TEMPERATURE SENSOR

Abstract: Temperature sensor based on temperature dependence of electric charge carrier traps in the energy band of a solid material, which is subjected to the temperature to be measured. The charge carrier traps are first filled with electrical charges. Thereafter a quantity having a predetermined relation to the emitted charge or to the rest of the charge is measured. The change in the measured quantity has a time constant corresponding to a given temperature. The time constant is indicated and the temperature is calculated.

Description: TEΠPERATURE SENSOR Background of the invention This invention relates to a temperature sensor based on temperature dependence of electric charge emission from charge carrier traps in the energy bandgap of a solid ma- terial, which is subjected to the temperature to be measure For many applications of temperature measurements, it is of great interest to be able to use a method which does not re ly on a calibration procedure and combines good reproducibi with long term stability. Contemporary temperature sensors, which use electrical methods, e.g. thermocouples, thermistors, and pn-diodes give signals magnitude of whichis a function of the temperat Changes in the contact resistance and/or aging phenomena alter the signal level and severe errors may be introduced in the temperature measured. The present invention, given by the characteristics stated in patent claim No 1 , is an improvement compared with the sensors dicsussed above. Further improvements are given by the additional patent claims. Summary of the invention A semiconductor or insulator which has an energy bandgap is preferably used as a sensor element. The sensor ele¬ ments are made in such a way that impurity centers, acting as charge carrier traps, and at leastd one space charge region are incorporated in the material. The traps are fileld with electrical charges, electrons or holes, by "pumping" the sensor element by applying an electrical pulse e.g. by short circuiting a reversed biased pn-diode before initiating a temperature measurement. The charge emission from the traps is measured after the filling pulse. This can be done in several different ways, depending on the implementation of the sensor element. If the sensor is manufactured as a diode for electrical measurements, the charge emission can be measured as a chang in the .apitance of the diode. The diode can be a pn-diode, a MIΞ-capacitor (MIS = Metal Isolator Semiconductor), or a Schottky diode. The time constant of the capacitance change of the diode is a measure of the temperature. Brief description of drawings The innovation is described in more detail below and referri to the enclosed drawings. Fig. 1, shows an embodiment of the sensor when the change of the capacitance of a pn-diode is detected and the tem¬ perature of the surroundings is calculated and displayed. Fig. 2a shows the pumping signal applied to the embodiment of the present invention. The detected capacitance of em- bodiment of the invention is presented in Fig. 2b. Fig. 2c shows the relation between the time constant of the changes in the signals shown in Fig. 2b and the temperature. Description of preferred embodiment In the embodiment, shown in Fig.1, a diode is connected in parallel with a pulse generator 2 in series with a controlla current switch S2. A MIS-capacitor, a Schottky diode or any other element with a space charge region e.g. a transistor, can be used instead of the shown pn-diode. A control logic unit 4 is connected to an output of the capacitance meter 3, to an activation input of the capacitance meter 3, to an activation input of the pulse generator 2 and to the ac¬ tivation inputs of the current switches S * and S^. A measurement is initiated by closing the current switch Ξ. by a signal from the control logic unit 4. The pulse generator 2 is activated to give an output signal (shown in Fig. 2a) by an activation signal from the control logic unit 4. This output signal is a pulse sequence consisting of long measurement pulses with short pulses in between (filling pulses). The "pumping" of the diode takes place during the filling pulses. It is possible to control one or more of the parameters suc as the pulse frequency, the length of the measurement pulse and the pulse pause from the control logic unit 4. The control logic unit 4 keeps the current switch Ξ-, opened order to prevent damage to the capacitance meter 3 during the pumping procedure in the pulse pause when the diode is short circuited. During the measurement pulses the control logic unit keeps the current switch S-, closed. The control logic unit 4 is implemented preferably by a micro-computer and the output signal from the capacitance meter 3 is sampl with a certain frequency. Fig. 2b shows the capacitance of the diode during several measurement periods. The control logic unit 4 samples the output signal of the capacitance meter 3 at time t., t ? - t and t, and thus the corresponding capacitance values C . , C ? C-,, and C. are obtained. The course of events during a measurement is as follows: The impurity centers in the space charge region of the diode are fileld during the pulse pause 2 of the pulse generator 2. During the measurement pulse the filled im¬ purity traps emit the electrons or holes and the capacitanc of the diode changes correspondingly. By measuring the ca¬ pacitance of the diode, an output signal is a function of the temperature, as shown in Fig. 2b. Alternatively, it is possible to measure the capacitance in the time period between t-, and t-., which must be separated from t, and t defining the pumping pulse. In the last alternative the signal is sampled at least twice inside every time window t-, - t-. The time constant is converted into temperature by the control logic unit 4 giving an out¬ put signal to a digital or analog display. Different materials may be chosen for the diode to obtain different effective temperature ranges, since every mate¬ rial has a specific relation between the time constant of the capacitance transient and the temperature. Se¬ veral diodes 1, 1' of different types can alternatively be connected in the circuit by the switch S-., as shown in Fig. 1. In order to fully make use of this possibility, the control logic unit can automatically control the pulse generator 2 in such a way that the time window, giving the best resolution in the temperature, may be used fo adapt to the temperature range of interest. The innovation is thus based on the principle that after a filling pulse, resulting in a certain occupation of majority or minority traps, a capacitance transient is detected. The decay rate of the transient is directly related to the temperature of the atmosphere. Since se¬ veral minority and majority traps can exist in the same semiconductor material the same sensor element can be used for temperature measurements in several temperature ranges. Such semiconductor materials could be: A1P, GaP, GaAs, InP, InAs, In^A^P. In^Ga^, Ga^A^As, In^Ga^S, Ga^A^As InAsj P , GaAs. j _ Py, where 0_<_ x_<_1 and 0 <_ y j c_1 , or Ge, Si, C or ZnTe, ZnSe, CdTe, CdSe or CdS, SiC, GaN. These mate¬ rials are doped with impurities or treated in a proper way to produce charge carrier traps. If Si is used it can be doped with S, Se or Te and if GaAs is used it can be doped with Fe. The emission rates of electric charges from the charge carrier traps, after the pumping (filling pulse), are related to the energy difference between the energy position of the trap and the valence band (hole-emission) or the conduction band (electron emission) by an exponenti relation. Two major types of filling pulses exist, namely: a majorit carrier pulse, which decreases momentarily the reverse vol age bias of the diode and the traps become occupied by ma¬ jority carriers which pulse is of the type shown in Fig. 1 and Fig. 2a-2c. The other type of occupational pulse is a minority carrier pulse which momentarily forward bias the diode, and the traps become occupied by minority carriers. The last alternative is not presented in any figure but is nevertheless, a possibility within this innovation. The circuit in Fig. 1 is interchangable in such a way tu provide the possibility, that a pulse from the pulse gen¬ erator 2 may inject minority charge carriers. This pulse i more or less of the same form as the pulse shown in Fig. 2 but with the difference that the injection pulse is above the abscissa. The capacitance transient in this case is upside down, compared to the one shown in Fig. 2b, in such a way that the peak is located at a high capacitance value and the capacitance signal decreases with the time. is the time constant of the transient, i.e. the inverted value of the emission rate, and has a well defined relatio to the temperature. This is shown in Fig. 2c, where logM/ versus 1/T is plotted. T is the temperature in (Kelvin). Log(1/f) is more or less a linear function of 1/T. The control logic unit 4 in Fig. 1 computes the value of log(1/O from the sampled values from the capacitance meter 3, and T is then computed from the graph in Fig. 2c. The signal to the indication unit 5 is suitabley computed from the mean value of T obtained from two or more measure ments. However, it is also possible to have a running in¬ dication of the temperature, especially when the indicatio unit 5 is an anlog display instrument. The length of the filling pulse can be 100ns - 1ms e.g. 5us and the spacing between the pulses may be 5 us - 1s. Thus, it is possiboe to follow a rather fast change in the temperature with the sensor in accordance with the present innovation even if the control logic unit computes a mean value of several measurements of the temperature. The circuit shown in Fig. 1 can be made with contemporary techniques as an integrated circuit. Alternatively, the elements 1, 1', 2, 3, S. , S-, can be made as one unit, fab¬ ricated as an IC, with a control logic unit being a one-chi microprocessor. The semiconductor component, in the example above, is a pn-diode, a MIS capacitor, or a Schottky diode and has been pumped by an electrical pulse. The systems described above, with reference to the drawings may be varied in many ways within the scope of the inventio

