Datasheet
V
be
= K ln (N)
k T
q
I
F
= I
S
e
V
be
KV
t
V
t
=
k T
q
I
F
= I
S
e - 1
V
be
KV
t
LM95221
SNIS134B –MAY 2004–REVISED MARCH 2013
www.ti.com
DIODE NON-IDEALITY
Diode Non-Ideality Factor Effect on Accuracy
When a transistor is connected as a diode, the following relationship holds for variables V
BE
, T and I
f
:
where
•
• q = 1.6×10
−19
Coulombs (the electron charge),
• T = Absolute Temperature in Kelvin
• k = 1.38×10
−23
joules/K (Boltzmann's constant),
• η is the non-ideality factor of the process the diode is manufactured on,
• I
S
= Saturation Current and is process dependent,
• I
f
= Forward Current through the base emitter junction
• V
BE
= Base Emitter Voltage drop (2)
In the active region, the -1 term is negligible and may be eliminated, yielding the following equation:
(3)
In the above equation, η and I
S
are dependant upon the process that was used in the fabrication of the particular
diode. By forcing two currents with a very controlled ratio (N) and measuring the resulting voltage difference, it is
possible to eliminate the I
S
term. Solving for the forward voltage difference yields the relationship:
(4)
The voltage seen by the LM95221 also includes the I
F
R
S
voltage drop of the series resistance. The non-ideality
factor, η, is the only other parameter not accounted for and depends on the diode that is used for measurement.
Since ΔV
BE
is proportional to both η and T, the variations in η cannot be distinguished from variations in
temperature. Since the non-ideality factor is not controlled by the temperature sensor, it will directly add to the
inaccuracy of the sensor. For the Pentium 4 and Mobile Pentium Processor-M Intel specifies a ±0.1% variation in
η from part to part. As an example, assume a temperature sensor has an accuracy specification of ±1°C at room
temperature of 25 °C and the process used to manufacture the diode has a non-ideality variation of ±0.1%. The
resulting accuracy of the temperature sensor at room temperature will be:
T
ACC
= ± 1°C + (±0.1% of 298 °K) = ±1.4 °C (5)
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