Microelectronics Reliability 45 (2005) 1742–1745
www.elsevier.com/locate/microrel
A novel fast and versatile temperature measurement system for
LDMOS transistors
A. Tazzoli *, G. Meneghesso, E. Zanoni
Department of Information Engineering, Università di Padova, Via Gradenigo 6/b, 35131 Padova, Italy
Abstract
This paper describes a fast and versatile system developed to measure indirectly the junction temperature of
LDMOS transistors which can be easily adapted to other kind of devices. The system takes less than 20us to make the
measurement, and the polarization for the self-heating of the device is user-selectable for the time and for the value of
biasing. Furthermore, the system can be integrated with other stress system and be used to monitor the temperature of
the device under test, in order to control an otherwise uncontrollable increase of device temperature.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
The evaluation of the junction temperature of
solid-state devices subjected to self-heating can be
obtained by means of theoretical analysis or by
experimental characterization.
Theoretical analysis is based on the resolution of
more or less complex analytical equations and
numerical analysis techniques. Experimental
characterization of the device/chip/junction/surface
temperature of packaged or unpackaged devices takes
both direct, infrared micro radiometry, or liquid
crystals and thermo graphic phosphorus, or, to a lesser
extent, thermocouples, and indirect measurements of
electrical parameters. Direct techniques, although
typically more accurate, require that the surface of the
operating device is directly accessible. On the contrary,
electrical techniques for measuring the temperature of
semiconductor devices can be performed on fully
packaged devices, using a temperature sensitive
electrical parameter of the structure, which can be
characterized and calibrated with respect to
temperature and subsequently used as the temperature
indicator [1-2].
The paper reports the description of a novel in its
conception, fast and very versatile, temperature
measurement system, used to evaluate the increase in
temperature due to self-heating of commercial LDMOS
transistors, to control the temperature during DC or
pulsed electrical stress. Moreover, it can also be used
to measure the junction temperature of any other kind
of devices (diodes, BJT, JFET…).
Starting from the classical relation between
voltage and current of an ideal diode (Eq. 1),
ID
ª § qV · º
I S «exp¨ D ¸ 1»
¬ © nkT ¹ ¼
(1)
the voltage can be obtained (Eq. 2),
* Corresponding author. augusto.tazzoli@dei.unipd.it
Tel: +39 (049) 827 7791; Fax: +39 (049) 827 7699
0026-2714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.microrel.2005.07.101
VD
·
nkT § I D
ln ¨¨
1¸¸
q
© IS
¹
(2)
A. Tazzoli et al. / Microelectronics Reliability 45 (2005) 1742–1745
Fig. 1. Structure of a twin N-channel LDMOS.
with the saturation current proportional to the intrinsic
concentration of carriers ni2 (Eq. 3),
I S v ni2
§ E ·
BT 3 exp¨ G ¸
© kT ¹
(3)
From the last equation, the heavy negative
dependency of the voltage drop upon the diode with
the increasing of the temperature is clear [3].
The basis of the thermal characterization is quite
simple: i) any intrinsic diode present in the device (the
Body-Drain diode in LDMOS) must be fully
characterized at different temperatures; ii) the device is
biased in order to produce self heating; iii) the intrinsic
diode is hence used as a temperature probe by
measuring the voltage drop consequent to self heating;
comparing this voltage with a voltage-temperature map
previously made in a climatic chamber it is possible to
indirectly estimate the temperature of the device under
test [4-6]; iv) the time between the end of the selfheating regime and the measuring of the intrinsic diode
660
1743
must be as short as possible in order to avoid device
cooling down; v) the current adopted to measure the
intrinsic diode must be as low as possible to avoid
further self-heating. Figure 2 shows an example of
calibration curve of the LDMOS intrinsic diode (BodyDrain) carried out at different temperatures. All devices
tested exhibit the same behaviour and also very similar
values for the calibration curve, despite differences in
other electrical parameters.
Usually, in thermal-characterization circuit
proposed in literature, the measurement of the intrinsic
diode following the self heating, [see iii) above], is
biased at constant current. At the contrary, in order to
simplify and make the measurement system as fast as
possible, we have biased the diode at constant voltage
limiting the current with a series resistor. The voltagetemperature conversion map takes into account the
effect of the series resistor and in this way the system
doesn’t need a successive calibration or correction
session.