Claims: CLAI MS 1. A temperature sensor comprising: - a body (1) of a solid material with an energy bandgap with electric charge carrier traps and incorporating at least one space charge region as a detection element* 5 - an activation apparatus (2) connected to said body, which apparatus during the first time period supplies electric charges to charge carrier traps in said body so as to completely or almost completely fill traps in said space region with chargesj 10 - a detection apparatus (3) which during the second time p after said first time period detects a quantity which is u ambiguously related to the magnitude of the thermally emit charge from traps or to remaining charge in traps, said quantity having a time constant dependent on temperature 15 during the period said activation is not activej - an apparatus (4) which converts said detected quantity into a signal giving information about the temperature. 2. Temperature sensor as claimed in claim 1, in which the 20 said body (1) of solid material is a pn-diode. 3. Temperature sensor as claimed in claim 1, in which the said body (1) of solid material is a MIS-capacitor. 25 4. Temperature sensor as claimed in claim 1, in which the said body (1) of solid material is a Schottky diode. 5. Temperature sensor according to any of the previous cla in which said activation apparatus (2) is a pulse generato 30 and the detector apparatus (3) is a capacitance meter. 6. Temperature sensor according to any of the previous cla in which said sensor is a part of an integrated circuit.

Inventors: KLEVERMAN MATS, KLEVERMAN, MATS
Applicants: DELTASENSE INSTR PTY LTD, KLEVERMAN MATS, DELTASENSE INSTRUMENTS (PTE) LTD., KLEVERMAN, MATS
Application year: 1985
Main classification: G01K7
IPC codes: G01K7