Measurement of junction temperature confirms package thermal design

Junction temperature of packaged laser diodes and high-power LEDs affects output wavelength, spectrum, power, and reliability.

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Junction temperature affects light-emitting-diode (LED) and laser-diode performance in many ways. Light output center wavelength, spectrum, power magnitude, and diode reliability are all directly dependent on the junction temperature. The very high power densities found in laser diodes—which reach 1000 W/cm2 or more in the junction area—provide additional thermal management issues. Thus, the thermal design of the diode itself and the packaging in which it is encased becomes crucial to the overall performance of the device. Validation of thermal design and assembly repeatability requires the ability to measure junction temperature.

There are basically three different methods for making laser-diode junction-temperature measurements. All three methods have been in use for more than two decades by various manufacturers and research laboratories. Two of the methods use the light output of the device for an indirect measurement of junction temperature. The third method is more traditional, considering its application to other types of diodes, and is based on one of the device's electrical characteristics for an indirect measurement of junction temperature.

Optical test methods

The output power (PO) of a laser diode is linearly proportional to its junction temperature (TJ). This attribute can be used to determine TJ if the relationship between TJ and the power output is known in advance. Unfortunately, this relationship is dependent not only on the process-dependent characteristics of the laser-diode chip but also on the ability to get rid of the heat generated at the diode junction. The latter is a function of the chip package and the combination of chip attachment technique and material. These variables make empirical determination the best approach to deriving the PO vs. TJ relationship. A typical value of the relationship slope reciprocal (referred to as KP) is about 2.5°C/mW in the 25°C to 50°C range for a 25-mW output laser device.1 The exact value is specific to the particular diode construction and materials. Once KP is known, the junction temperature can be determined as follows:

TJ = TA + ΔTJ

where ΔTJ = (PO - POi)(KP)

therefore TJ = TA + (PO - POi)(KP).

The other optical method makes use of the fact that the light-output spectrum, primarily the center wavelength (λo), shifts linearly with device junction temperature with a positive slope. This method is particularly useful for narrow-spectrum laser devices. As with the previous method, a calibration procedure is necessary to obtain the reciprocal of the slope, referred to as Kλ. The value of Kλ is typically about 3°C/nm but the specific value is heavily dependent on the laser-diode construction.2 Once KP is known, the junction temperature can be determined as follows:

TJ = TA + ΔTJ

where ΔTJ = (λO - λO)(Kλ)

therefore TJ = TA + (λO - λOi)(Kλ).

Reference 7 discusses both these methods in more detail.

Electrical test method

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FIGURE 1. The usual practice is to select a measurement current (IM) value that is right around the knee of the diode forward current–voltage characteristic curve.
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Light-emitting diodes, like most other semiconductor-junction diodes, have a forward voltage characteristic that can be used for temperature sensing. The forward-bias current, referred to as measurement current (IM) or sense current (IS), must be large enough to turn the junction on over the range of temperature of the measurement but not so large as to cause significant self-heating of the junction. The usual practice is to select a current value that is right around the knee of the diode forward characteristic (see Fig. 1). The actual value is dependent on the specific device under test (DUT); 1 to 10 mA is sufficient for most light-emitting diodes, although high-power output diodes can require 50 mA or more. As a general rule of thumb, the ratio of heating current (IH) to IM is between 25:1 and 100:1; the exact value is dependent on the DUT and the test conditions during the TJ measurement.

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FIGURE 2. Characterizing the forward voltage (VF) temperature relationship (left) requires a system comprising current source, voltmeter, a thermocouple meter, and a multichannel electronic switch (right).
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Calibration of the forward-voltage (VF) temperature characteristic requires a setup similar to that shown in Fig. 2. The low value of IM ensures that the environment and junction temperatures are the same. The relationship between VF and temperature is, for most practical purposes, very linear. Note that because calibration requires only a constant current and a voltage measurement capability, multiple units can be calibrated at the same time in a batch mode.

