Littelfuse AN9767.1 Application Note

Littelfuse Varistors - Basic Properties,

Terminology and Theory

What Is A Littelfuse Varistor?

Varistors are voltage dependent, nonlinear devices which
have an electrical behavior similar to back-to-back zener
diodes. The symmetrical, sharp breakdown characteristics
shown in Figure 1 enable the varistor to provide excellent
transient suppression performance. When exposed to high
voltage transients the varistor impedance changes many
orders of magnitude from a near open circuit to a highly
conductive level, thus clamping the transient voltage to a
safe level. The potentially destructive energy of the incoming
transient pulse is absorbed by the varistor, thereby
protecting vulnerable circuit components.

PER VERT

DIV 1mA

PER HORIZ
DIV 50V

I

PER STEP

gm PER DIV

9991 yluJetoN noitacilppA

Littelfuse Varistors are available with AC operating voltages
from 2.5V to 6000V. Higher voltages are limited only by
packaging ability. Peak current handling exceeds 70,000A
and energy capability extends beyond 10,000J for the larger
units. Package styles include the tiny multilayer surface
mount suppressors, tubular devices for use in connectors,
and progress in size up to the rugged industrial device line.

AN9767.1

Physical Properties

Introduction

An attractive property of the metal oxide varistor, fabricated
from zinc oxide (ZnO), is that the electrical characteristics
are related to the bulk of the device. Each ZnO grain of the
ceramic acts as if it has a semiconductor junction at the
grain boundary. A cross-section of the material is shown in
Figure 2, which illustrates the ceramic microstructure. The
ZnO grain boundaries can be clearly observed. Since the
nonlinear electrical behavior occurs at the boundary of each
semiconducting ZnO grain, the varistor can be considered a
“multi-junction” device composed of many series and parallel
connections of grain boundaries. Device behavior may be
analyzed with respect to the details of the ceramic
microstructure. Mean grain size and grain size distribution
play a major role in electrical behavior.

V

FIGURE 1. TYPICAL VARISTOR V-I CHARACTERISTIC

The varistor is composed primarily of zinc oxide with small
additions of bismuth, cobalt, manganese and other metal
oxides. The structure of the body consists of a matrix of
conductive zinc oxide grains separated by grain boundaries
providing P-N junction semiconductor characteristics. These
boundaries are responsible for blocking conduction at low
voltages and are the source of the nonlinear electrical
conduction at higher voltages.

Since electrical conduction occurs, in effect, between zinc
oxide grains distributed throughout the bulk of the device, the
Littelfuse Varistor is inherently more rugged than its single P­N junction counterparts, such as zener diodes. In the
varistor, energy is absorbed uniformly throughout the body of
the device with the resultant heating spread evenly through
its volume. Electrical properties are controlled mainly by the
physical dimensions of the varistor body which is sintered in
various form factors such as discs, chips and tubes. The
energy rating is determined by volume, voltage rating by
thickness or current ow path length, and current capability
by area measured normal to the direction of current ow.

100µ

FIGURE 2. OPTICAL PHOTOMICROGRAPH OF A POLISHED

AND ETCHED SECTION OF A VARISTOR

10-89

1-800-999-9445 or 1-847-824-1188 | Copyright © Littelfuse, Inc. 1999

Application Note 9767

Varistor Microstructure

Varistors are fabricated by forming and sintering zinc
oxide-based powders into ceramic parts. These parts are
then electroded with either thick film silver or arc/flame
sprayed metal. The bulk of the varistor between contacts is
comprised of ZnO grains of an average size “d” as shown in
the schematic model of Figure 3. Resistivity of the ZnO is
<0.3 Ω -cm.

CURRENT

FIGURE 3. SCHEMATIC DEPICTION OF THE

MICROSTRUCTURE OF A METAL-OXIDE
VARISTOR. GRAINS OF CONDUCTING ZnO
(AVERAGE SIZE d) ARE SEPARATED BY
INTERGRANULAR BOUNDARIES

Designing a varistor for a given nominal varistor voltage, V
is basically a matter of selecting the device thickness such
that the appropriate number of grains, n, are in series
between electrodes. In practice, the varistor material is
characterized by a voltage gradient measured across its
thickness by a specific volts/mm value. By controlling
composition and manufacturing conditions the gradient
remains fixed. Because there are practical limits to the range
of thicknesses achievable, more than one voltage gradient
value is desired. By altering the composition of the metal
oxide additives it is possible to change the grain size “d” and
achieve the desired result.

A fundamental property of the ZnO varistor is that the
voltage drop across a single interface “junction” between
grains is nearly constant. Observations over a range of
compositional variations and processing conditions show a
fixed voltage drop of about 2V-3V per grain boundary
junction. Also, the voltage drop does not vary for grains of
different sizes.