It is important to note how to play with the results
obtained with this kind of indirect temperature
measurement, in particular with multi-finger devices.
In fact, LDMOS transistors are constituted of several
cells (for example, Ericsson PTF 10107A are
constituted of 180 fingers), everyone with an intrinsic
diode. During the calibration all diodes are at the same
temperature, but being all diodes in parallel and
because of the not probable equal self-heating of all the
area of the device, the total current will be split
between them as a function of their local temperature.
For this reason, the measured temperature must not be
A/D PIC CONVERTER
610
560
510
460
410
360
15
35
55
75
95
115
135
Temperature [ºC]
Fig. 2. Example of a calibration curve for a LDMOS
intrinsic diode. Interpolation clearly indicate a linear trend.
PIC represent the output of the 10-bit AD converter of the
adopted microcontroller.
Fig. 3. Schematic of the temperature measure system.
DUT is highlighted at the center of the scheme.
1744
A. Tazzoli et al. / Microelectronics Reliability 45 (2005) 1742–1745
70
40
60
1
0.8
30
0.6
Drain Voltage
20
Id [A]
Vd [V] - Power [W] - Temp [°C]
50
1.2
0.4
Temperature [ºC]
Polarization = 1.5ms
Duty Cycle = 98.5%
Vg = 5.8V
60
50
40
30
20
Temperature
Dissipated Power
10
0.2
Drain Current
0
000E+0 20E-3
10
Duty Cycle = 82%
Duty Cycle = 90%
Duty Cycle = 95.7%
Duty Cycle = 97.8%
Duty Cycle = 98.5%
0
0
40E-3
Room Temperature
60E-3 80E-3 100E-3 120E-3
Time [s]
Fig. 4. Measurement of the junction temperature, Vd, Id,
and dissipated power for an open-package LDMOS biased
with a Duty Cycle of 98.5% for the first 60ms and then left
cooling.
considered the true temperature but a temperature
between the maximum and the average one.
2. Measurement system description
The measurement system is constituted by a Hp
8114A pulse generator, a Hp 3631A power supplier, a
pc, and a self-made circuit that controls all the
operations of the system, see Figure 3. The heart of the
circuit is based on the Microchip PIC 16F877A. The
microcontroller, clocked at 20Mhz, controls the correct
synchronization between the energy pulse, provided by
the pulser used in External Trigger Mode and the
power supplier, and the measurement of the device
temperature by switching two opto-couplers and a
NMOS transistor.
In this way, it is possible to bias a device at any
values of Drain and Gate voltage, and for any time,
starting from some hundreds of nanoseconds to hours,
at the contrary of others more limited circuit presented
in literature.
In addition, the circuit was designed to allow the
polarization of the device not only with square shaped
energy pulses, but with any shaped ones, using an
arbitrary waveform generator, usually amplified for
power pulses. This feature can be useful to run the
measurement with triangular shaped pulse to obtain
inductive like self-heating or other behaviours [4].
The internal A/D converter of the microcontroller
is also used to measure the Drain and the Source
voltage during the energy pulse. Only in 20 us the
000E+0
25E-3
50E-3
75E-3
100E-3
125E-3
Time [s]
Fig. 5. Comparison between different value of Duty Cycle
for the polarization of the same transistor.
microcontroller makes the result and, thanks to its 10
bits resolution, the temperature can be evaluated with a
better resolution compared to other systems based to
classical 8 bits Digital Oscilloscope. Furthermore, the
current flowing through the device is measured by the
voltage drop up a 1 ohm sense resistor. In this way it is
possible to evaluate the
instantaneous power
dissipated by the device and so the self-heating.
A very important feature of this system is the very
short time spent to switch between the self-heating
stage and the temperature measurement one. In fact,
the measurement must be performed as fast as possible
after the energy pulse, otherwise, especially dealing
with short pulse, the device could have enough time to
cooling itself too much. The microcontroller only in
200 nanoseconds control the switches and, considering
the switching time of the NMOS and of the two optocouplers, the temperature measurement is not delayed
more than 1 us. This guarantees that the temperature
measurements are far less than one unit per cent from
the real values.
