Reference

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Pubi'i • cations

IMBSIR 78-1559

A111DL4 MfiTE3D

MR Assessment of High- Power

Thyristor Technology

R. I. Scace

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

October 1978

Final

Prepared for department of Energy

L nr division of Electric Energy Systems

/Vashington, D.C. 20545

100

.U56

78-1559

MAY 1 4 197?

NBSIR 78-1559

AN ASSESSMENT OF HIGH-POWER

THYRISTOR TECHNOLOGY

R. I. Scace

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

October 1978

Final

Prepared for Department of Energy

Division of Electric Energy Systems

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kraps, Secretsry

Dr. Sidney Harman, Und»r S»crmtmy

Jordan J. Baruch, Assisfnt Sacrafary for Sc^nca and Tochnotogy

NATIONAL BUREAU OF STANDARDS, Emaat AmMar, Director
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TABLE OF CONTENTS

Page

Abstract 1

1. Historical Outline 1

2. Device Design 3 2.1 Voltage Blocking Characteristics 3 2.2 Turn-on and Turn-off Mechanisms 5 2.3 dV/dt and dl/dt Properties 7 2.4 Conclusions 9

3. Thermal Problems 10

3.1 Conclusions 12

4. Materials Problems 13

4.1 Resistivity Nonuniformity 13

4.2 Carrier Lifetime 14

4.3 Defects 15

4.4 Conclusions 17

5. Processing 18

5.1 Blocking Junction Formation 18

5.2 Blocking Junction Passivation 19

5.3 Lifetime Control 20

5.4 Conclusions 22

6. Assembly and Electrical Test 22

7. Summary 24

Acknowledgment 25

References 26

iii
LIST OF FIGURES

Page

1. Cross-section of a thyristor in the forward-blocking

off-state, showing the junction configuration and the

depletion layer aroiand junction J2 33

2. Equipotential lines in a reverse-biased p'^-n junction

with a contoured surface 33

3a- Experimental and theoretical I-V curves for a 1200-V

triac 34

3b. Experimental and theoretical I-V curves for a 3500-V

thyristor 34

3c. Experimental and theoretical I-V curves for a 2500-V

thyristor 35

4. Theoretical curve which illustrates the relative importance of the various physical mechanisms, including heat

flow, on the I-V characteristics 35

5. p-n-p-n device with shorted emitter 36

6. A thyristor for double-sided cooling 36

7. Thyristor with double-sided air-cooled heat sinks 37

8. Spreading resistance measurements in silicon slices;

distance between measurement points 20 ym 37

IV
An Assessment of High-Power Thyristor Technology

Robert I. Scace

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

Abstract

The current state of the technology for designing and

making high-power thyristors is reviewed. The discussion is

confined primarily to thyristors having voltage ratings of

2000 V or greater and current ratings above a few hundred am- peres. Particular attention is given to describing the factors which limit the performance of such devices and to discussing recent work aimed at solving these problems.

Key Words: High current switches; high voltage switches;

SCR; semiconductor devices; semiconductor measurements; silicon devices; silicon controlled rectifiers; solid-state switches; technology assessment; thyristors.

1. Historical Outline

The thyristor or silicon controlled rectifier (SCR) was originally

devised by Moll et al

.

at Bell Laboratories [1].* Its commercial

development was undertaken in the mid 1950s by General Electric,

followed quickly by a number of other manufacturers. The term thyristor

covers a large class of semiconductor devices, all of which characteristically have high and low impedance states. The change from high to low

impedance can be initiated by avalanche breakdown of one p-n junction,

by control current into an auxiliary electrode, by light, temperature,

or other means. Once in the low impedance state, thyristors remain in

that condition until the current density through the device falls below

some threshold value. At this point, the high impedance state is

reestablished. Thyristors may have from two to four terminals and may

be composed of from three to five alternating layers of p- and n-type

*References are -given only to selections from the voluminous literature

on thyristors.

1
conductivity. Some can be switched into the low impedance state for

only one direction of current, while others are symmetrical. Strictly

speaking, silicon controlled rectifiers are four-layer unidirectional

triode thyristors, only one of the class of all thyristors. The

discussion here will be concerned entirely with SCRs.

From the beginning, the application of thyristors was primarily

at high power levels, since at lower power levels transistors offer the

advantages of being able to be modulated and to be returned to a high

impedance condition under complete control of the base electrode. The

relatively simple structure of the SCR, and the fact that suitable

processing techniques were available to produce that structure, led in

the early 1960s to rapid increases in the size (and therefore the

current-carrying capacity) and voltage rating of commercial thyristors.

By the mid 1960s, about 10 years after the initial work by Moll,

thyristors having voltage ratings of 1000 V and current handling

capacities of over 100 A were beginning to appear.

The Department of the Navy played 'a key role in this development

by its early recognition that these devices would offer advantages for

shipboard power systems. Navy device development programs at both

General Electric and Westinghouse during the early and middle 1960s

promoted a much more rapid advance of the technology than would

otherwise have been possible. Through patent and technology licenses,

the know-how from these two companies was relatively quickly transferred to others in both Europe and Japan.

For the most part, the technology was acquired and practiced by

companies whose principal business was the manufacture of large

electrical equipment. No successful entry into the thyristor business

with other than relatively low power devices has been made by any

company whose principal business is the manufacture of signal-level

semiconductor devices. This is apparently due to two causes. First,

electrical equipment manufacturers saw very early the high-power thyris2
tor as having the potential for becoming an important component for

their products. That potential has been realized. Second, the avenues

of marketing thyristors and the kinds of application engineering support

which the manufacturer gives to his customers were significantly different than those appropriate for more conventional signal-level devices.

The companies listed in table 1 make up the high-power thyristor

manufacturing community today. With the exception of National

Electronics, Power Semiconductors, and Semikron, all of these firms

use their own products extensively in the electrical systems which

they manufacture. The three firms just mentioned are companies which

specialize in power device manufacture. Nearly all of the companies

listed in table 1 also sell their components to equipment manufacturers; only ASEA and Als thorn do not.

