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Semiconductor

 
 

Introduction to Semiconductor Temperature Sensors

The semiconductor (or IC for integrated circuit) temperature sensor is an electronic device fabricated in a similar way to other modern electronic semiconductor components such as microprocessors. Typically hundreds or thousands of devices are formed on thin silicon wafers. Before the wafer is scribed and cut into individual chips, they are usually laser trimmed.

Semiconductor temperature sensors are available from a number of manufacturers. There are no generic types as with thermocouple and RTDs, although a number of devices are made by more than one manufacturer. The AD590 and the LM35 have traditionally been the most popular devices, but over the last few years better alternatives have become available.

These sensors share a number of characteristics - linear outputs, relatively small size, limited temperature range (-40 to +120°C typical), low cost, good accuracy if calibrated but also poor interchangeability. Often the semiconductor temperature sensors are not well designed thermally, with the semiconductor chip not always in good thermal contact with an outside surface. Some devices are inclined to oscillate unless precautions are taken. Provided the limitations of the semiconductor temperature sensors are understood, they can be used effectively in many applications.

The most popular semiconductor temperature sensors are based on the fundamental temperature and current characteristics of the transistor. If two identical transistors are operated at different but constant collector current densities, then the difference in their base-emitter voltages is proportional to the absolute temperature of the transistors. This voltage difference is then converted to a single ended voltage or a current. An offset may be applied to convert the signal from absolute temperature to Celsius or Fahrenheit.

In general, the semiconductor temperature sensor is best suited for embedded applications - that is, for use within equipment. This is because they tend to be electrically and mechanically more delicate than most other temperature sensor types. However they do have legitimate application in many areas, hence their inclusion.

Types of semiconductor sensors

A summary of available semiconductor temperatures sensors is presented below, followed by more detail on some of the more popular devices. The sensors can be grouped into five broad categories: voltage output, current output, resistance output, digital output and simple diode types.

Most of these sensors employ a modified forward diode voltage drop method to measure temperature, however bulk silicon reseistance and more recently infrared with thermopile.

1. Voltage Output Temperature Sensors

The following sensors provide a voltage outputs signal with relatively low output impedance. All require an excitation power source and all are essentially linear.

Sensor
Manuf.
Output
Tolerance
(range)
Package
Comments
AD22100
Analog Devices
22.5mV/°C at 5V
250mV offset
±2°C & ±4°C
(-50 to +150°C)
TO-92
SO-8
Output ratiometric with supply voltage - good with ratiometric ADC's
AD22103
Analog Devices
28mV/°C (at 3.3V),
250mV offset
±2.5°C
(0°C to +100°C)
TO-92
SO-8
Output ratiometric with supply voltage
LM135
LM235
LM335
National Semi, Linear Tech
10mV/°K or
10mV/°C
±2.7°C to ±9°C
(-55°C to 150°C
-40°C to 100°C)
TO-92
TO-46
Zener like operation with scale trim pin, 400µA
LM34
National Semi
10mV/°F
±3°F & ±4°F
(-20°C to 120°C)
TO-46
TO-92
SO-8
Needs a negative supply for temperatures < -5°C
LM35
National Semi
10mV/°C
±1°C & ±1.5°C
(-20°C to 120°C)
TO-46
TO-92
SO-8
Needs a negative supply for temperatures < 10°C
LM45
National Semi
10mV/°C
500mV offset
±1°C & ±1.5°C
(-20°C to 120°C)
TO-46
TO-9
SO-8
LM35 with 500mV output offset
LM50
National Semi
10mV/°C
500mV offset
±3°C & ±4°C
(-40°C to 125°C)
TO-46
TO-92
SO-8
Low cost part, 500mV off set, easy to use
LM60
National Semi
6.24 mV offset
±3°C & ±4°C
(-40°C to 125°C)
SOT-23
Supply voltage down to 2.7V
S-8110
S-8120
Seiko Instruments
-8.5 mV/°C
(note neg. TC)
±2.5°C & ±5°C
(-40°C to 100°C)
SOT-23
SC-82AB
Very low 10µA operating current
TC102
TC103
TC1132
TC1133
Telcom Semi
10 mV/°C
±8°C
(-20°C to 125°C)
SOT-23
TO-92
.
TMP35
Analog Devices
10 mV/°C
±3°C ±4°C
(10°C to 125°C)
TO-92
SO-8
SOT-23
Similar to LM35 plus shutdown for power saving (not in TO-92)
TMP36
Analog Devices
10 mV/°C
500 mV offset
±3°C ±4°C
(-40°C to 125°C)
TO-92
SO-8
SOT-23
Similar to LM50 plus shutdown (not in TO-92)
TMP37
Analog Devices
20 mV/°C
±3°C ±4°C
(5°C to 100°C)
TO-92
SO-8
SOT-23
High sensitivity
LM94021
LM94022
programmable
±2.5°C
(-50°C to 150°C)
SC80
Low power, easy to use
FM20
-11.77 mV/°C
±5°C
-55°C to 130°C
SOT23
Low power
FM50
10 mV/°C
±3°C
-40°C to 125°C
SOT23
Similar to LM50

The LM34 and LM35 parts are prone to oscillation if sensor cable capacitively loads their output. The symptom is an offset in the sensors output - something which is not always obvious. It is wise to always include the manufacturer's recommended resistor - capacitor network close to the sensor.

