ISSN 10637397, Russian Microelectronics, 2012, Vol. 41, No. 7, pp. 383–386. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © Yu.F. Adamov, A.G. Sibagatullin, I.O. Shurchkov, 2011, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Elektronika, 2011, No. 4, pp. 14–18.
Heterostructural bipolar transistors (HBT) are
used in highperformance integrated circuits requiring
a high current density (up to 10 mA/
). HBTs sur
transistors in the admissible cur
rent density by a factor of 10 and in the cutoff amplifi
cation frequency by a factor of 5 . In a mode with a
low current density (0.1 mA/
), HBTs have no fun
damental advantage in performance compared with
usual silicon transistors since their performance is lim
ited by barrier capacities of the structure and external
resistors in this mode. An increase in the current den
sity of the HBT by a factor of 10 leads to a proportional
increase in power density and local heating . The
problem of accounting for the transient thermal pro
cesses becomes very urgent.
To simulate thermal processes in electronic cir
cuits, the method of electrothermal analogy is used.
The heat source is represented as the current source,
while the temperature is represented as voltage. There
are elements for accounting the electrothermal cou
pling in the known BJT504t model for the HBT with
the base in the SiGe layer, such as the effective thermal
resistance and the effective heat capacity of the physi
cal structure of the transistor (Fig. 1).
The thermal flow is distributed in space, and the
transient heating process cannot be described by two
concentrated elements. Using the BJT504t model, we
can only evaluate the time for attaining the established
mode. The circuit delay for the input signal frequency of
about 1 MHz can be computed only with a large error.
MEANS TO SIMULATE THE HBT
The CAD means allow us to construct the modified
HBT model with the distributed structure of heat
resistances and capacities based on the BJT504t model
(Fig. 2) . The model takes into account the thermal
energy flows into the silicon substrate to the case base
and into the metallization system. Model parameters
can be determined using the available CAD means or
approximate analytical calculations.
Figure 3 represents the computed time depen
dences of the temperature of the collector junction
upon switching on the power of 4 mW in the HBT with
m in size found using the starting
BJT504t model and the modified model.
The main means to decrease the heat resistance of
the HBT is the separation of the emitter into several
sections. This allows us to reduce the static component
of heat resistance by a factor of more than 3. The pos
sibilities to separate the emitter are limited by the
A local increase in the HBT temperature primarily
affects the voltage of the emitter–base
and the current amplification. The variation in the
voltage of the emitter junction leads to instability of
the zero bias voltage of the differential cascades. The
variation in the current amplification violates the
operation of the blocks based on the current mirror.
OF DESIGNING ANALOG BLOCKS
A decrease in the influence of the electrothermal
coupling in radio engineering blocks operating only
with highfrequency signals is attained by the exclu
sion of galvanic couplings between the cascades and by
the independent stabilization of the operational mode
of each cascade (Fig. 4). It should be noted that when
simulating the established periodic mode, it is neces
Correction Methods of the Electrothermal Coupling in Analog Blocks
Based on Heterostructure Bipolar Transistors
Yu. F. Adamov, A. G. Sibagatullin, and I. O. Shurchkov
Institute for Design Problems in Microelectronics, Russian Academy of Sciences,
ul. Sovetskaya 3, Zelenograd, Moscow, 124681 Russia
Received March 15, 2011
The problem of taking into account the thermal processes in highperformance microcircuits
based on heterostructural bipolar transistors (HBT) is considered. A new thermal model of the HBT is sug
gested, which differs from the known BJT504t model by the addition of thermal resistance and thermal
capacity. The means to decrease the thermal resistance of the HBT and to correct the electrothermal coupling
in the comparator are suggested. Simulation of the comparator using the simplified model of the transistor in
a wide temperature range showed that the use of compensation makes it possible to attain a decrease in the
magnitude of hysteresis by a factor of 10.