2 . Device Design

A good general reference to the theory and design of thyristors

may be found in Gentry et aJ. [2]. The principles of operation of

thyristors, and a few approximations to quantitative design, were

well understood in 1964 when their book was published. The rationale

for designing devices to withstand a given voltage or to carry a particular current has not changed significantly since then.

However, the development of a complete, quantitative model describing

the operation of thyristors in detail is still continuing. The

significant technological issues which are currently being addressed

will be described in greater detail in the paragraphs to follow.

2.1 Voltage Blocking Characteristics

The off-state voltage across a thyristor is supported by a reverse-biased p-n junction. Large thyristors have an essentially

plane-parallel junction configuration, as illustrated in figure 1.

The shaded region represents the depletion layer, which is the space

surrounding the blocking junction from which essentially all mobile

charges have been removed by the applied voltage.

3
While figure 1 correctly shows the depletion layer in the interior

of the device, it does not show the effects which occur at the perimeter

where the junction reaches the surface. These are shown in figure 2,

where the surface of a single p-n junction has been beveled in order

to spread the depletion layer over a significantly larger distance

across the surface than it would otherwise occupy. This must be done

because the breakdown electric field in silicon is 230 kV/cm, while

in air or in other dielectrics which could be applied to the outside

of the silicon the breakdown field is considerably less than that.

Beveling reduces the field at the surface and thus reduces the likelihood of arcing across the surface of the device

The presence of electrical charges or surface states at or near

the surface of the silicon also affects the distribution of electric

field there. Both the geometrical effects and the influence of

surface charge must be taken into account in any analysis to determine

the proper contour of the wafer edge to provide stable, reproducible

voltage blocking characteristics. Davies and Gentry [3] reported the

first efforts toward a solution of this mathematically unpleasant

problem. They used a finite-difference method to calculate numerically

a potential distribution which satisfied Poisson's equation within the

semiconductor and, since they assumed no net charge to exist on or outside the surface of the semiconductor, which satisfied Laplace's

equation outside the semiconductor. Once a self-consistent potential

distribution was established, the electric field could be determined

at any point. Figure 2 is an example of a field distribution computed

in this way. Surface bevels of various angles were analyzed, and the

results showed that the surface electric field could be reduced (by

choice of a suitable bevel angle) to a degree that avoided surface

breakdown altogether.

Typically, a 6-deg angle is used on the forward-blocking junction

and a 60-deg angle is used on the reverse-blocking junction of presentday thyristors. In a high-voltage- device 300 ym thick, an annular space

4
up to 3 mm wide is required for such a bevel; this region does not

contribute to the current carrying capability of the device. In

large devices this significantly reduces the effective area.

Better solutions to this problem are sought. Kao and Wolley [4]

described a technique for using field-limiting rings to distribute

the electric field across the surface of planar high-voltage junctions.

An etching technique was described by Temple and Adler [5]

,

and Temple

has used ion implantation for this purpose [6]

.

Each method had its

difficulties, and most, if not all, of today's high-power thyristors

are made with beveled edges.

2.2 T\irn-on and Turn-off Mechanisms

An ideal switch would turn on or off instantaneously.

Thyristors are not ideal switches, and they often require several

tens of microseconds to change their state. The requirements of the more demanding applications have led to a number of investigations

aimed at understanding and controlling the turn-on and turn-off

mechanisms more precisely. Thyristors switch on only after a period

of time which is determined initially by diffusion of minority

carriers across the base regions (nl and p2 of fig. 1) of the device.

The part of the thyristor nearest the gate electrode connection

switches on first, and this condition propagates laterally across the

device until, after an additional period of time, the entire thyristor

has been switched to its low impedance state.

Until recently, these two turn-on phenomena (initial switching

and on-state spreading) were investigated independently because of

the complications of calculations in two dimensions [7-10] . More

recent work relies heavily on computer analysis [11-14] , with the most

complete of these studies [14] showing the early stages of turn-on

in great detail graphically in three-dimensional computer plots . The

model employed in [14] takes into account Shockley-Read-Hall and

Auger recombination, band-gap narrowing, and carrier-carrier

scattering. Calculations of the on-state voltage drop and power

5
dissipation using this model [15] show excellent agreement with

experimental results. Figure 3 illustrates this for three different

devices, and figure 4 shows the effects of removing various mechanisms

from the calculation. In figure 4, the curve labeled "nominal case thermal conductance (G = 50)" is the same as the curve for the 2500-V

thyristor in figure 3.

Turn-off begins at the time when the current in the device falls

below the holding current, which may be 100 mA or less for a 100-A

rms-rated thyristor. (The decrease in current is entirely a function

of the circuit of which the thyristor is a peirt.) At this instant, the

depletion region about J1 of figure 1 begins to be established, if it is

assumed that the thyristor is entering the reverse-blocking condition.

The excess charge carriers (electrons in pi, holes in nl) must be

removed before the depletion layer can expand. This removal is

accomplished mainly by recombination, at a rate determined by the

minority-carrier lifetimes in regions nl and pi.

High-power thyristors require careful control of minority-carrier

lifetime if their performance is to be optimized. In addition to

affecting both turn-on and turn-off behavior as mentioned above, the

lifetime significantly influences reverse leakage currents, dl/dt

characteristics*, on-state voltage drop, .and trigger sensitivity.

Unfortunately, the measurement of lifetime as a function of position

in solid-state devices is one of the most difficult to perform.

Recent measurements of lifetime on charge-carrier concentration in

cross-sections of thyristors have been made using electron irradiation

[6], the recombination radiation at 1.1 ym [17], and free-carrier

absorption of laser radiation at 3.4 ym [18]. Specially prepared

devices are required to make measurements by all of these techniques.

*This property, although related to turn-on, is specified separately

from turn-on time. A discussion of dl/dt characteristics is given

in the following section.

