Brief Information on Semiconductor Thermoresistors Used for Determining the Temperature of Measuring Wires

G. N. Gushchin

(Presented by the scientific seminar of the departments of geophysical prospecting methods, geodesy, and mine surveying)

A. Information on semiconductor thermoresistors. Thermosensitive resistors (TS) are bulk semiconductor resistors whose magnitude decreases sharply as temperature increases. Whereas in metals the temperature coefficient of resistance \(\alpha\) is \(0.4\%—0.5\%\) per \(1^\circ\mathrm{C}\), in thermoresistors this coefficient is \(3\%\div -6\%\) per \(1^\circ\mathrm{C}\).

The strongly expressed dependence of the resistance of a TS on temperature makes it convenient for use as a temperature sensor. The advantages of thermoresistors are:

1. High temperature sensitivity. Figure 1 shows the nature of the dependence of resistance on temperature for a TS (1) and a metallic conductor (2). This graph indicates that semiconductor temperature sensors make it possible to obtain, in comparison with metals, a higher order of temperature sensitivity. As an example, the following data may be cited: when the temperature is raised from 0 to \(100^\circ\mathrm{C}\), the resistance of platinum increases by 35%, whereas in semiconductors it decreases by 20–70 times (depending on the temperature coefficient of the TS) [1].

Fig. 1. 1 — platinum lead wires. 2 — MT-54 thermoresistors. 3 — glass ampoule.

1. Small dimensions and, consequently, low thermal inertia of the TS. The degree of thermal inertia is judged by the value of the so-called time constant. The time constant is taken to be the time during which the temperature of the TS rises by 63% when it is transferred from an air medium at a temperature of \(0^\circ\mathrm{C}\) to an air medium at a temperature of \(100^\circ\mathrm{C}\).

2. Extreme simplicity of the device (for example, Fig. 3).

3. High stability of TS characteristics over time. To illustrate the stability of TS, Fig. 2 gives the characteristics of the change in resistance after heating thermoresistors of two main types currently manufactured by industry in various structural designs, namely: TS KMT-1 and MMT-4.

The graphs (Fig. 2) indicate the high stability of the thermistor, which at the maximum operating temperatures does not exceed 1.5%.

1. Durability and the complete absence of any need for special maintenance during operation.

2. With the aid of a thermistor it is possible to organize remote and centralized temperature measurement reliably and simply.

Fig. 2

The large resistance value of the thermistor makes it possible to completely disregard the resistance of the lead wires, contact resistances, and contact EMFs, which makes it possible to measure temperature at a considerable distance from the measuring panel.

That interruption of the integrity of the contact of the lead wires and a change in the value of the resistance of the latter do not have a noticeable effect on the readings of the thermistor can be verified by the following simple experiment. Let us cut one of the lead wires and then reconnect it. We shall then find that the readings of the thermistor do not change. Next, we heat the same lead wire with an alcohol lamp at the point of contact (connection), while leaving the second lead wire at room temperature. The readings of the thermistor do not change even when the first lead wire is placed in the flame of the alcohol lamp.

The temperature dependence of resistance is the principal characteristic of thermal resistors. With increasing temperature in semiconductors, the mobility of current carriers—electrons and holes—increases sharply. At the same time the resistance decreases just as sharply. The dependence of the resistivity \(\rho\) of semiconductors on temperature over a not too wide temperature interval can always be expressed by the exponential law

\[ \rho = A_{\text{sp}} e^{\frac{B}{T}}, \tag{1} \]

where \(A_{\text{sp}}\) and \(B\) are constant quantities depending on the physical properties of the material;

\(T\) is the temperature in degrees of the absolute scale.

It follows from this that the magnitude of the electrical resistance \(R_T\) (in ohms) at temperature \(T\) can be determined as

\[ R_T = A e^{\frac{B}{T}}, \tag{2} \]

where the constant

\[ A = A_{\text{sp}} \frac{l}{S} \tag{3} \]

depends on the physical properties of the material and the dimensions of the thermistor; \(l\) is the distance between the electrodes in cm;

\(S\) is the cross-sectional area in sq. cm.

The value of the constant \(B\) is determined experimentally by measuring the resistance of the TR at two temperatures \(T\) and \(T_0\)

\[ B=\frac{2.303 \Delta \lg R}{\Delta \frac{1}{T}}, \tag{4} \]

where

\[ \Delta \lg R=\lg R_T-\lg R_{T_0} \]

and

\[ \Delta \frac{1}{T}=\frac{1}{T}-\frac{1}{T_0}. \]

If the temperature coefficient of resistance \(\alpha\) is defined as is usually accepted,

\[ \alpha_T=\frac{1}{R}\frac{dR}{dT}, \tag{5} \]

then from equation (2) it follows that \(\alpha_T=-\frac{B}{T^2}\).

