).
 Ratings: Voltage: V CE < 3.3kV, Current,: I C < 1.2 kA currently available.
 Latest: HVIGBT 4.5 kV/1.2 kA.
 Switching frequency up to 100 KHz.
 Typical applications: 20-50 KHz.
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37. IGBT Equivalent Circuit 37

38. IGBT Characteristics Curve 38
 IGBT are classified as
 (i) Non Punch Through IGBT (NPT IGBT) or symmetrical IGBT.
 (ii) Punch Through IGBT (PT IGBT) or asymmetrical IGBT.
 IGBTs having n + buffer layer are termed as Punch Through (PT IGBT)s.
 IGBTs not having n + buffer layer in known as Non Punch Through (NPT IGBT)s.
 A symmetrical IGBT is one having equal forward and reverse breakdown voltages which
are normally used in AC applications.
 In the asymmetrical IGBT, the reverse breakdown voltage is less than the forward
breakdown voltage which are normally used in DC circuits where the device does not
need to provide support in the reverse direction.

39. IGBT Characteristics Curve 39

40. IGBT – Merits & Demerits 40
 Merits
1. Voltage controlled device
2. Less On state loss
3. High switching frequency
4. No commutation circuit.
5. Gate has full control over operation
6. Flat temperature coefficient.
 Demerits:
1. Static charge problem.
2. Costlier than BJT and MOSFET.

41. IGBT – Comparison 41
Device
Characteristic
Power
Bipolar
Power
MOSFET
IGBT
Voltage Rating High <1kV High <1kV Very High >1kV
Current Rating High <500A Low <200A High >500A
Input Drive
Current, hFE
20-200
Voltage, VGS
3-10V
Voltage, VGE
4-8V
Input Impedance Low High High
Output Impedance Low Medium Low
Switching Speed Slow (uS) Fast (nS) Medium
Cost Low Medium High

42. Gate turn-off Thyristor (GTO)
 Behave like normal thyristor, but can be turned off using gate signal
 However turning off is difficult.
 Need very large reverse gate current (normally 1/5 of anode current).
 Gate drive design is very difficult due to very large reverse gate current at turn off.
 Ratings: Highest power ratings switch:
 Voltage: V AK < 5 kV; Current: I A < 5 kA. Frequency < 5 KHz.
 Very stiff competition:
Low end-from IGBT. High end from IGCT
42

43. Classification - GTO
1. Asymmetrical GTO:
 The Asymmetrical type GTOs are the most common type on the market.
 This type of GTOs are normally used with a anti-parallel diode.
 They do not have high reverse blocking capability.
 They are used in Voltage Fed Converters.
2. Symmetrical GTO:
 The symmetrical type GTOs have an equal forward and reverse blocking capability.
 They are used in Current Fed Converters.
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44. Construction - GTO
 Almost similar to SCR.
 In this, n + layer at the cathode end is highly doped to obtain high emitter efficiency.
 Dur to this, breakdown voltage of the J3 junction is low (20 to 40 V).
 The doping level of p type gate is highly graded as trade off between high emitter
efficiency (low doping required) good turn off properties (high doping required).
44

45. Construction - GTO
 Junction between p+ anode and N base is called anode junction.
 Heavily doped p+ anode region is required to obtain high efficiency on turn on property.
 But turn Off property is affected.
 This problem is solved by introducing heavily doped n+ layers at regular intervals in p+
anode layer.
 This is called anode shorted GTO structure.
 This causes the electrons to travel from base N to anode metal contact directly without
causing hole injection from p+ anode.
 But reverse blocking capacity of GTO is reduced and speeds up turn Off mechanism.
45

46. Working - GTO
 The Gate turn off thyristor (GTO) is a four layer PNPN power semiconductor
switching device that can be turned on by a short pulse of gate current and can be
turned off by a reverse gate pulse.
 This reverse gate current amplitude is dependent on the anode current to be turned off.
 There is no need for an external commutation circuit to turn it off.
 So inverter circuits built by this device are compact and low-cost.
 The device is turned on by a positive gate current and it is turned off by a negative gate
cathode voltage.
46

47. GTO Characteristics Curve 47

48. GTO V – I Characteristics: 48
 It is similar to SCR during turn on.
 The 1st quadrant characteristics are similar to that of SCR.
 Latching current and holding current are considerably higher than SCR.
 The gate drive can be removed if anode current is more than the holding current.
 It is though not recommended as cathode is subdivided into small finger elements
causing the anode current to go below the holding current hence destroying the device.
 GTO is turned off by applying reverse gate current in either ramp or step mode.
 The dashed line in the figure shows i-v trajectory during the turn OFF for an inductive
load.
 dV/dt triggering is avoided by placing a rated resistor between gate and cathode or by
means of a reverse bias voltage.