6
None of these measurement methods is as yet adaptable to day-to-day

production-line use. If the distribution of carriers within operating

thyristors can be determined, local lifetime and diffusion length data

can be deduced. These data can then be used in computer models from

which improved device designs can be developed.

More information is needed about the detailed properties of

recombination centers in thyristors. Some of this knowledge can be

obtained by thermally stimulated current and capacitance measurements

.

The temperature dependence of the capture cross-sections of recombination centers is not known in many cases, and evidence exists [19]

for a recombination center of unknown origin with a ratio of highlevel to low-level lifetime of greater than 12. For comparison, the

ratio for gold is 4 [16] . A high ratio is desirable to improve the

trade-off between short turn-off time and low on-state voltage drop.

With today's computer models providing improved understanding of the

role of lifetime as a fiinction of position in the operation of thyristors, the realization of improved devices must then depend upon the

development of processing techniques which permit precise tailoring

of lifetime within the device to meet the requirements of the models

and upon the development of suitable means to measure the resulting

lifetime as a function of position to verify that the desired lifetime

distribution has been achieved.

2 . 3 dV/dt and dl/dt Properties

In many applications, thyristors in the off-state are exposed to high rates of rise of applied forward voltage. This is

particularly true in inverter circuits where waveforms are frequently

not sinusoidal. If no design precautions are taken, the charge

displaced from the forward blocking junction as the depletion layer

widens can cause the current density in the device to exceed the

threshold for switching, and the device will turn on in the absence of

a gate trigger signal. This effect is controlled by the use of the

shorted emitter [20]

.

7
As figure 5 shows, both the emitter n-region (n2) below the cathode

terminal and the neighboring p-region (p2) are connected to the

cathode terminal. Emitter junction J3 is thereby short-circuited, and

these emitter shorts provide a path for the displaced charge in the

upper p-region (p2) to pass to the cathode terminal without crossing

the cathode emitter junction. It also may be seen that a considerable

lateral current in the p-base exists as a result. In effect, the

emitter efficiency is made to be zero at low current densities, but

as the lateral current develops an appreciable voltage drop in the

p-region, the portions of the emitter farthest from the shorts become

forward biased and normal electron injection can take place. At high

current densities, the emitter efficiency is high, being little

affected by the shorts. The shorts consume a small portion of the

active area, commonly a few percent, and do not appreciably decrease

the current-handling capabilities of the device. Since this dV/dt

effect can be isolated from the remainder of the device design, early

analytical attempts at understanding and solving the design problem

were reasonably successful [21] . This early analysis neglected the

effects of conductivity modulation and the fact that the emitter

efficiency is not unity, complications which would have made an analytical solution impossible. These can, of course, be modeled

successfully on today's computer models.

A much more difficult problem relates to the limitations on current-handling capability during the early stages after the device

has been switched on. As briefly mentioned above, a small region near

the gate electrode switches into the conducting state first, and this

condition then spreads laterally across the device at a speed of

approximately 50 m/s [22] until the device is completely turned on. The spreading velocity of the turned-on region is a function of the

current density, the temperature, the distribution of emitter shorts,

the sheet resistance of the bases (nl and p2 in fig. 5) and the

distribution of recombination centers.

8
since only a fraction of the area of the device is turned on for some time following triggering, the current-carrying capability of the

thyristor during that interval is limited. The maximum rate of current

rise which the device can handle repetitively and the peak pulsecurrent rating are functions of time ; both are dependent upon the

spread of the turned-on region. Numerical and experimental analyses

of the pulse-current rating of thyristors during this period show that

junction temperatures in excess of 1000°C exist during pulses having

durations in the range of 10 to 100 ys [24] . Evidently these very

high instantaneous temperatures do not have an adverse effect on the

device

.

The studies referred to in this section have provided at least

the minimal understanding required for the development of specialpurpose firing arrangCTients . Devices have been designed to have

especially high or low sensitivity to gate triggering, to have a gate

amplifying region to turn on a larger area soon after firing to

improve dl/dt characteristics, or to be triggered by light [25-28]

.

Light- triggered devices offer attractive advantages for high-voltage dc

terminal (HVDC) systems, in part because the trigger source can be at

ground potential while the thyristor is at a potential of tens or hundreds of kilovolts. The design of such devices requires a difficult

trade-off to be made, since high sensitivity to light but low sensitivity to dV/dt effects must exist concurrently, and additional information

on the details of gate-region operation appears to be needed before

these trade-offs can be optimized.

2.4 Conclusions

Many design questions which could only be dealt with

empirically a few years ago can now be answered quantitatively with

the aid of computer models. Design of a thyristor for a specific

application requires that trade-offs be made. For example, higher

voltage ratings require reduced current ratings for a given diameter

of device, since the on-state voltage drop must be increased. Higher

dV/dt ratings require further sacrifice of current rating, because

9
emitter shorts reduce the active area of the device. In some cases,

the optimum set of trade-offs for a given application may raise the

cost of the thyristor itself in order to permit savings to be made

elsewhere in the equipment.

A new generation of questions can now be posed, such as: Can current ratings for one line-frequency period or less

(surge current) be improved in comparison with steady-state

ratings?

Considering that substantial lateral currents exist inside

thyristors, can these be employed to cause the on-state to

spread more rapidly and thus improve dl/dt ratings?

Are there other means to improve di/dt?

If one could arbitrarily control minority carrier lifetime

locally within the device, what beneficial effects could be

expected?

As experience is gained in the use of models, such questions could

profitably be explored. Intuition and empiricism are not adequate to

deal with the quantitative behavior of such an apparently simple but,

in fact, very complex device.

3. Thermal Problems

The ratings of high-power semiconductor devices in general are limited by problems of heat removal. The steady-state power dissipation of a 50-mm diameter thyristor is comparable to that of an electric range burner on high heat [15] . An appreciation of the

thermal problem can be gained when one considers that this quantity

of heat must be removed from the thyristor while limiting the maximum

steady-state junction temperature to 125°C. Most devices larger than

30 mm in diameter, and all larger than 50 mm, are enclosed in packages

which provide for the extraction of heat from both sides of the device.