2. Current Output Temperature Sensors

The current output sensors acts as a high-impedance, constant current regulator typically passing 1 micro-amp per degree Kelvin and require a supply voltage of between 4 and 30 V.

Sensor
Manuf.
Output
Tolerance
(range)
Package
Comments
AD590
Analog Devices
1µA/°K
±5.5°C & ±10°C
(-55°C to +150°C)
TO-52
An old favorite, but need to watch cable leakage currents
AD592
Analog Devices
1µA/°K
±1°C & ±3.5°C
(-25°C to +105°C)
TO-92
A more precise AD590
TMP17
Analog Devices
1µA/°K
±4°C
(-40°C to +105°C)
SO-8
Thermally faster AD590
LM134
LM234
LM334
National Semi
Programmable
0.1µA/°K to 4µA/°K
±3°C & ±20°C
(-25°C to +100°C)
TO-46
TO-92
Not well specified, but with calibration can be effective.

3. Digital Output Temperature Sensors

The digital temperature sensor is the first sensor to integrate a sensor and an analog to digital converter (ADC) on to a single silicon chip. In general, these sensors do not lend themselves for use with standard measuring devices because of their non standard digital interfaces. Many are designed specifically for the thermal management of microprocessor chips. A selection of representative devices is presented below:

Sensor
Manuf.
Output
Tolerance
(range)
Package
Comments
LM95071
National Semi
14 bit SPI
±2°C
(-45°C to 150°C)
SOT-5

High resolution
(0.03°C)
2.4-5.5V operation

LM56
National Semi
2 comparators with setable thresholds
±3°C & ±4°C
(-40°C to 125°C)
SOP-8
MSOP-8
Thermostat with two outputs with hysteresis
LM75
National Semi
I2C Serial,
9 bit or 0.5°C resolution
±3°C
(-55°C to +125°C)
SOP-8
MSOP-8
Addressable, multi drop connection. Better suited to embedded systems
TMP03
TMP04
Analog Devices
Pulse width modulation
(mark-space ratio)
±4°C
(-25°C to 100°C)
TO-92
SO-8
TSSOP-8
Nominal 35 Hz output with 1:1 mark-space ratio at 25°C
DS1620
DS1621
National Semi
2 or 3 wire serial, 0.5°C resolution
±0.5°C
(0°C to 70°C)
±5°C
(-55°C to 125°C)
SOP-8
DIP-8
Also has digitally programmed thermostat output. ±0.03°C resolution possible
DS1624
Dallas
2 wire serial
0.3°C resolution
±5°C
(-55°C to 125°C)
SOP-8
DIP-8
Addressable, multi drop connection. Also has 256 bits of EEPROM
DS1820
Dallas
1 wire serial
0.5°C resolution
±0.5°C
(0°C to 70°C)
±5°C
(-55°C to 125°C)
Modified
TO-92
SSOP-16
Good un-calibrated tolerance over 0-70°C range.
DS1821
Dallas
1 wire serial
1°C resolution
±1°C
(0°C to 70°C)
±2°C
(-55°C to 125°C)
Modified TO-92
TO-220
SO-8
Has a thermostat mode.
DS2435
Dallas
1 wire serial
0.5°C or 1°C resolution
±4°C
(0°C to 127.5°C
-40°C to 85°C)
TO-92
modified
Also builds a time / temperature histogram
TCN75
Telcom Semi
I2C Serial,
9 bit or 0.5°C resolution
±3°C
(-55°C to +125°C)
DIP-8
SOP-8
TSSOP-8

Second source for LM75

FM75
SMBus
12 bit / 0.07°C
resolution
±4°C
-40°C to 125°C
MSOP8
Variable resolution, threshold output

The Analog Devices parts are interesting. They employ a sigma-delta ADC that produces continuous pulse stream output with a mark-space ratio, which is proportional to the temperature. This makes for easy interfacing to a microprocessor and also for isolating by optical or other means. The same signal could also be passed through a low pass filter to generate an analog voltage.

The Dallas DS2435 goes beyond that of a sensor plus ADC by providing simple data reduction using an eight bin time / temperature histogram with definable bin boundaries. It appears to have been specifically designed for battery management, but other application could include food transport monitoring, machine use monitoring. This sensor demonstrated the way of the future in sensor technology where sensor, ADC, memory and microcontroller are integrated to form an application specific task very cost effectively.