The calibration constant, K Factor (or just K) is typically in the range of 0.5°C/mV for silicon-based p-n junctions but can vary greatly depending on the specific LED construction and materials, typically being in the 0.3°C/mV to 0.8°C/mV range. The exact value of K is also dependent on IM. Although the slope is negative, K is always stated as a positive number:

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The batch calibration approach typically takes less than two hours to complete, depending on the type and size of the temperature-controlled environment.

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FIGURE 3. The electrical test method (ETM) for diode junction temperature measurements is based on a three-step operation using the test set up shown (left) First, IM is applied and the diode under test (DUT) junction voltage is measured—the measurement value is referred to as VFi; second, IM is replaced with a desired amount of heating current (IH) for a time duration (tH) consistent with the steady-state or transient data required. During this time the diode voltage (VH) is measured for determining the amount of power (PH) being dissipated in the diode. Third, IH is removed and quickly replaced with IM and a final junction voltage measurement is be made—this voltage is referred to as VFf (right).

The electrical test method (ETM) for diode junction temperature measurements uses a three-step sequence of applied current levels to determine a change in junction voltage (ΔVF) under measurement current (IM) conditions (see Fig. 3).3, 4, 5 As discussed with the two previous methods, the ETM also requires the use of a temperature sensor (TA) placed on the diode mounting surface or package to determine TJ in absolute terms. This sensor, in conjunction with monitoring of VF under IM conditions before the start of the test, is also used to determine whether temperature equilibrium conditions exist before the start of the test. Without power applied to the diode, the VF reading will settle down to a value corresponding to the temperature of the external sensor. The value of TJ can then be calculated as follows:

TJ = TA + ΔTJ

where ΔTJ = (VFi - VFf)(K)
therefore

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Thermal resistance

An application for LED and laser-diode junction-temperature measurements is to determine the thermal resistance of the device. Thermal resistance is defined as the temperature difference across a heat-flow path divided by the power dissipation that caused the temperature difference.6 For most laser diodes, the heat is produced at the junction and flows through a single path to the back side of the diode to the mounting surface. Thus, one end of the path is always the junction (J) and the other is the surface upon which the diode is mounted (M) or the bottom mounting surface of the package (referred to as C for case). The thermal resistance symbol is either θJM or θJC. Generically, thermal resistance is stated as θJX, where X is the reference point that must be defined.

The same data obtained from any of the methods discussed, with the addition of PH and the appropriate selection of heating time (tH), can also be used to calculate the diode thermal resistance. For the ETM, the thermal resistance is calculated as follows:

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The value of tH and the environmental conditions determine the meaning of the thermal resistance X subscript. For greatest accuracy on high-power-conversion light-emitting diodes, the previous equation has to take into account the portion of applied electrical power (PH) that is converted to optical output power (PO). Hence, a more exact value of thermal resistance is:

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Accurate knowledge of LED and laser-diode junction temperature is necessary to predict and specify parametric and reliability performance. Of the three measurement approaches, the ETM approach offers a relatively simple technique to determine junction temperature without requiring optical instrumentation. Being electronic in nature, the ETM also offers the opportunity for measurement automation, making possible the addition of TJ measurements to the normal suite of parametric measurements made in LED and LD development and production environments.

REFERENCES

  1. Product Catalog, Melles Griot, (2002), pp. 45.2-45.3
  2. Product Catalog, Melles Griot, (2002), pp. 45.2-45.3
  3. Mil-Std 750, Method 3101, U.S. Dept. of Defense.
  4. J. J. Hughes, D. B. Gilbert, and F. Z. Hewrylo, RCA Review 46, 200 (June 1985).
  5. B. Siegal, Electro-Optical systems Design, Kiver Publications, 47 (November 1981).
  6. JESD51-1, EIA JEDEC,.
  7. B. Siegal, Proc. PhoPack 2002, (July 2002).

BERNIE SIEGAL is president of Thermal Engineering Associates, 612 National Ave., Mountain View, CA 94043-2222; e-mail bsiegal@thermengr.com.

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