POWER SEMICONDUCTOR DEVICES

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Contents:1.1 Introduction1.2 Diode Characteristics1.3 Reverse Recovery Characteristics1.4 Power Diode Types- A] General purpose diode B] Fast recovery diode C] Schottky diode

1.5 A] Series connection of diodes

B] Parallel connection of diodes

1.6 A] Gate Turn off SCR [GTO]

B] Power Metal Oxide Semiconductor Field

Effect Transistor [MOSFET]

C] Insulated Gate Bipolar Transistor [IGBT]

1.1 IntroductionMany applications have been found for

diodes in electronics engineering circuits.

A diode acts as a switch to perform various functions, such as switches in filters, regulators,etc.

Power diodes can be assumed as ideal switches for most applications but practical diodes have certain limitations.

1.2 Diode Characteristics

Symbol

Diode Characteristics

Above fig. shows the symbol and characteristics of pn- junction diode .

The complete V-I characteristics obtained by combining the forward and reverse characteristics of diode.

Forward characteristics is the graph of VF verses IF. During forward bias forward voltage is small and less than cutting voltage. Therefore IF through diode is also small.

The voltage at which forward diode current start increasing rapidally is known as cutting voltage of diode. During the some period forward vtg becomes greater than cutting vtg.Therefore current increases rapidally.

Reverse characteristics is a graph of reverse voltage verses reverse current. Reverse saturation current flows due to minority carriers.

As reverse voltage is increase reverse saturation current remains constant.

As reverse voltage reaches breakdown voltage value large current flows through diode due to avalanche effect .Operation of diode in breakdown region should be avoided because it may damage diode due to excessive power dissipation.

1.3 Reverse Recovery Characteristics

The current in a forward-biased junction diode is due to net effect majority & minority carriers.

Once a diode is a forward conduction mode and then its forward current is reduced to zero the diode continues to conduct due to minority carriers that remain stored in the pn-junction and the bulk semiconductor material. This time is called the “Reverse Recovery Time” of the diode.

Figure shows two reverse recovery characteristics of junction diodes. The soft-recovery type is more common.

The reverse recovery time is denoted as trr and is measured from the initial zero crossing of diode current to 25%of maximum reverse current Irr.

The trr consist of two components ta and tb. Variable ta is due to charge storage in the depletion region of the junction and represents the time between the zero crossing and the peak reverse current Irr. The tb is due to charge storage in the bulk semiconductor material

The ratio tb/ta is known as the softness factor . For practical purpose ,one needs be concerned with the total recovery time trr and the peak value of the reverse current IRR.

trr= ta + tb .

The peak reverse current can be expressed in reverse di/dt as

IRR= ta di/dt

Reverse recovery time trr may be defined as the time interval between the instant the current passes through zero during the changeover from forward conduction to reverse blocking condition and the moment the reverse current has decayed to 25% of its peak reverse value IRR.

It can be noted from the above equations that reverse recovery time trr & peak reverse recovery current Irr depend on storage charge Qrr & reverse di/dt.

The storage charge is depend on the forward diode current If. The peak reverse recovery current Irr, reverse charge Qrr & SF are all of interest to the circuit designer, & these parameters are commonly included in the specification sheets of diode.

1.4 Power Diode Ideally….a diode should have no reverse recovery

time. However, the manufacturing cost of such a diode may increase.

In many applications, the effects of reverse recovery time is not significant, and inexpensive diodes can be used.

Depending on the recovery characteristics and manufacturing techniques, the power diodes can be classified into the three categories.

General purpose

diode

Fast recovery diode

Sch

ottky

diode

Types of Power Diode

1.4(a) General Purpose diode The general purpose rectifier diodes have relatively high reverse recovery

time, typically 25µs; and are used in low speed applications, where recovery time is not critical. e.g. Diode rectifiers.

These diodes covers current rating from less than 1A to several thousands of amperes, with voltage rating from 50 V to 5Kv.

These diodes are generally manufactured by diffusion. However, alloyed types of rectifiers that are used in welding power supplies are most cost-effective & rugged, & their rating can go up to 1500 V, 400 A.

1.4(b) Fast Recovery Diode The fast recovery diodes have low recovery time, normally less than 5 µs.