Thus, the value of \(\alpha\) is determined by the value of the constant \(B\), with the temperature coefficient inversely proportional to the square of the absolute temperature of the TR; i.e., as the temperature increases, the absolute value of \(\alpha\) decreases. This character of the change in resistance is inherent in all thermoresistors, regardless of their design.

Very often TRs are hermetically sealed, as a result of which they can be used under conditions of increased humidity and in liquids.

In thermistors used for determining the temperatures of measuring wires ([4], [5]), the temperature sensor is the MT-54 semiconductor microthermoresistor, proposed by V. G. Karmanov.

Structurally, the body of the MT-54 resistor is a cylinder (Fig. 3), with cross-sectional dimensions of \(0.3 \times 0.5\) mm, made from a mixture of oxides of several metals: manganese, cobalt, titanium, nickel, etc. The thermoresistor is sealed into a glass bulb whose outer diameter is 0.8 mm.

Fig. 3

The main technical characteristics of the MT-54 are as follows: nominal ohmic resistance at \(0^\circ\text{C}\)—4016 ohms, at room temperature—800 ohms; temperature coefficient at \(20^\circ\text{C}\), 3.6%; time constant equal to 10 seconds; permissible temperature-measurement range from \(-70^\circ\text{C}\) to \(+150^\circ\text{C}\).

B. Remarks concerning the manufacturing technology of MT-54 semiconductor thermoresistors. During the period of investigating the possibility of using semiconductor thermoresistors as thermometers, we used MT-54-type microthermoresistors produced first by the experimental workshops of the Leningrad Agrophysical Institute (5 specimens), and then by the Leningrad Experimental Plant of Semiconductor and Ultrasonic Instruments (20 specimens).

The results of the investigations carried out showed that, in the manufacture of MT-54 resistors, it is necessary:

1. First of all, to insulate the platinum contact leads (1, Fig. 3). The fact that the contact leads of the resistors have not yet been insulated, despite their serial production, creates certain inconveniences in their practical use.

2. One of the most important characteristics of a thermistor is its aging characteristic, which shows how constant the resistance of the thermistors remains over time at a given temperature.

The clear majority of the MT-54 resistors with which we had to work were poorly aged, which adversely affects the stability of their readings and makes it necessary to carry out additional obligatory aging.

We carried out additional thermal aging of the MT-54 resistors by heating them in a physical thermostat to 50°C for 4–5 days, after which the stability of the resistors increased sharply. But for individual MT-54 resistors such additional thermal aging alone proved insufficient. This was revealed as follows. When calibrating the instrument by the method described in [6], after determining the position of the zero of the temperature scale, a gradual (sometimes continuous, sometimes stepwise) drift suddenly began to be observed; its magnitude reached 4–7°C of the previously calibrated scale. Only later, after repeated calibration (5–7 times), i.e., after preliminary passage of current through the resistors and the accompanying change in its temperature (heating and cooling), did the position of the zero of the scale and of individual degrees become stable. This phenomenon gives grounds to suppose that artificial aging of MT-54 resistors should be carried out under those regimes and loads under which the given thermal resistor will operate.

Thus, artificial aging of MT-54 thermal resistors intended for use as a sensor in recording the temperature of a measuring wire should be carried out in two stages. In the first stage of aging, the thermal resistor must be kept at +50°C for 4–5 days. In the second stage of aging, repeated calibration of the thermistor should be performed by the method described in [6]. It is recommended to repeat the calibration of the thermistor until the last calibration temperature scales agree, within the specified accuracy, with the initial ones.

Such a procedure makes it possible to ascertain how constant the thermistor readings remain over time when its temperature changes, and at the same time makes it possible to refine the temperature scale.

LITERATURE

1. Thermal resistors. Semiconductor resistors with a large temperature coefficient. 1957.

2. I. T. Sheftel. Semiconductor heat-sensitive resistors. 1956.

3. The use of semiconductors in instrument making. Proceedings of the conference, edited by N. I. Chistyakov. Mashgiz. Moscow, 1958.

4. M. V. Postnikov, Ya. V. Shvartsman. Instrument for determining the temperatures of measuring wires (second model). Collection of Works of TISI, Vol. IV, 1958.

5. M. V. Postnikov, G. N. Gushchin. Determination of the temperature of measuring wires by a semiconductor micro-thermal-resistor. Proceedings of Higher Educational Institutions—Geodesy and Aerial Photography, Issue 4, Moscow, 1964.

6. G. N. Gushchin. Method for calibrating thermistors used to determine the temperature of measuring wires. Izv. TPI, Vol. 118; 1963.