It follows, then, that the varistor voltage will be determined by
the thickness of the material and the size of the ZnO grains.
The relationship can be stated very simply as follows:

ELECTRODES

INTERGRANULAR
BOUNDARY

d

N

Varistor Voltage, V

Where, n = average number of grain boundaries

and, varistor thickness, D = (n + 1)d

where, d = average grain size

The varistor voltage, V

(DC) = (3V)n

N

between electrodes

VNd×

-----------------

≈

3

, is defined as the voltage across a

N

varistor at the point on its V-I characteristic where the transition
is complete from the low-level linear region to the highly
nonlinear region. For standard measurement purposes, it is
arbitrarily defined as the voltage at a current of 1mA.

Some typical values of dimensions for Littelfuse Varistors are
given in Table 1.

TABLE 1.

VARISTOR

VOLTAGE

VOLTS MICRONS

150V

RMS

25V

RMS

NOTE: Low voltage formulation.

AVERAGE

GRAIN SIZE

20 75 150 1.5

80 (Note) 12 39 1.0

GRADIENT

V/mm AT

n

1mA mm

DEVICE

THICKNESS

Theory of Operation

,

Because of the polycrystalline nature of metal-oxide
semiconductor varistors, the physical operation of the device
is more complex than that of conventional semiconductors.
Intensive measurement has determined many of the device’s
electrical characteristics, and much effort continues to better
define the varistor's operation. In this application note we will
discuss some theories of operation, but from the user’s
viewpoint this is not nearly as important as understanding
the basic electrical properties as they relate to device
construction.

The key to explaining metal-oxide varistor operation lies in
understanding the electronic phenomena occurring near the
grain boundaries, or junctions between the zinc oxide grains.
While some of the early theory supposed that electronic
tunneling occurred through an insulating second phase layer
at the grain boundaries, varistor operation is probably better
described by a series-parallel arrangement of
semiconducting diodes. In this model, the grain boundaries
contain defect states which trap free electrons from the
n-type semiconducting zinc oxide grains, thus forming a
space charge depletion layer in the ZnO grains in the region
adjacent to the grain boundaries [6].

Evidence for depletion layers in the varistor is shown in
Figure 4 where the inverse of the capacitance per
boundary squared is plotted against the applied voltage per
boundary [7]. This is the same type of behavior observed

10-90

Application Note 9767

for semiconductor abrupt P-N junction diodes. The
relationship is:

2V

1

-------

2

C

Where V

V+()

b

--------------------------=

qε sN

is the barrier voltage, V the applied voltage, q the

b

electron charge, ε s the semiconductor permittivity and N is
the carrier concentration. From this relationship the ZnO
carrier concentration, N, was determined to be about
2 x 10

17

per cm

3

[7]. In addition, the width of the depletion
layer was calculated to be about 1000 Angstrom units.
Single junction studies also support the diode model [9].

It is these depletion layers that block the free flow of carriers
and are responsible for the low voltage insulating behavior in
the leakage region as depicted in Figure 5. The leakage current
is due to the free flow of carriers across the field lowered
barrier, and is thermally activated, at least above about 25

(1014)

/cm

4

4

3

2

0 0.4 0.8 1.2

V

PER BOUNDARY

A

VARISTOR RESEMBLES A SEMICONDUCTOR
ABRUPT-JUNCTION REVERSED BIASED DIODE
Nd ~ 2 x 10

17

3

/cm

1

-------------

2n2

c

FIGURE 4. CAPACITANCE-VOLTAGE BEHAVIOR OF

o

C.

Figure 5 shows an energy band diagram for a ZnO-grain
boundary-ZnO junction [10].
biased, V

, and the right side is reverse biased to V

L

depletion layer widths are X
barrier heights are φ
is φ

. As the voltage bias is increased, φ

O

φ

is increased, leading to a lowering of the barrier and an

R

L

The left-hand grain is forward

and X

L

and φ

. The zero biased barrier height

R

, and the respective

R

is decreased and

L

. The

R

increase in conduction.

The barrier height φ

of a low voltage varistor was measured

L

as a function of applied voltage [11], and is presented in
Figure 6. The rapid decrease in the barrier at high voltage
represents the onset of nonlinear conduction [12].

φ

φ

B

0

V

E

C

E

E

E

V

FIGURE 5. ENERGY BAND DIAGRAM OF A

LφL

f

X

L

I

δ

ZnO-GRAINBOUNDARY-ZnO JUNCTION

φR

V

X

R

φ

F

0

R

Transport mechanisms in the nonlinear region are very
complicated and are still the subject of active research. Most
theories draw their inspiration from semiconductor transport
theory and the reader is referred to the literature for more
information [3, 5, 13, 14, 15]

o

φ

1.0

⁄

L

φ

0.8

0.6

0.59=

o

0.4

φ

0.2

0

NORMALIZED THERMAL BARRIER

0

FIGURE 6. THERMAL BARRIER vs APPLIED VOLTAGE

.