They are used in dc to dc & ac to ac converters circuits, where the speed of recovery is often of critical importance. These diodes covers current rating of voltage from 50 V to around 3 Kv.

For voltage rating above 400 V, fast recovery diodes are generally made by diffusion and recovery time is controlled by platinum or gold diffusion. For voltage rating below 400V, epitaxial diodes provide faster switching speed than those of diffused diodes.

1.4(c) Schottky Diode The charge storage problem of a PN junction can be limited in a schottky

diode. It is accomplished by setting up a “Barrier Potential” with a contact between a metal & semiconductor.

A layer of metal is deposited on a thin epitaxial layer of N type silicon. The potential barrier simulates the behavior of a pn junction.

The rectifying action depends on the majority carrier only, & as a result there are no excess minority carriers to recombine. The recovery effect is due solely to the self-capacitance of the semiconductor junction.

1.5(a) Series connection of diode There are some application which requires high voltage

(HVDC) transmission but the single power diode cant satisfy the high voltage requirement. Hence the power diodes are connected in series.

The fig shows circuit diagram of series connected diode in reverse biased condition.

Fig. Series connection of Diode

In forward biased condition, diodes conduct equal amount of current & forward voltage.

But in reverse biased condition reverse voltage across each diode is different because of leakage current (due to minority charge carriers).

To avoid this problem resistance is connected across of each diode as shown in fig.

Fig. Series connection of diode using resistor

Further due to transient at the time of switch of load or initial switching there is problem of unequal voltage distribution. Hence a capacitor is connected across diode.

Fig. Modified Series connection of diodes

If there is high resistance the capacitor will damage. So

Rs as current limiting resistance.

1.5(b) Parallel Connection of Diode

In certain high power application higher current is required But single diode can not achieve the current capabilities. Hence to obtain high current the diodes are connected in parallel.

Fig. Parallel connection of Diode

The fig shows circuit diagram of parallel connected diode. To increase the high current capabilities.

Here the uniform current sharing is achieved by

connecting the resistance in series with the diode. Further the inductance are connected in each of diode in series manner.

Fig. Modified Parallel connection of diode

1.6(a) GTO- Gate Turn Off SCR

The major disadvantage of SCR is that once they are turned on, the gate will not have any control on their operation. SCRs once start conducting can be turned off by using external turn off circuits known as the commutation circuits.

Therefore, a new type of SCR is called gate turn off (GTO) SCR is developed which can be turned off by using a negative gate current pulse.

Gate Turnoff SCR Symbol

Basic Structure of GTO

Construction of GTOThe basic structure of GTO is as shown in the figure.

It is basically a four layer structure similar to conventional SCR.

As shown in the circuit symbol of GTO, it is three terminal device. Gate is control terminal.

Note the two arrows marked on the gate terminal. They indicate that the gate current for GTO can be either positive or negative. (whereas in SCR gate current is only positive.)

There are few significant differences between GTO & conventional SCR. They are as follows:

1. The gate & cathode terminal of GTO are highly interdigitated as compared to those in SCRs with various types of geometrical forms.

2. The cathode areas are usually formed by etching away the silicon from cathodes so that they appear as islands.

3. The difference is in anode region. In the P type anode layer (P1 layer) the n+ regions penetrate at regular intervals. These n+ region i.e. the base layer n1. this results in the so called “Shorted Anode Structure” of GTO.

Specifications of GTO

Sr. No. Description Value

1. On-state Voltage (Vt) 3.4 V

2. Turn On Time 1µS

3. Turn Off Time 5 to 30 µS

4. Turn off Gain 3 to 5

5. Turn on Gain 600

Applications of GTO

1. Inverters.

2. Uninterruptable power supplies (UPS).

3. DC motor drives.

1.6(b) Power MOSFETThe long form of MOSFET is Metal Oxide

Semiconductor Field Effect Transistor.

A BJT is a current controlled device. The collector current is dependent on the base current hence current is highly dependent on the junction temperature. This is a serious disadvantage of power BJT.

To overcome this advantage, we can use voltage controlled device such as a power MOSFET.

Features of power MOSFET

Low input current.

It is current controlled device.

Switching speed is very high.

Switching speed are of the order of nanoseconds.

They do not have the problem of second breakdown.