Figure 6 shows a typical design of such an assembly in cross-section,

at approximately full scale. Figure 7 shows how this assembly is

incorporated into a further assembly for exchange of heat into an air

stream

10
The double-sided cooling approach was taken in response to the heat

dissipation problems encountered with stud-mounted packages when devices

passed about 25 mm in diameter in the mid 1960s [29]

.

Not only did the improved cooling allow an increase in the current

rating of the device for a given size, but a thermal expansion problem

was alleviated. The thermal expansion coefficient of silicon is

significantly lower than that of copper and is matched within a factor of 2 by only two metals, tungsten and molybdenum, as table 2 shows. The thyristor element (see fig. 6, part 2) is composed of a metallurgically bonded sandwich of silicon with a tungsten or molybdenum plate one or two millimeters thick on either side.

In previous designs, this sandwich subassembly was bonded to a copper stud, usually with a brazing alloy having a high shear strength.

Upon cooling from the brazing temperature, the copper contracted more

than the adjoining metal plate, and much more than the silicon. The

stresses thus created frequently caused the brittle silicon to crack,

destroying the device. The two-sided assembly provides for pressure

contacts between the copper conductors (fig. 6, part 3) and the

thyristor siibassembly . Lateral differential expansion can be accom- modated by sliding at the pressure contacts, thus reducing the stresses

on the silicon.

With today's larger thyristors the differential expansion problem

has returned, but now the difference between the thermal expansion

coefficients of silicon and tungsten (or molybdenum) is the source of

stress. The sandwich of silicon and the two refractory metal plates

is not symmetrical; the cathode plate is smaller in diameter than the

anode plate because the periphery of the silicon has been beveled.

As the diameter of the device increases , the total dimensional

mismatch between the silicon and the metals increases, sufficiently

increasing the bending moments on the assembly that fracture of the

silicon can occur. If the assembly is to remain flat, the metal

members must be increased in thickness to a centimeter or more. Since

neither tungsten nor molybdenum has a thermal conductivity as high ^s

11
copper, this causes a thermal conductivity penalty to be paid. Furthermore, both tungsten and molybdenim are expensive. In present-day

large thyristors these back-up plates frequently cost more than the

silicon.

In a different design approach now being taken to overcome this

problem, the silicon is not attached to any back-up plate. Thin (1- to

2-mm) tungsten or molybdenum plates between the silicon and the copper

conductors are used in a package like that of figure 6, but none of

these pieces is metallurgically bonded to its neighbors. The thermal

expansion problem is alleviated, because each of these joints permits

sliding to take place to accommodate differences in dimensions. There

are two problems with this approach. One is that the number of dry

interfaces which have relatively large thermal impedance is now

doubled. With the Scindwich construction, the interfaces between the

copper conductors of figure 6 and the thyristor element were the only

dry joints. With the new design, there is also a dry joint between

the silicon and each back-up plate. The second problem lies in the

fact that the silicon slice is completely unsupported during

manufacture. Silicon is as brittle as glass, and these slices are

fragile. Extreme precautions must be taken in handling to avoid

breakage.

One further problem exists with two-sided cooling assemblies. The

electrical and thermal paths between the heat sinks and the device and

the dry interfaces within the device package are all held together by

an externally applied force. The force, about 70 kg/cm^ (1000 Ib/in^)

is generally provided by heavy springs in an external framework. The

structure adds cost and size to the assembly. In many applications,

the increased size presents problems.

3.1 Conclusions

Heat management will continue to be a major design consideration for thyristors. Exotic methods for heat removal, such as heat

pipes or the use of the very few materials having thermal conductivities

greater than that of copper (e.g., type II B diamond), cannot often be

12
used, either because they are far too costly or for basic physical

2 reasons. The thermal flux from the silicon slice is typically 300 W/cm , used.* three times the flux at which water cooling can be * It is

necessary to transport the heat to a surface of larger area to reduce

the heat flux to a more tractable level, and solid copper is the best

practical medium for this purpose. It appears that no significant

breakthrough is possible in this direction.

4 . Materials Problems

The upper limits of thyristor ratings have historically been

imposed by the characteristics of available float-zoned silicon.

Applications for very large devices have always been sufficient in

number that, with every advance in float-zoned crystal technology, new

thyristor generations have appeared. For example, the largest diameter

thyristor which has been publicly announced is made by Mitsubishi, with

a rating of 4000 V, 2500 A. This device is made from a 100-mm diameter

slice of silicon, which is commercially available on a pilot basis only

from Wacker Chemitronic in Germany. This section deals with those

properties of today's silicon which impose significant limitations on high-power thyristor performance.

4.1 Resistivity Nomoniformity

Many thyristor properties depend critically on the characteristics of the starting silicon. Although semiconductor-grade

silicon is an extraordinarily pure material, there are trace quantities

of impurities present which were not deliberately introduced during

the crystal growing process. Oxygen is present in relatively high

quantities in Czochralski silicon; its concentration is much lower in

float-zoned silicon. Under certain conditions of heat treatment,

otherwise electrically inactive oxygen can form groupings within the

2 *At about 100 W/cm a film of water vapor develops, separating the

liquid from the solid surface. This gaseous layer radically increases

the thermal impedance.

13
silicon crystal which add electrons in an uncontrolled fashion and thus

affect the resistivity [30]. Vacancies in the silicon lattice (places

in the silicon lattice where no atom is present) are essential to the

process of oxygen donor formation. Because of the importance of

resistivity control in thyristor manufacture, if the designed

properties of the device are to be realized, oxygen must be held to a low level in the silicon crystal at all stages of manufacture.

