Power Semi – Conductor

Devices

ER. FARUK BIN POYEN

ASST. PROFESSOR

DEPT. OF APPLIED ELECTRONICS AND INSTRUMENTATION ENGINEERING

FARUK.POYEN@GMAIL.COM

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

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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

Power Semi – Conductor Devices Ratings 4

Semiconductor Symbols 5

The Shockley Equation – Diode Law

• The trans-conductance curve is characterized by the following equation:

𝑰𝑫 = 𝑰𝑺 𝒆

𝑽𝑫ɳ𝑽𝑻 − 𝟏

• ID is the current through the diode, IS is the saturation current and VD is the appliedbiasing voltage.

• VT is the thermal equivalent voltage and is approximately 26 mV at room temperature.The equation to find VT at various temperatures is:

• 𝑽𝑻 =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 isconstructed. It somewhat varies with diode current. For a silicon diode is around 2 forlow currents and goes down to about 1 at higher currents

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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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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.

8

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.

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Types of Diodes 10

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

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

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

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.

14

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.

15

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

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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 𝑺𝒓 = 𝒕𝟐 − 𝒕𝟏 𝒕𝟏 − 𝒕𝟎

17

Power Transistor Types

Bipolar Junction Transistor (BJT)

Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Insulated Gate Bipolar Transistor (IGBT)

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Transistors 19

Bipolar Junction Transistor 20

Three terminal, three layer, two junction semiconductor device.

Emitter, Base and collector form the three terminals.

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.

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.

V – I Curve - BJT 23

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 .

BJT: NPN & PNP 25

BJT Darlington pair

𝜷 =𝑰𝑪

𝑰𝑩𝟏= 𝑰𝑪𝟏+𝑰𝑪𝟐

𝑰𝑩𝟏=

𝑰𝑪𝟏

𝑰𝑩𝟏+

𝑰𝑪𝟐

𝑰𝑩𝟏= 𝜷𝟏 +

𝑰𝑪𝟐

𝑰𝑩𝟐

𝑰𝑩𝟐

𝑰𝑩𝟏

= 𝜷𝟏 + 𝜷𝟐𝑰𝑩𝟏+𝑰𝑪𝟏

𝑰𝑩𝟏= 𝜷𝟏 + 𝜷𝟐. 𝟏 + 𝜷𝟏 = 𝜷𝟏 + 𝜷𝟐 + 𝜷𝟏𝜷𝟐

Normally used when higher current gain is required.

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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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MOSFET – Types 28

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.

29

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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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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MOSFET Characteristics 32

The current equation for MOSFET is given as

𝐼𝐷 = 𝑢𝑛𝐶𝑜𝑥𝑊

2𝑉𝐺𝑆 − 𝑉𝑇𝐻 𝑉𝐷𝑆 −

1

2𝑉𝐷𝑆2

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.

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

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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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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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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IGBT Equivalent Circuit 37

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.

IGBT Characteristics Curve 39

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.

IGBT – Comparison 41

Device

Characteristic

Power

Bipolar

Power

MOSFETIGBT

Voltage Rating High <1kV High <1kV Very High >1kV

Current Rating High <500A Low <200A High >500A

Input DriveCurrent, 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

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

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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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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).

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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.

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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.

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GTO Characteristics Curve 47

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.

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.

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.

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 requiredfor normal SCR.

4. Gate drive circuit losses are more.

5. Its reverse voltage blocking capability is less than the forward voltage blocking capability.

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.

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.

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Insulated Gate-Commutated Thyristor (IGCT)

Specifications GTO IGCT IGBT

Full Form Gate Turn-Off ThyristorInsulated 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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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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