Even More Forward Bias
•
If we know the voltage and current
for a diode at any point on it’s I-V
curve we can find any other
operating point:
•
Alternatively in Log base 10:
•
Here we can see that for a decade
change in current, the voltage drop
across the diode will change by
2.3nV
T
, for a diode with n=1 this is
approximately 60mV at room temp.
•
If the temp increases, then the
voltage drop will increase for a fixed
current.
(
)
T
nV
V
V
e
I
I
1
2
2
1
−
=
1
2
1
2
ln
I
I
nV
V
V
T
=
−
1
2
1
2
log
3
.
2
I
I
nV
V
V
T
=
−
Log base 10

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Cut-in Voltage
•
With the exponential dependence of the current on the
applied voltage, the I-V curve in forward bias appears
to start off with zero current near zero bias and then
suddenly increase substantially.
•
Of course there is a current near zero bias, but it is
extremely small (pA or nA)
•
The cut-in voltage is typically around 0.5V. This
voltage drop will have consequences for circuits using
diodes as we shall see later.
•
A diode that is “fully conducting” is typically operating
with biases of 0.6 to 0.8 Volts.

Reverse Bias and Breakdown
•
The reverse bias region is much simpler than the forward
bias region.
–
If we go back to the diode equation, we can see that negative
values for the applied bias result in an effectively constant
current:
–
Perhaps you can see where the term saturation current came
from, in reverse bias the current appears to saturate.
–
In reality though, the current in reverse bias is dominated by
leakage currents, due to imperfections in the diodes. These
leakage currents are about 1000x larger.
•
Just like the forward bias region, even larger negative
biases lead to breakdown. This is not covered in this
model
*
.
•
The voltage that this occurs at is called the breakdown
voltage V
ZK
where the Z stands for Zener (we’ll talk about
that later) and the K stands for “knee” meaning the point
where the slope changes dramatically on the I-V curve.
* It’s worth mentioning here a general issue. Just about every
equation you see is an approximation. You should always
keep in mind what its limits are!
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
1
T
nV
v
S
e
I
i
0
S
I
i
−
≅

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