Until recent years, float-zoned silicon has characteristically had

a variation of resistivity from the center of the slice to the periphery

of as much as 50%. Since the blocking voltage limit of a p-n junction

depends on the lowest resistivity anywhere in the junction area, this

meant that the nominal resistivity of the silicon and the nominal

design voltage had to be much higher than required by theory for the

actual voltages achieved. With the advent of neutron-transmutationdoped (NTD) silicon [31-34] , thyristors can be produced with at least

15% higher blocking voltage ratings without changing the processing in

any way. This change is attributable entirely to the increased

uniformity of resistivity across the diameter of the starting silicon

slice. This improved unformity is noticed not only on a large scale,

but also in the fine details of resistivity as shown in figure 8.

These fine-scale variations in resistivity (striations) have been shown

to cause irregularities in the profile of high voltage p-n junctions

[35, 36]. Such irregularity leads to increases in the local electric

field and to decreased breakdown voltage.

4.2 Carrier Lifetime

The minority-carrier lifetime of conventional float-zoned

silicon has' been more than sufficient for high-voltage thyristor use

for at least ten years. Minimum values in the neighborhood of 200 ys are generally specified when material is purchased and values of 1 ms or

' greater are typical of as-grown material. In order to maintain low

on-state voltage drops, the charge-carrier lifetime in the completed

device must be kept relatively high *[37] . With suitable care taken

14
during processing, values of several tens of microseconds usually exist

within the completed device. Only when this must be sacrificed to

improve the switching speed of the device are deliberate steps taken to

reduce the lifetime [38] . Discussion of ways in which this is done will

be found in a later section.

For NTD silicon, the situation is different. During irradiation in

the nuclear reactor, the silicon crystal structure is considerably

disarranged [31,32], and the electrical properties of the material are changed drastically. In order to return the resistivity and lifetime

to useful values, the crystal must be annealed. The annealing process

has been studied by a number of workers in recent years [39-42] . The

resistivity value stabilizes after an anneal of less than 1 h at 500 °C,

whereas the lifetime reaches its final value only after annealing at

ten^jeratures of 800°C or higher. The lifetime recovery appears to

depend significantly on the cleaning procedure used after the crystal

has been irradiated and before it has been annealed. Lifetimes of 50

to 100 ys can be achieved by this means. These values appear adequate, even though they are substantially less than the lifetime in typical

float-zoned silicon. Some improvement in NTD einnealed lifetime can be

expected as more is learned about making NTD silicon and its annealing

mechanisms

.

4.3 Defects

Because of the large scale of their structure, power thyristors are not as sensitive in processing to scxne small-scale surface defects

which would cause great difficulty in integrated circuit processing.

However, the residual impurity levels (particularly of heavy metals,

oxygen, and carbon) must be closely controlled. Structural defects

appear to serve as nucleation sites for clusters of random impurities,

which then can influence the distribution of the electric field in the

depletion and lead to reduced voltage breakdown characteristics.

15
Elements such as iron, copper, nickel, and other heavy metals have

been found to affect the minority carrier lifetime eind to increase the

reverse-biased leakage current of p-n junctions . These elements

probably are incorporated during the device manufacturing process and

can be controlled by maintaining suitably clean conditions. Most of

these stray impurities have been observed to collect preferentially in

the neighborhood of dislocations or other crystal imperfections where

the lattice distortions permit them to be more soluble. Careful hightemperature processing techniques reduce the incidence of processinduced structural imperfections in the silicon and thus tend to reduce

the effects of the presence of these random impurities.

Carbon has long been known to be present in semiconductor silicon

at levels sometimes exceeding a few parts per million*. Since carbon

(like silicon) is tetravalent, the assumption was generally made that it was incorporated in the silicon lattice substitutionally and contributed no electrical effects. It was regarded as an analog of a silicon

atom. In the last few years, high concentrations of carbon have been

discovered to correlate well with the existence of striations in the

crystal as well as with precipitates of a-quartz and y-Fe2^2> t43] . The presence of these precipitates is of course undesirable, and their

distribution is found to be quite nonuniform within the crystal. It is

postulated that the presence of carbon, which was grown into the crystal,

produces lattice strains which promote the formation of these precipitates by diffusion of other impurities within the crystal in such a manner as to relieve the carbon-induced strain. The nonuniform distribution of carbon has also been shown to lead to nonuniform con- centrations of gold which had been introduced to control lifetime if

the average density of carbon is greater than 10^^ cm“^. ’This implies

that the lifetime from point to point within the device will vary in an

*One part per million in silicon equals 5 x atoms per cubic

centimeter

16
uncontrolled manner. The same workers found that carbon levels of about

10^^ cm ^ cause rounded reverse blocking characteristics and that car- bon in amounts even below 5 x 10^^ cm~^ caused problems in annealing of

NTD silicon.

Studies of the effects of unwanted impurities and the mechanisms

by which these effects arise are made particularly troublesome by the

very low concentrations at which they are significant. It is desirable

to detect the presence of such impurities as oxygen and carbon at levels

well below 10^^ cm~^, but at present this is only marginally possible.

Infrared transmission measurements have been widely used for this, but

thick specimens are required. Device wafers cannot be used because

they are too thin. Transmission electron microscopy and electron

resonance techniques are used for studying radiation- induced defects

in NTD silicon and the many precipitation products which can occur. Thermally stimulated current and capacitance techniques are in use to

develop a collection of signatures of various deep-level defects in

silicon. In principle, such a catalog could be used to identify un- wanted defects to aid in process control. A good review of currently

available measurement technology is given in reference [45]. The

inability to observe and measure imperfections and impurity levels at

extremely low values has been and continues to be a limitation in

understanding and controlling the effects of such defects.

4 . 4 Cone lus ions

The ability to make devices at good yields that perform

according to their design is limited by the presence of a variety of

structural defects and unwanted impurities which are either present in

the silicon slice or are put there during wafer processing operations.

The very low concentrations at which these defects are significant

causes great difficulty in their detection and control. In many cases,

the detailed mechanisms by which these defects act to limit the

performance of the device are not known.

