AN2386
Application note
How to achieve the threshold voltage thermal coefficient
of the MOSFET acting on design parameters
Introduction
Today, the MOSFET devices are used mainly as switches in electronic circuits. In such
operational conditions, the MOSFET device works in switch on and switch off modes.
However, in some applications, as in audio amplifiers or air conditioning, the MOSFET
works in a linear zone. The MOSFET works in a linear zone when either it is subject to a
high voltage, or a high current passes through the device. As it is well known in literature,
during the linear zone operation mode the MOSFET could fail if a thermal run-away occurs.
The failure conditions depend on either of the internal structure of MOSFET or of the
package used. The threshold voltage thermal coefficient (TVTC) is one of the big elements
that could bring the MOSFET to fail. TVTC is achieved deriving the MOSFET threshold
voltage against the temperature. TVTC is a negative coefficient because of when the
temperature increases the threshold voltage decreases. When TVTC increases in absolute
value, the MOSFET becomes thermally instable and a failure could occur. Therefore, in
order to understand if a MOSFET device can be used in an application working in linear
zone in safety conditions, a device with a low TVTC value must be considered and, thus, it is
important to achieve a theoretical expression for it.
June 2006
Rev 1
1/30
www.st.com
Contents
AN2386
Contents
1
2
MOS structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Some considerations on V
TH
and TVTC equations and real examples .
14
Case of DEVICE3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3
4
5
6
2/30
AN2386
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Cross section view of a MOS capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Energy band diagram of an ideal MOS capacitor under thermal equilibrium.. . . . . . . . . . . . 5
Energy band diagram and charge distribution in an ideal MOS capacitor in accumulation condition
. . 6
Energy band diagram and charge distribution in an ideal MOS capacitor in accumulation condition.
. 6
Energy band diagram and charge distribution in an ideal MOS capacitor in inversion condition
. . . . 7
DEVICE1 V
TH
- comparison between simulated and measured data . . . . . . . . . . . . . . . . 16
DEVICE1 TVTC - comparison between simulated and measured data . . . . . . . . . . . . . . . 16
Weight of single term of (Equation
42)
on TVTC at different temperatures . . . . . . . . . . . . 17
DEVICE2 V
TH
- comparison between simulated and measured data . . . . . . . . . . . . . . . . 18
DEVICE2 TVTC - comparison between simulated and measured data . . . . . . . . . . . . . . . 19
DEVICE1 V
TH
simulated data - comparison between different N
g
. . . . . . . . . . . . . . . . . . . 19
DEVICE1 TVTC simulated data - comparison between different N
g
. . . . . . . . . . . . . . . . . 19
DEVICE1 V
TH
simulated data - comparison between different N
a
. . . . . . . . . . . . . . . . . . . 20
DEVICE1 TVTC simulated data - comparison between different N
a
. . . . . . . . . . . . . . . . . 20
DEVICE1 V
TH
simulated data - comparison between different tox. . . . . . . . . . . . . . . . . . . 20
DEVICE1 TVTC simulated data - comparison between different tox . . . . . . . . . . . . . . . . . 21
DEVICE1 V
TH
simulated data - fixing T and acting on N
g
. . . . . . . . . . . . . . . . . . . . . . . . . 21
DEVICE1 TVTC simulated data - fixing T and acting on N
g
. . . . . . . . . . . . . . . . . . . . . . . . 21
DEVICE1 V
TH
simulated data - fixing T and acting on N
a
. . . . . . . . . . . . . . . . . . . . . . . . . 22
DEVICE1 TVTC simulated data - fixing T and acting on N
a
. . . . . . . . . . . . . . . . . . . . . . . . 22
DEVICE1 V
TH
simulated data - fixing T and acting on t
ox
. . . . . . . . . . . . . . . . . . . . . . . . . 22
DEVICE1 TVTC simulated data - fixing T and acting on t
ox
. . . . . . . . . . . . . . . . . . . . . . . . 23
V
TH
simulated data - comparison between DEVICE1 and the new device . . . . . . . . . . . . 23
TVTC simulated data - comparison between DEVICE1 and the new device . . . . . . . . . . . 23
V
TH
simulated data considering a p-gate doped MOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
TVTC simulated data considering a p-gate doped MOS . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Energy band diagram at low and high doping concentration . . . . . . . . . . . . . . . . . . . . . . . 25
DEVICE3 V
TH
- comparison between simulated and measured data . . . . . . . . . . . . . . . . 26
DEVICE3 TVTC - comparison between simulated and measured data . . . . . . . . . . . . . . . 26
3/30
List of tables
AN2386
List of tables
Table 1.
Table 2.
Table 3.
Main electrical parameter simulated by DEVICE1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Main electrical parameter simulated by DEVICE2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4/30
AN2386
MOS structure
1
MOS structure
As it is well known, a MOS structure is composed by three layers: the first one is metal or
heavily doped polycrystalline silicon, the second one is an insulator of SiO
2
and the third
one is the semiconductor (see
Figure 1.).
Figure 1.
Cross section view of a MOS capacitor
Considering an ideal MOS system with a p-doped semiconductor, the energy band diagram
can be illustrated as in
Figure 2.
Figure 2.
Energy band diagram of an ideal MOS capacitor under thermal
equilibrium.
q
Φ
m
is the work function (energy that needs to extract an electron from the metal); q
Φ
B
is
the energy difference between the oxide conduction band and the metal Fermi energy level
(metal-to-oxide barrier energy); q
Φ
sc
is the work function of the semiconductor; q
χ
is the
energy difference between the vacuum level and the conduction band edge.
When a negative voltage (V
g
) is applied on the gate terminal respect to the semiconductor,
the Fermi level of the metal raises of qV
g
compared to the semiconductor side. In moving
from the semiconductor to the metal, the vacuum level must bend up gradually to
5/30
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
