TEMPERATURE SENSING APPARATUS

Abstract: It is desirable to be able to sense the temperature of a semiconductor element to detect, for example, overheating of that element. A temperature sensing apparatus is disclosed which comprises a polysilicon device, (7), functioning as a temperature transducer and integrated with a semiconductor element (1). A silicon oxide layer (5) separates the temperature transducer device (7) from the element (1).

Description: This invention relates to temperature sensing apparatus, and in particular to temperature sensing apparatus comprising at least one semiconductor device for sensing temperature. It is desirable to be able to sense the temperature of a semiconductor element for a variety of purposes. For instance, it is a problem with many sorts of semiconductor element that overheating of the element may, if prolonged, lead to the destruction of that element. In particular, any MOS switch has a limit to its capability to dissipate power which, if exceeded, may result in the overheating and self-destruction of the MOS switch. The power dissipated by a switch is the product of the voltage across the switch and the current through the switch: the switch is normally used in conjunction with a load which, under normal operating conditions, limits the current through the switch and the voltage across it to a value within the power dissipation capability of the switch. Should the load be inadvertently short circuited, the total supply voltage will be across the switch which may result in the power dissipation capability being exceeded with the consequent overheating and destruction of the switch. It is known to provide thermal switches for circuit components (IBM Technical Disclosure Bulletin Vol 23 no. 3 August 1980 pages 1118 to 1119). Further, it is know to fabricate semiconductor devices with an integral temperature sensor. EP-A-0174686 discloses the use of a conductive layer disposed on the lower surface of the substrate of a MOS device. This compensates for the influence of temperature dependent charges at the silicon oxide layer. JP-A-57111425 discloses a circuit which incorporates a diode for detecting the temperature of a bipolar transistor: the two components being integrally fabricated. The diode is forward biased and the variation of the voltage measured to give a measure of the temperature. According to the present invention, a temperature sensing apparatus comprises a semiconductor element; at least one semiconductor device for sensing the temperature of the semiconductor element and integral with the semiconductor element; and an output circuit connected to the semiconductor device and operative to provide an output indicative of the temperature of the semiconductor element; characterised in that the semiconductor element is a MOS switch, and a dielectric layer is provided between the MOS switch and the semiconductor device to provide both good electrical isolation and good thermal contact between the MOS switch and the semiconductor device. Consequently, the temperature of the MOS switch, may be sensed and overheating readily detected. Preferably, the semiconductor device may be integrated in polysilicon and the dielectric layer comprises silicon nitride and/or silicon dioxide. Consequently, the semiconductor device for sensing temperature may be positioned within a few microns of the element, thus providing a compact and rapidly reacting temperature sensor. The semiconductor device may consist of essentially a uni-polar junction device. For instance, a polysilicon diode or resistor may be utilised or, alternatively, a MOS transistor fabricated in polysilicon. Conveniently, the output means may be arranged to provide an output dependant on the current flowing through the device which may constitute an indication that the temperature of the MOS switch exceeds a predetermined threshold. An example of the invention will now be described with reference to the drawing in which Figure 1 depicts a schematic cross section through a MOS structure having a temperature sensing apparatus in accordance with one embodiment of the present invention. Referring now to Figure 1, a MOS switch 1, the temperature of which is to be sensed, is arranged in a silicon substrate 2, and is used to switch current supplied on a conductor 3 to a load 4. Heat is dissipated from the switch 1, when on, to the silicon substrate 2. A thin silicon oxide layer 5 bounding a portion of the silicon substrate 2 in proximity to the switch 1, allows both electrical isolation and thermal contact between the switch 1 and the semiconductor device 7 which is used for sensing the temperature of the switch 1. The device 7 may comprise a chain of junction devices, 7, integrated in polysilicon 6. In the present example, the chain consists of series connected polysilicon diodes 7, of which two are illustrated. An output circuit, comprising a voltage source 9, is connected to apply a voltage across the chain of diodes 7 via conductor 8. A current detection device 10 is arranged to detect current flowing through the devices 7. The voltage source 9 applies a fixed voltage which is below the voltage threshold required to drive the diodes 7 into conduction, under normal operating conductions. The voltage threshold is, however, temperature dependant, and decreases with increasing temperature. Consequently the diodes 7 will become conducting should sufficient heat from the switch 1 be conducted to the diodes 7. In such circumstances, the current detection device 10 may provide an output signal on conductor 11 to turn off MOS switch 1. It will be apparent that the precise form of the semiconductor devices 7, the voltage applied by source 9, the proximity of the devices 7 to the switch 1 and the thermal conductivity of the interspaced portion between switch 1 and device 7 are at least some of the factors to be taken into account in arranging for the output circuit to provide a predetermined output indicative of the temperature of the switch 1, and, in particular, of the overheating of the switch 1 above a predetermined level. If a semiconductor device is utilised having temperature dependent charge transport properties not characterised by an abrupt conduction threshold, for instance a thermistor, then the temperature sensing apparatus may be arranged so that the current detection circuit 10 provides an output 12 indicative of the actual temperature of the switch 1, in dependence, for instance upon the amount of current flowing or the voltage applied by voltage source 9. Further, particular arrangements of semiconductor devices 7 may be utilised which are arranged to allow current flow in an output circuit only when the devices are below a predetermined temperature.

Claims: Temperature sensing apparatus comprising a semiconductor element (1); at least one semiconductor device (7) for sensing the temperature of the semiconductor element (1) and integral with the semiconductor element (1); and an output circuit (9, 10) connected to the semiconductor device (7) and operative to provide an output indicative of the temperature of the semiconductor element (1); characterised in that the semiconductor element (1) is a MOS switch, and a dielectric layer (5) is provided between the MOS switch (1) and the semiconductor device (7) to provide both good electrical isolation and good thermal contact between the MOS switch (1) and the semiconductor device (7). Temperature sensing apparatus as claimed in Claim 1 wherein the semiconductor device (7) is integrated in polysilicon. Temperature sensing apparatus as claimed in Claim 2 wherein the dielectric layer (5) comprises silicon nitride and/or silicon dioxide. Temperature sensing apparatus as claimed in any preceding claim, wherein the semiconductor device (7) is a unipolar junction device. Temperature sensing apparatus as claimed in Claim 4 wherein the unipolar junction device consists of either a diode, a MOS device or a resistor. Temperature sensing apparatus as claimed in any preceding claim wherein the output circuit (9, 10) further includes means operable to turn off the MOS switch (1) at a predetermined temperature of the MOS switch (1). Temperature sensing apparatus as claimed in any preceding claim wherein the output circuit (9, 10) comprises a voltage source (9) for applying a predetermined voltage to the semiconductor device (7).

Inventors: JENKINS, JAMES OLIVER
Applicants: SILICONIX LIMITED
Application year: 1987
Main classification: G01K7
IPC codes: G01K7, H01L23, G01K1, H01L21, H01L27