17
5. Processing

This section will not present a complete description of the process

for making high power thyristors but will discuss certain steps in

which difficulties occur. Manufacturing processes for these devices

employ many of the same techniques used for making integrated circuits,

but the scale of the operations is frequently quite different. Therefore, the problems which are encountered tend to be unique to the manufacture of high power devices. Furthermore, the discussion is not

intended to describe the process used by any particular manufacturer

but to be representative of such processes. Details of processes for

producing the desired junction configuration in the silicon wafer and

for assembling the wafer into its final housing are regarded as proprietary, and little published literature exists on these topics.

It should be noted that several processes which are in wide use

for making other kinds of semiconductors are rarely used in making

high-power thyristors. Such techniques as epitaxial growth, ion

implantation, and planar blocking junctions have been tried for experimental devices but have so far not been found to be useful in highvoltage, high-current thyristors for both economic and technical reasons.

5.1 Blocking Junction Formation

The first diffusion step, in which the high-voltage blocking

junctions (J1 and J2 of fig. 1) are formed, is crucial to obtaining

the desired voltage rating of the finished thyristor. The breakdown

voltage rating of the blocking junctions is directly related to the

resistivity of the starting silicon and to the gradient of the impurity

concentration at the junction. High voltage requires high resistivity

silicon (100 to 200 Q.’cm) and a shallow doping gradient, which implies

a deep junction diffusion. Junction depths of 75 to 100 ym are common. This requires long diffusion times (up to 100 h) and high temperatures,

usually 1250°C; gallium and aluminum are commonly used for the diffusing

species. It is important that no crystal damage, such as dislocations,

be introduced by this diffusion process and also that extreme cleanliness be maintained to avoid reducing the minority-carrier lifetime in

18
the silicon by inadvertent introduction of contaminating elements.

Several gettering methods are used to help reduce the effects of

stray contaminants. The silicon slices may be given a lapped surface

before diffusion. If this is done properly, this surface will develop

a superficial network of dislocations which does not penetrate the

silicon appreciably, but in which unwanted elements are more soluble.

Or, the diffusion conditions may be such that a thin liquidus exists on

the surface, in which the random impurities are more soluble. After

diffusion, the outer region is etched away.

If it is found that the junction breakdown voltage is too low or that reverse leakage currents are too high after diffusion, there are

no tests that can be used in a production environment available to

determine the cause of the problem. Some feeling for the sensitivity

required of such tests can be gained from the following: One cubic

centimeter of silicon contains 5 x 10 atoms, and if it is n-type with

a resistivity of 200 fi*cm, then 2.5 x 10^^ of those atoms have been

replaced with donor impurities. The donor concentration is thus one-half

part per billion. Significant stray impurity concentration levels are comparable

.

In the event of trouble, one would like to identify the kind of

the defect and its concentration as an aid to finding its source and

correcting the difficulty. The lack of suitable measurement techniques

means that much process control and correction in thyristor manufacturing is empirical. Very likely much of the gettering of wafers is

done purely as a safety measure and may not really be necessary.

Empiricism and unnecessary process steps are costly. It is expected

that work on thermally stimulated current now being undertaken will help

relieve this process control problem.

5.2 Blocking Junction Passivation

After all high-temperature processing has been done and the

wafer edge has been contoured cuid etched, the freshly etched edge of

the wafer must be protected against subsequent contamination by some kind of dielectric coating. Further, as described earlier, this region

19
is exposed to extremely high electric fields which would break down any

gaseous environment. It would be desirable to have a reproducible,

inorganic coating applied to the surface. Success in finding such a

material and a suitcible process for applying it has been extremely

limited. One requires a material (perhaps a glass) which can be applied

at a temperature sufficiently low that the metal—silicon contacts are

not affected, which contains no ingredients which would adversely affect

characteristics of the p-n junction, which is impervious to moisture or other deleterious fluids, and which has a reproducible amount of

electric cheirge within it. This last chciracteristic affects the interface state density at the surface of the silicon and strongly influences the distribution of the electric field at the boundary between

the silicon and the insulating film. Lacking such an inorganic

passivating layer, most manufacturers use a silicone rubber compound.

With sufficient care in characterizing the silicone rubber and in

applying it, reasonably reproducible results can be obtained. This

step remains one of the least understood and poorest controlled steps

in the manufacture of these devices, since it is based on empirical

methods only.

5.3 Lifetime Control

Nothing so far has been said about the means of controlling

the minority-carrier lifetime within the device to any specific value.

•Thyristors for line frequency operation frequently do not require any

lifetime-controlling steps to be taken. Devices which are to operate

at higher frequency generally do. Unless specific steps are taken, a large thyristor may take several tens of microseconds to turn off after

the current through it decreases to zero. In increasing numbers of

applications, thyristors are required to turn off more rapidly and

specific steps to control the lifetime must be taken. Two techniques

are in general use, diffusion of a lifetime-controlling impurity and

irradiation with electrons or gamma rays.

Gold [46] , and to a lesser extent platiniim [47] , are the impurities

generally used for lifetime control by diffusion. This diffusion

20
follows the emitter diffusion and precedes the contact metallization

steps. This operation is difficult to control, because one does not

know the lifetime in the device at that stage. Before the lifetimecontrolling diffusion is done, one needs to know how much additional

lifetime-controlling impurity to add to achieve the desired result. Too

much gold, for instance, will raise junction leakage currents to an intolerable degree and adversely affect the conducting voltage drop.

Too little gold will fail to reduce the lifetime to the desired level.

The way this problem is dealt with in practice is to select a sample of a few wafers from a processing lot, give them a diffusion for

lifetime control under conditions based on exp>erience, and then complete

the processing of the samples (metallization, backup plate attachment,

beveling, and passivation) so that their turnoff time can be measured.

The remainder of the lot is held aside until the results of the measurement on the sample are available. Then a final selection of the

conditions for the lifetime-controlling diffusion is made and the

balance of the lot is processed.

There are obvious disadvantages to this procedure. The main part

of the lot is delayed, which adds to the cost of inventory. It is

assumed that the lot is uniform enough that the samples are representative, and that the process is under sufficient control that the

diffusion on the sample and then the diffusion of the main lot will behave in the same way, even if a period of a few days or weeks has

elapsed. It would be far better to have a way of measuring lifetime or another parameter correlated with turn-off time prior to the

lifetime-controlling diffusion, but the means to do this are not

available

.