4. Resistance Output Silicon Temperature Sensors

The temperature - versus - bulk resistance characteristics of semiconductor materials allow the manufacture of simple temperature sensors using standard silicon semiconductor fabrication equipment. This construction can be more stable than other semiconductor sensor, due to the greater tolerance to ion migration. However other characteristics (see below) require that care be taken in using these sensors.

Sensor
Manuf.
Output
Tolerance
(range)
Package
Comments
KTY81
KTY82
KTY83
KTY84
KTY85
Phillips
1K or 2K at 25°C, +0.8%/°C
See below
±1°C to ±12°C
(-55°C to +150°C
some to 300°C)
SOD-70,
SOT-23
SOD-68
SOD-80
Bulk resistance of silicon. Keep excitation current >0.1mA and < 1mA
KYY10
KTY11
KTY13
Siemens
1K or 2K at 25°C, +0.8%/°C
See below
±1°C & ±3.5°C
(-50°C to +150°C)
TO-92
modified
Bulk resistance of silicon.

The silicon temperature sensor's resistance is given by the equation:

R = Rr ( 1 + a.( T - Tr ) + b.( T - Tr )2- c.(T - Ti)d )

where Rr is the resistance at temperature Tr and a, b, c and d are constants. Ti is an inflection point temperature such that c = 0 for T < Ti.

The resistance of some of these semiconductor sensors is dependent on the excitation current (due to current density effects in the semiconductor) and the polarity of the applied voltage. As with other non-passive temperature sensors, self-heating can induce errors.

There are a number of specialist cryogenic temperature sensors that use resistive semiconductor sensor elements made from silicon and germanium.

5. Diode Temperature Sensors

The ordinary semiconductor diode may be used as a temperature sensor. Cheap and nasty! The diode is the lowest cost temperature sensor and can produce more than satisfactory results if you are prepared to undertake a two point calibration and provide a stable excitation current.   Almost any silicon diode is ok. The forward biased voltage across a diode has a temperature coefficient of about 2.3mV/°C and is reasonably linear. The measuring circuit is simple as shown to the right.

The bias current should be held as constant as possible - using constant current source, or a resistor from a stable voltage source.

Without calibration the initial error is likely to be too large - in the order of ±30°C - the largest of all the contact type temperature sensors. This initial error is greatly reduced if sensor grade parts are used.

One advantage of the diode as a temperature sensor is that it can be electrically robust - tolerant to voltage spikes induced by lightning strike. This is particularly true if power diodes (e.g. the common 1N4004) are used and a second back to back diode is used to limit power dissipation during high peak currents.

The transistor sensor is used in diode mode by connecting the base and collector together. If this is not done, the sensor is wired between base and emitter and the excitation current reduced by a factor of about 100. The result is a very low power, sensitive and linear sensor. The simplicity and performance of the sensor is under valued.

To improve the performance of the diode as a temperature sensor, two diode voltages (V1 and V2) can be measured at different currents (I1 and I2), typically selected to be about 1:10 ratio. The absolute temperature can be calculated from the equation:

T = (V1 - V2) / (8.7248x10-5 ln( I1 / I2))

The result is in Kelvins (K). This is the method employed by most integrated circuit temperatures sensors and explains why some output a signal proportional to absolute temperature.

Accuracy of semiconductor sensors

As can be seen from the above information, the "out of the box" or interchangeability accuracy of most semiconductor temperature sensors is not particularly good. In addition the raw sensing element is generally packaged in a standard case for electronic devices, which is less than ideal for precision temperature measurement. However, despite these shortcomings, the sensors are sensitive, reasonably linear and very usable.

If individual sensors are calibrated, significantly better measurement accuracy is possible. Typically, a two point calibration will yield a five-fold better accuracy and a three point calibration will yield a ten-fold improvement over the full temperature range. If the temperature range is limited, even better accuracies are possible. Due to the sensitivity of some sensors, they can be very good in measuring small temperature changes (as opposed to absolute measurement).

Which is the most appropriate semiconductor sensor?

A difficult question to answer, but the following selection process may help:

  • Decide on the accuracy required
  • Decide on the temperature range
  • Decide on a budget
  • Define the measuring instruments input capabilities

Select any sensor that satisfies all of the above (you will be lucky if one does), otherwise:

  • If accuracy is deficient, you will need to calibrate the sensor. Will a single point offset correction be sufficient with any sensor? Select that sensor. Otherwise:
  • Will a two point calibration provide adequate accuracy by correcting for offset and slope? Select that sensor.

Once you have decided that calibration will be required, the selection becomes easier. It makes little difference if the initial un-calibrated error is large or small. The nature of the deviation from the ideal response curve becomes the most important factor. If this deviation is a simple linear function, a two-point calibration will yield excellent results. If the deviation is more complex, a multipoint calibration will be required, followed by the fitting of a polynomial or a series of linear segments.