Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 187

TABLE 4.2 Classification of Thermistors

Clarification of Application

Thermistor Characteristics Temperature

NTC thermistor
(negative-
temperature-
coefficient thermistor)

NTC thermistor has the negative
temperature coefficient with
semilog characteristic.

Medium temperature
use -50-300°C
Low-temperature
use -100-0°C
High-temperature
use 200-700°C

R

T

Log R

1/ T

PTC thermistor
(positive-temperature-
coefficient thermistor)

PTC thermistor has positive
temperature coefficient with
the switching behaviors.

-50-150°C

R

T

CTR (critical-
temperature resistor)

CTR has the negative temperature
coefficient with the switching
behaviors.

0-150°C

R

T

when temperature rises. Among the various kinds of semiconductors, a ther-
mistor has the appropriate resistivity, temperature coefficient, and stability.
The origin of the name thermistor stems from the fact that the oxidic com-
pound sintered material was developed at Bell Laboratories in the United
States and was called a thermal-sensitive resistor , announced as a thermistor.
Then, Philips, in the Netherlands, developed a positive-temperature-coefficient
thermistor, with its electric resistance suddenly increasing at some tempera-
ture. Further, in Japan, the critical-temperature resistor (CTR) thermistor, with
its electric resistance suddenly decreasing, was developed. Therefore, the ther-
mistor with an electrical resistance increase is called a positive-temperature-
coefficient thermistor (PTC), while the conventional-type thermistor with an
electrical resistance decrease is called a negative-temperature-coefficient ther-
mistor (NTC). The characteristics of these thermistors are given in Table 4.2.

Principle of NTC Thermistor In this section, we describe in detail the ther-
mistor thermometer, which has been mainly used for clinical thermometry. The