Radiation techniques [48, 49] for lifetime adjustment offer the

advantage that the electrical properties of the device can be determined

before irradiation and the radiation dose tailored to produce more

accurately the desired control over lifetime. For this reason, radiation is being used more frequently today than in the past, in spite of

the industry's relatively limited experience with it. One disadvantage

to radiation control of lifetime is that the facilities to do this are

21
not available in most semiconductor manufacturing houses, due to the

expense and the problems of managing a radioactive facility. One is

therefore faced with the problem of moving semi-finished, expensive

materials to another place, frequently under the control of others,

for a critical process step.

5.4 Conclusions

Development is continuing on refinements of the shape of the

edge of the wafer to control surface electric fields without undue

sacrifice of current-carrying area. Major efforts are req[uired to

reduce the empiricism now needed for effective passivation of the

blocking junctions around the periphery of the device and for the

execution of lifetime-controlling diffusion. Large cost improvements

could result from reduction of the need for a hermetic external package.

Much-improved precision in process control would pay large dividends

especially in the manufacture of thyristors for HVDC equipment. The

design specifications for these devices have very tight tolerances

imposed by the need for proper current and voltage sharing among hundreds of series-parallel connected thyristors. Improved in-process

measurement will improve yields and thus reduce thyristor costs which

comprise a large part of the cost of HVDC terminals.

6. Assembly and Electrical Test

Because the blocking junctions are passivated with a material which

is not impervious to water, the thyristor subassembly is packaged in . a hermetic housing. This is a relatively complex metal and ceramic

assembly, providing for high-current electrical connections and largearea thermal connections as previously described. In addition,

arrangements must be made for a low-current connection for the gate

signal. The final seal for the assembly is done by a peripheral weld,

joining two metal flanges together. This can be done either as a copper

compression weld or by a helium arc-welding process. Two problems

exist. First, the assembly must be truly hermetic. Thyristors are used

in industrial equipment with a design life of many years. One function

of the hermetic housina is to maintain a dry, contaminant-free

22
environment for the thyristor element itself. Since a typical device

housing may contain a half-dozen independently made joints, the chances

of occurrence of a leak are considerable. Si±>assemblies of the

housing must be checked by helium mass spectrometry for freedom from

leaks, and the finished assembly must be checked as well. The assurance of hermeticity has always been a difficult problem in the

semiconductor industry.

The second problem is that of cost. The housing is made of a precisely formed high-alumina ceramic, nickel-iron parts, and copper

contact pieces. If it were possible to find a suitable inorganic

jionct ion-coating material such as that described in section 5, the

large and expensive housing would not be required; a molded plastic

encapsulation could be devised which would be suitable. At present,

this is not the case, and a silicon wafer which, in its finished form,

is worth a few tens of dollars is the active part of an assembly whose

cost is a few hundred dollars.

Electrical characterization of high power thyristors is described

in Chapter 4 of reference [2]. These principles have not changed. significantly, but the body of data gathered from field experience, which

is useful to determine the applicability of these characterization

methods, has grown [50]. Work on improving the techniques of characterizing thyristors is continuing, particularly with respect to thermal

[51, 52] and electrical [53] transients which are important in large

systems

.

With the continuing increase in the size and power-handling

capability of thyristors, the magnitude of the investment in test

equipnent has increased substantially. Equipment to test these devices

is all custom designed and is therefore expensive. In evaluating a new device design, a large number of specimens must be tested to destruction.

This is also costly. There are no adequate predictors of life, so devices must be tested under maximum rated conditions for long periods

to be sure that no failure mechanism is present which will cause the

life of the device to fall short of the equipment design life.

23
At present, there seems little prospect that technic^es of product

evaluation and quality control will change significantly in the future.

7 . Sxjtimary

The state of the art of high power thyristor technology has been

reviewed. In recent years, three principal developments have spurred

the development of these devices to voltage and current levels

significantly above those existing previously. These are:

a) the development of neutron-transmutation-doped silicon as a commercial material;

b) improved, but still incomplete, understanding of the sources

of defects in silicon, including the formation of oxygen

donors, the association of carbon with striations in silicon,

and the role of carbon in the formation of precipitates of

other impurities; and,

c) most significantly, the development of detailed computer

models which permit the analysis of existing devices and the

design of new ones with a degree of accuracy impossible until

only recently.

None of the above advances can be considered as finished work, but they

are significant developments over the state of knowledge of, say, a half -decade ago.

Many problems remain. In device design, the use of the most

advanced computer models remains within those firms which have

developed them; the availability of these tools to the device manufacturing industry is by no means uniform. The models need to be applied

to the design of advanced gating arrangements , such as optically

sensitive ones, where the available energy to stimulate the thyristor

is limited and where (in HVDC) the thyristor's other characteristics

cannot be traded off in favor of high gate sensitivity. Improved

thyristor performance can be obtained by reducing the thermal impedance

of the cooling arrangements.

The next step in thyristor current ratings awaits the more general

24
availability of high-quality 100-mm diameter silicon. The level of

carbon in all float-zoned silicon must be reduced and its distribution

in the crystal made more un'iform. The lifetime of NTD silicon must be

raised and the annealing mechanisms understood more completely.

Processing improvements revolve principally around the removal of

empiricism as a process control tool. The control of process-induced

defects and unwanted impurities, the techniques of lifetime control,

and the passivation of the blocking junctions all would be improved if

more objective techniques were available. Significant package cost

reduction also requires advancement in passivation methods

.

Many of these problems will yield only with the aid of new or improved measurement techniques. Methods are needed for the detection

and identification of the whole spectjrum of defects: crystalline

imperfections down to single-atom vacancies, unwanted elements

(especially carbon), and precipitates of various kinds. Better means

of lifetime control can only be developed if new lifetime measurement

tools are available, preferably with good spatial resolution.

