Energy Capability of LDMOS as a Function of
Ambient Temperature
Adarsh Basavalingappa*, Anumeha, Gene Sheu
Department of Computer Science and Information Engineering, Asia University, 500, Lioufeng Rd., Wufeng,
Taichung 41354, Taiwan
E-mail: adarsh.b88@gmail.com*
Abstract—Energy capability of a LDMOS device
structure is shown to have nonlinear relationship with
ambient temperature. Analytical model for energy
capability has been discussed and is in good agreement
with the simulation results. The dependency of critical
temperature on ambient temperature is shown.
Keywords-laterally diffused MOS(LDMOS); critical
temperature; thermal failure; energy capability; ambient
temperature; SOA; TCAD simulation
I. INTRODUCTION
LDMOS has become the most important member of
power semiconductor devices because of its robustness,
power capability, cost, reliability, high voltage, high
frequency applications, and also because of its superior
performance with respect to linearity and efficiency.
However device heating under transistor operation results
in performance degradation or even thermal runaway.
Hence understanding the thermal effects, which is one of
the principle failure mechanisms, is very important for
designing and optimizing the device to have high energy
handling capability.
Safe operating area is a fundamental limiting property
of the LDMOS devices as claimed by work of Hower et
al. [1], [2]. It was shown by [3] that these devices will be
affected by electro-thermal instabilities as both electrical
and thermal effects are coupled in nature unlike the
claims of [4] where the device will be affected by
electrical or thermal instability.
Localized heat source concept is considered and
analytical model for thermal breakdown has been
developed in the past [5]. The major complexity in this
method is the strong temperature dependence of the
material parameters thermal conductivity (K) and
thermal diffusivity (D). Using the appropriate effective
values of K and D is very important [6]. Previous works
of Hagino et al. [7] and Khemka et al. [8] have assumed
constant values for K and D for all the ambient
temperature conditions and have concluded that energy
capability has a linear dependency over ambient
temperature. In this paper the LDMOS energy capability
has been studied making use of TCAD software
Sentaurus [9] and we see nonlinear relationship between
energy capability and ambient temperature. The
calculated energy capability values from the proposed
analytical model are in good agreement with the
simulation results.
II. ANALYTICAL MODELING
An analytical heat flow model is necessary for
calculating both electrical and thermal safe operating
area. We follow the Green function approach of Dwyer
et al. [5].A thin rectangle of dimension a x b on a semi-
infinite silicon surface at z=0 is assumed to represent the
heat source. A three-dimensional view of the heat flow
region is shown in Fig. 1. Thermal failure of a semi-
infinite piece of semiconductor subjected to a
rectangular power pulse is assumed to occur when the
critical temperature is reached, where the excess carrier
generated due to thermal effects exceeds the majority
carrier concentration. For a surface heat source of lateral
dimensions a x b in a semi-infinite medium (ab), power
to failure depends on the time scale with respect to the
characteristic diffusion times ta, tb associated to these
dimensions, where these times represent the time
required for the thermal gradient to reach equillibrium in
the dimesions a and b respectively [5] and are given by
following formulae
and
(1)
Where D is the thermal diffusivity (cm
2
/s).
Since b a, the device reaches complete thermal
equilibrium after a time of approximately t
a
.
The energy capability of the device E can be calculated
in Joules by using the equation
(2)
where Po is magnitude of the constant pulse power (W),
t is the period (s), T
C
is the critical temperature (K), T
O
978-1-4673-0192-3/12/$31.00 ©2012 IEEE 65