Symbol of power MOSFET

Structure of power MOSFET

Construction of a power MOSFET

The power MOSFET’s are generally enhancement type. In order to increase the voltage rating of the enhancement MOSFET, a drift layer is included as shown in the figure.

The power MOSFET has the vertically oriented four layer structure of alternate p & n layer.

The symbol of power MOSFET is as shown in the figure.

1. The vertically oriented structure is used to minimize area of current flow. This reduces on state resistance & therefore on state loss. The p type middle layer termed as “body”. Then n- layer is “drift region” & is lightly doped as compared to drain & source layers. This drift region determines breakdown voltage of power MOSFET.

2. The direction of arrow on body indicates that direction of current flow if body source pn junction were forward biased by removing the short link between body & structure. Therefore n channel MOSFET has a p type body region & the arrow points into MOSFET symbol.

3. The simplified structure of enhancement MOSFET is as shown in the figure.

4. The gate terminal is not connected directly to the semiconductor, instead there exists an oxide layer SiO2 between the metal & semiconductor. The oxide layer acts as a layer of dielectric between the metal & semiconductor to form a MOS capacitance at the input of MOSFET. This MOS capacitance does not exist in low power JFET. The input capacitance of MOSFET is large. The SiO2 oxide layer locates the gate terminal from body layer & gives the device insulating properties.

Principle of Operation With gate to source voltage VGS=0 the MOSFET is

equivalent to two back to back diodes connected as shown in figure.

The basic structure of MOSFET is very much similar to BJT. The only difference is presence of MOS capacitor that isolates gate from body region.

When VDS is applied, MOSFET turns on.

Applications of power MOSFET

1. In the high frequency

inverters & choppers.

2. In the switching mode power supplies

(SMPS).

3. In UPS & ac motor control.

4. In stepper motor

controllers & brushers' motor

drives.

1.6(c) Insulated Gate Bipolar Transistor

The BJT has advantage of low on state power dissipation, but it cannot be switched at faster rates due to longer turn-off time.

The power MODFET can be switched at much higher frequency but has a drawback of higher on state power loss.

Thus to develop a new device having best qualities of both BJT & MOSFET by combining BJTs & MOSFETs monolithically on same chip.

Feature of IGBT

Low on state voltage drop.

Low on state power loss.

It has higher switching frequency than that of a power BJT.

IGBT has the best qualities of BJT & MOSFET.

Symbol of IGBT

Basic structure of IGBT

Construction of IGBTIGBT uses vertically oriented structure to

maximize the area available for current flow.

This will reduce resistance offered to the current flow & hence on state power loss taking place in the device.

IGBT also uses highly inter digitated gate-source structure in order to reduce possibility of source/emitter current crowding.

The doping layers used in different layers of IGBT are similar to those used in comparable layers of vertical MOSFET structures except for body region.

The main difference in structure of IGBT as compared to that of MOSFET is existence of p+ layer that form’s drain of IGBT.

This device also uses n- type grain drift layer which improves its breakdown voltage capacity, this is same as that in case of power MOSFET’s.

The n+ buffer layer is essential for operation of IGBT & Some IGBTs are made without it.

This layer improves operation of IGBT in two important aspects- 1) It reduces on state voltage drop across device. 2) It shortens turn-off time.

Drawback is that presence of buffer layer reduces reverse blocking capacity of IGBT.

The circuit symbol for n channel IGBT is as shown in the figure. IGBT his a three terminal device, gate being control device.

The symbol shown in fig. is essentially same as that of n channel MOSFET, but with addition of an arrow in drain lead pointing into body of the device indicating an injecting contact.

The symbol indicates that IBT is basically a BJT with MOSFET gate input. This symbol indicates that the IGBT has output characteristics similar to power BJT & input characteristics similar to power MOSFET.

Principle of operationThe principle of operation of IGBT is

similar to that of a MOSFET. The only difference is that there is no “conductivity modulation” of drift layer in MOSFET.

Therefore the on state resistance & hence on state power loss is very high in MOSFET. In IGBT however the conductivity modulation takes place & reduces on state voltage across IGBT.

Application of IGBT

Switching mode power

supplies (SMPS)

UPS system

AC motor control

Chopper

Inverters