49. GTO V – I Characteristics: 49
 A symmetric GTO has higher reverse blocking capability than an asymmetric type.
 After a small reverse voltage (20 to 30 V) GTO starts conducting in the reverse
direction due to anode short structure.

50. GTO Turn OFF Current Gain: 50
 The Turn Off Current Gain of a GTO is defined as the ratio of anode current prior to
turn off to the negative gate current required for turn off.
 It is typically very low (4 or 5).
 It means a 6000A rating GTO requires 1500A gate current pulse.
 However, the gate pulse duration and the power loss due to the gate pulse is small.
 It can be supplied by low voltage power MOSFETs.
 This gate turn off capability is advantageous because it provides increased flexibility in
circuit application.
 Now it becomes possible to control power in DC circuits without use of elaborated
commutation circuitry.

51. Summary GTO 51
 Advantages of GTO:
1. High blocking voltage capabilities.
2. High over current capabilities during turn off. .
3. Exhibits low gate currents.
4. Fast and efficient turn off.
5. Better static and dynamic dv/dt capabilities.
6. Enhanced Safe Operating Area during turn off.
 Disadvantages:
1. Magnitude of latching, holding currents is several times more than thyristors.
2. On state voltage drop and the associated loss is more.
3. Due to multi cathode structure of GTO, triggering gate current is higher than that required
for normal SCR.
4. Gate drive circuit losses are more.
5. Its reverse voltage blocking capability is less than the forward voltage blocking capability.

52. Summary GTO 52
 Applications of GTO:
1. Motor drives,
2. static VAR compensators (SVCs)
3. AC/DC power supplies with high power ratings.
4. DC circuit breakers
5. Induction heating
6. Low power applications.
7. DC choppers.

53. Insulated Gate-Commutated Thyristor (IGCT)
 Among the latest Power Switches.
 Conducts like normal thyristor (latching), but can be turned off using gate signal,
similar to IGBT turn off; 20 V is sufficient.
 Power switch is integrated with the gate-drive unit.
 Ratings:
Voltage: V AK < 6.5 kV; Current: I A < 4 kA.
 Frequency < 1 KHz. Currently 10kV device is being developed.
 Very low on state voltage: 2.7V for 4kA device.
53

54. Insulated Gate-Commutated Thyristor (IGCT)
Specifications GTO IGCT IGBT
Full Form Gate Turn-Off Thyristor
Insulated Gate Commutated
Thyristor
Insulated Gate Commutated
Thyristor
Advantages
• Controlled turn-off ability.
• Relatively high overload
capacity.
• Series connection
possibility.
• Working frequency of
hundreds of Hz.
• Controlled turn-off ability.
• Relatively high overload
capacity.
• Low on-state losses.
• Working frequency of kHz.
• Series connection possibility.
• High cyclic resistance.
• Controlled turn-off
ability.
• Minimum working
frequency up to 10 kHz.
• Very low control power.
Disadvantages
• Higher on-state losses.
• High control power.
• Very high on-state losses.
• Relatively low cyclic
resistance.
Applications
• High power drives
• Static compensators
• Continuous supply sources
• Induction heating sources
• High power drives
• Supply inverter sources for DC
transmissions
• Big frequency converters
• Choppers
• Continuous supply
sources
• Statical compensators and
active filters
• Switching sources
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55. References
 Chapter 1; Power Electronics and Drives (Version 3-2003). Dr. Zainal Salam, UTM-JB.
 Introduction to Power Electronics - A Tutorial, Burak Ozpineci, Power Electronics and
Electrical Power Systems Research Center, Oak Ridge National Laboratory, US Dept. of
Energy.
 http://www.completepowerelectronics.com/
 Power Electronics A to Z/POWER SEMICONDUCTOR DEVICES/Comparison of
MOSFET with BJT
 http://www.rfwireless-world.com/Terminology/GTO-vs-IGCT-vs-IGBT.html
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