Junction temperature, short for
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
junction temperature, is the highest
operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
of the actual
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
in an electronic device. In operation, it is higher than case temperature and the temperature of the part's exterior. The difference is equal to the amount of heat transferred from the junction to case multiplied by the junction-to-case
thermal resistance
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance.
* (Absolute) thermal resistance ''R'' in kelvin ...
.
Microscopic effects
Various physical properties of semiconductor materials are temperature dependent. These include the diffusion rate of
dopant
A dopant, also called a doping agent, is a trace of impurity element that is introduced into a chemical material to alter its original electrical or optical properties. The amount of dopant necessary to cause changes is typically very low. When ...
elements,
carrier mobilities and the thermal production of
charge carrier
In physics, a charge carrier is a particle or quasiparticle that is free to move, carrying an electric charge, especially the particles that carry electric charges in electrical conductors. Examples are electrons, ions and holes. The term is used ...
s. At the low end, sensor diode noise can be reduced by cryogenic cooling. On the high end, the resulting increase in local power dissipation can lead to
thermal runaway
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
that may cause transient or permanent device failure.
Maximum junction temperature calculation
Maximum junction temperature (sometimes abbreviated TJMax) is specified in a part's datasheet and is used when calculating the necessary case-to-ambient thermal resistance for a given power dissipation. This in turn is used to select an appropriate
heat sink
A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, th ...
if applicable. Other cooling methods include
thermoelectric cooling
Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side o ...
and
coolant
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosio ...
s.
In modern processors from manufacturer such as
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
,
AMD
Advanced Micro Devices, Inc. (AMD) is an American multinational semiconductor company based in Santa Clara, California, that develops computer processors and related technologies for business and consumer markets. While it initially manufactur ...
,
Qualcomm
Qualcomm () is an American multinational corporation headquartered in San Diego, California, and incorporated in Delaware. It creates semiconductors, software, and services related to wireless technology. It owns patents critical to the 5G, 4 ...
, the core temperature is measured by a network of sensors. Every time the temperature sensing network determines that a rise above the specified junction temperature (
), is imminent, measures such as clock gating, clock stretching, clock speed reduction and others (commonly referred to as thermal throttling) are applied to prevent the temperature to raise further. If the applied mechanisms are not compensating enough for the processor to stay below the junction temperature, the device may shut down to prevent permanent damage.
An estimation of the chip-junction temperature,
, can be obtained from the following equation:
where:
= ambient temperature for the package ( °C )
= junction to ambient thermal resistance ( °C / W )
= power dissipation in package (W)
Measuring junction temperature (TJ)
Many semiconductors and their surrounding optics are small, making it difficult to measure junction temperature with direct methods such as
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
s and
infrared cameras
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
.
Junction temperature may be measured indirectly using the device's inherent voltage/temperature dependency characteristic. Combined with a
Joint Electron Device Engineering Council (JEDEC) technique such as JESD 51-1 and JESD 51-51, this method will produce accurate
measurements. However, this measurement technique is difficult to implement in multi-LED
series circuits
Two-terminal components and electrical networks can be connected in series or parallel. The resulting electrical network will have two terminals, and itself can participate in a series or parallel topology. Whether a two-terminal "object" is an ...
due to high common mode voltages and the need for fast, high
duty cycle
A duty cycle or power cycle is the fraction of one period in which a signal or system is active. Duty cycle is commonly expressed as a percentage or a ratio. A period is the time it takes for a signal to complete an on-and-off cycle. As a formu ...
current pulses. This difficulty can be overcome by combining high-speed sampling digital multimeters and fast high-compliance pulsed
current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources f ...
s.
Once junction temperature is known, another important parameter,
thermal resistance (Rθ), may be calculated using the following equation:
Junction temperature of LEDs and laser diodes
An
LED
A light-emitting diode (LED) is a semiconductor Electronics, device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy i ...
or
laser diode’s junction temperature (Tj) is a primary determinate for long-term reliability; it also is a key factor for
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electrom ...
. For example, a typical white LED output declines 20% for a 50 °C rise in junction temperature. Because of this temperature sensitivity, LED measurement standards, like
IESNA
The Illuminating Engineering Society (IES), formerly the Illuminating Engineering Society of North America (IESNA), is an industry-backed, not-for-profit, learned society that was founded in New York City on January 10, 1906. The IES's stated mi ...
’
LM-85 require that the junction temperature is determined when making photometric measurements.
Junction heating can be minimized in these devices by using the Continuous Pulse Test Method specified in LM-85. An L-I sweep conducted with an
Osram
Osram Licht AG is a German company that makes electric lights, headquartered in Munich and Premstätten (Austria). Osram positions itself as a high-tech photonics company that is increasingly focusing on sensor technology, visualization and tre ...
Yellow LED shows that Single Pulse Test Method measurements yield a 25% drop in
luminous flux
In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in th ...
output and
DC Test Method measurements yield a 70% drop.
See also
*
Safe operating area
For power semiconductor devices (such as BJT, MOSFET, thyristor or IGBT), the safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented ...
*
P-N Junction
*
Metal Semiconductor Junction
References
{{DEFAULTSORT:Junction temperature
Semiconductors