Measurement methods which are useful not only in the laboratory but in

a factory environment are of great potential value.

It is particularly desirable from a national viewpoint to have new measurement technology available uniformly to the entire industry.

Because of this fact, and also because the resources needed for this

kind of development exist in only a few of the largest firms in the

industry (whose motivation to develop new measurement technology has

been rather modest in the past) , it seems appropriate to consider a significant governmental role in the development of the needed measurement tools.

Acknowledgment

Figures 3, 4, and 8 are reproduced by permission of the Institute

of Electrical and Electronics Engineers, Inc. W. M. Bullis and F. F.

Oettinger provided a number of helpful and welcome suggestions for

improvement of this study.

25
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26
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.

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.

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27
26 . Temple, V. A. K. , Directly Light-Fired Thyristors With High di/dt

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28
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4

, 685-688 (1977).

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29
50 . Comstock, W. R. , and Locher, R. E. , High Current Diode and SCR

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Press Packages, PESC '78 Record, IEEE Power Electronics Specialists

Conference, Syracuse, NY, June 13-15, 1978, to appear.

• 30
Table 1

Manufacturers of Thyristors Rated > 200 A, 2000 V

U. S. Firms

General Electric Co.

International Rectifier Corp.

National Electronics, Inc. (division of Varian)

Power Semiconductors, Inc.

Westinghouse Corp.

European Firms

AEG-Telefunken

Almanna Svenska Elektriska AB (ASEA)

Associated Electrical Industries, Ltd.

Brown, Boveri et Cie AG

CGEE Als thorn

Semikron

Siemens AG

Westinghouse Brake and Signal Co. Ltd.

Japanese Firms

Hitachi, Ltd.

Mitsubishi Electric Corp.

Nippon Electric Co., Ltd.

Tokyo Shibaura Electric Co., Ltd. (Toshiba)

Companies visited by Electron Devices Division staff members during

1976 and 1977.

31
Table 2

* Thermal Expansion and Conductivity Data

Expansion Conductivity

Material ppm/®C W/cm*K

Ag 19.6 4.18

Cu 17.3 4.0

A1 24.1 2.38

BeO sintered, d = 2.8 5.3 2.3

W 4.6 1.7

Si 2.8 1.5

Mo 5.1 1.4

Ni 13.2 0.9

Sn 22.6 0.65

Low alloy steel (SAE 1020) 16.6 0.65

AI 2 O 3 , sintered, d = 3.8 6.2 0.40

MgO, sintered 11.7 0.40

18-8 stainless steel 16.4 0.14

Si02 , fused 0.56 0.014

From American Institute of Physics Handbook, second ed., McGraw-Hill,

1963.

32
CATHODE GATE

pi !

ANODE

Figure 1. Cross-section of a thyristor in

the forward-blocking off-state, showing the

junction configuration and the depletion

layer around junction J2.

Figure 2. Equipotential lines in a reverse- biased p~^-n junction with a contoured surface

(the relative scale is four vertical to one horizontal)

.

33
Figure 3a. Experimental and theoretical I-V curves for a 1200-V triac.

Figure 3b. Experimental and theoretical I-V curves for a 3500-V thyristor.

34
Figure 3c. Experimental and theoretical I-V curves for a 2500-V thyristor.

0 12 3 4 5 6 VOLTS Figure 4. Theoretical curve which illustrates

the relative importance of the various physical

mechanisms, including heat flow, on the I-V

characteristics

.

35

I
CATHODE GATE

ANODE

Figure 5. p-n-p-n device with shorted emitter.

1 2 3

4

5

1 WELDED JOINT 4 GATE CONNECTION

2 THYRISTOR SUBASSEMBLY 5 CERAMIC BODY

3 CONDUCTORS

Figure 6. A thyristor for double-sided cooling. The

conductors form an integral part of the capsule.

36
RESISTIVITY

p

(Qcm)

RtsiSTiviTY

^

(acnti

1 AUXILIARY CATHODE 4 HEAT SINKS

2 GATE CONNECTION 5 THYRISTOR DEVICE

3 COMPRESSION CLAMP

Figiire 7. Thyristor with double-sided air-cooled heat

sinks.

c^ 100 '•.•'/'O'. . /V

4.

H 100 >

*/ / ^ •

CCJITEW

t

I I I I I l i 0 4 a 12 46 < 20 24 26 32 36

t>tST4NCC AL0M6 WAFER OUMETER (mm)

(a)

(b)

Figure 8. Spreading resistance measurements in silicon slices; distance between measurement points 20 -m.

(a) Diameters across 1 1/2-in slices of 200-:7*cm n- type FZ (curve a) and neutron transmutation doped

silicon (curve b) . (b) Radius of a 57-mm slice after

neutron irradiation and annealing, showing small re- sidual striations and variations , due to too small

differences between initial and final resistivities.

37
NBS-114A (REV. S-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 78-1559

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE 5. Publication Date

An Assessment of High Power Thyristor Tecnnoiogy 6. Performing Organization Code

7. AUTHOR(S)

Robert I . Scace

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7216442

11. Contract /Grant No. EA-77-A-01-6010

; A021-EES amend.

2

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of Energy

13. Type of Report & Period Covered

Summary - no dates

Division of Electric Energy Systems

Washington, DC 20545

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

This work was conducted as a part of the Semiconductor Technology Program in the

Electron Devices Division of NBS

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) The current state of the technology for designing and maUcing high-power thyristors

is reviewed. The discussion is confined primarily to thyristors having voltage

ratings of 2000 V or greater, and current ratings above a few hundred amperes.

Particular attention is given to describing the factors which limit the performance

of such devices and to discussing recent work aimed at solving these problems

.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only die first letter of the first key word unless a proper name, separated by semicolons) current switches ; high voltage switches ; SCR; semiconductoji

devices; semiconductor measurements; silicon devices; silicon controlled rectifiers;

solid-state switches; technology assessment; thyristors.

18. AVAILABILITY Unlimited

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