The circuit is controlled by a pc through the
parallel port, used also to transfer the measured value
to the pc. The program, written in Visual Basic, is used
to set the duration of the energy pulse, the waiting time
between two measurement and the number of average
and to ease the construction of the calibration plot.
Furthermore, to avoid boring data manipulation, the
program is able to communicate with a spreadsheet to
create the graph of the measured value of temperature,
Drain-Source voltage, current and power dissipated.
Another feature of the system is its capability to stop
the polarization if the temperature goes over a preset
value. This feature can be used to monitor the junction
temperature of the device during long DC stress and
A. Tazzoli et al. / Microelectronics Reliability 45 (2005) 1742–1745
65
160
Original Package
Open Package
60
1745
140°C limit point
140
120
50
45
DC = 98.5%
40
DC = 95.7%
35
30
Vg = 5.8V
Dissipated Power = 7.8W
25
20
000E+0
25E-3
50E-3
75E-3
100E-3 125E-3
Time [s]
Temperature [°C]
Temperature [ºC]
55
100
Vg = 5.8V
Vd = 35V
Id = 400mA
Power = 14W
80
60
40
20
Ambient Temperature = 24°C
0
0
2000
4000
6000
8000
10000
Time [ms]
Fig. 6. Comparison between original and open-package
LDMOS biased with Duty Cycle of 95.7% and 98.5%.
can be very helpful to avoid self-destruction due to an
excessive self-heating.
3. Measurements
Figures 4 to 7 depict several plot of the increase of
the junction temperature during the self-heating stage
and during the following cooling. LDMOS tested were
either in closed package or in opened one, in order to
evaluate the differences in the heating and the
influence of the package. As expected open-package
devices dissipate the heat better than packaged ones,
but the difference is not so pronounced.
All measures were made biasing the Gate of the
device under test at 5.8V and the Drain at about 18V in
order to make the transistor dissipate about 7.8W. The
duration of the energy pulse was instead variable and,
being the duration of the temperature measurement
fixed to about 20us, the corresponding duty cycle starts
from about 80% to about 99%. In the last case,
depicted in Figure 7, the LDMOS can be considered
stressed like in a DC stress, but when the temperature
reached the preset maximum value, 140°C in this case,
the system turned off the power and left the device to
self-cooling to avoid self-destruction.
4. Conclusions
We have developed a new system capable to
indirectly measure the device temperature which have
the following main advantages with respects to other
already presented in literature: i) higher resolution in
temperature, Vd, and Id measurements; ii) fast speed in
Fig. 7. The measurement system can automatically stop
the biasing when the temperature goes over a preset value.
In this case the preset value was set at 140°C.
the switching topology stage (self-heating/temperature
measurements); iii) fast speed in temperature
measurement; iv) adaptability with any devices which
present an intrinsic diode; v) possibility of integrate
this circuit with other electrical stress system in order
to monitor and limit the maximum temperature.
References
[1] J. C. Whitaker, “The Electronics Handbook, Heat
Management”, CRC Press, IEEE Press, pp. 1133-1157
[2] R.Menozzi, J. Barrett, P. Ersland, A. Kingswood, “New
Methods for Easy DC Extraction Of The Thermal
Resistance Of Microwave Bipolar and FET Devices”,
Proceedings from WOCSDICE 2005, Session 4, pp. 8991.
[3] R. S. Muller, T. I. Kamins, “Device Electronics for
Integrated Circuits, 3rd edition”, Ed. Wiley, 2003, pp.
15–64.
[4] J.M. Bosc, P. Dupuy, J. Gil, J.M. Dorker, G.
Sarrabayrouse, “Thermal characterization of LDMOS
transistors
for
accelerating
stress
testing”,
Microelectronics Journal n. 31, 2000, pp. 747-752.
[5] Application Note: “Transient Thermal Resistance –
General Data And Its Use” , AN569, Motorola
Semiconductor Products Inc.
[6] Application Note: “Basic Semiconductor Thermal
Measurement”, AN1570, Motorola Semiconductor
Products Inc.