4

8

VOLTAGE (V)

12

16

Turning now to the high current upturn region in Figure 10,
we see that the V-I behavior approaches an ohmic
characteristic. The limiting resistance value depends upon
the electrical conductivity of the body of the semiconducting
ZnO grains, which have carrier concentrations in the range

17

of 10

to 10

18

per cm

3

. This would put the ZnO resistivity

below 0.3 Ω cm.

Varistor Construction

The process of fabricating a Littelfuse Varistor is illustrated in
the flow chart of Figure 7. The starting material may differ in
the composition of the additive oxides, in order to cover the
voltage range of product.

10-91

Application Note 9767

ZnO

MIXING

POWDER

PRESS

SINTER

ELECTRODE

MECHANICAL

ASSEMBLY

ENCAPSULATE

FIGURE 7. SCHEMATIC FLOW DIAGRAM OF LITTELFUSE

ADDITIVE OXIDES

(MAINLY BL

FINAL PRODUCT TO
ELECTRICAL TEST

VARISTOR FABRICATION

203

)

POWDER PREPARATION

FORM CERAMIC BODY

PACKAGE AS/IF REQUIRED

Device characteristics are determined at the pressing
operation. The powder is pressed into a form of
predetermined thickness in order to obtain a desired value of
nominal voltage. To obtain the desired ratings of peak
current and energy capability, the electrode area and mass
of the device are varied. The range of diameters obtainable
in disc product offerings is listed here:

Nominal Disc
Diameter - mm

3 5 7 10 14 20 32 34 40 62

Of course, other shapes, such as rectangles, are also
possible by simply changing the press dies. Other ceramic
fabrication techniques can be used to make different shapes.
For example, rods or tubes are made by extruding and
cutting to length. After forming, the green (i.e., unfired) parts
are placed in a kiln and sintered at peak temperatures in
excess of 1200

o

825

C, assisting in the initial densification of the

o

C. The bismuth oxide is molten above

polycrystalline ceramic. At higher temperatures, grain growth
occurs, forming a structure with controlled grain size.

Radials are also available with phenolic coatings applied
using a wet process. The PA series package consists of
plastic molded around a 20mm disc subassembly. The RA,
DA, and DB series devices are all similar in that they all are
composed of discs or chips, with tabs or leads, encased in a
molded plastic shell filled with epoxy. Different package
styles allow variation in energy ratings, as well as in
mechanical mounting. Figures 8 and 9 illustrate several
package forms.

Figure 9 shows construction details of some packages.
Dimensions of the ceramic, by package type, are given in
Table 2.

TABLE 2. BY-TYPE CERAMIC DIMENSIONS

PACKAGE

TYPE SERIES CERAMIC DIMENSIONS

Leadless
Surface Mount

Connector Pin CP Series 22, 20, 16 ID Gauge Tube

Axial Leaded MA Series 3mm Diameter Disc

Radial
Leaded

Boxed, Low
Profile

Industrial
Packages

Industrial Discs CA, NA Series 32mm, 40mm, 60mm

Arrester AS Series 32mm, 42mm, 60mm

Littelfuse multilayer suppressor technology devices.

†

CH, AUML
ML

†

MLN

ZA, LA, “C” III,
UltraMOV™
Series

RA Series 5mm x 8mm, 10mm x 16mm,

PA Series
HA Series
HB Series
DA, DB Series
BA, BB Series

, MLE

Series

†

,

5mm x 8mm Chip, 0603, 0805,

†

1206, 1210, 1812, 2220

†

5mm, 7mm, 10mm, 14mm,
20mm Diameter Discs

14 x 22 Chips

20mm Diameter Disc
32mm, 40mm Diameter Disc
34mm Square Disc
40mm Diameter Disc
60mm Diameter Disc

Diameter Discs, 34mm Square

Diameter Discs

Electroding is accomplished, for radial and chip devices, by
means of thick film silver fired onto the ceramic surface. Wire
leads or strap terminals are then soldered in place. A
conductive epoxy is used for connecting leads to the axial
3mm discs. For the larger industrial devices (40mm and
60mm diameter discs) the contact material is arc sprayed
aluminum, with an overspray of copper if necessary to give a
solderable surface.

Many encapsulation techniques are used in the assembly of
the various Littelfuse Varistor packages. Most radials and
some industrial devices (HA Series) are epoxy coated in a
fluidized bed, whereas epoxy is “spun” onto the axial device.

10-92