SEMICONDUCTOR DIODE

Third Approximation
 Third approximation can be used to account
more accurately for diode forward voltage drop
 The diode I-V characteristic is not vertical above
the knee voltage but actually slopes upwards to
the right
 This means that as more current flows through
the real diode, more voltage is dropped across it
 Once the diode conducts, it acts like a resistor



Department of EE 1

,Third Approximation….
Change in voltage is directly proportional
IF to the change in current
V
r
I
ΛI
This is resistance of the semiconductor
ΛV material (p and n regions)

VF This resistance is called forward or bulk
VB resistance of the diode (rB)
(Knee
voltage) V
rB 
I

Department of EE 2

, Third Approximation…
 The equivalent circuit is an ideal diode in series
with a battery of value VB and a resistor of value
rB
Ideal VB rB



 It takes at least 0.3V or 0.7V to turn on the diode. The diode
then acts like a resistor of value rB which drops additional
voltage depending on the amount of current drawn



Department of EE 3

, Estimating Bulk Resistance
IF If the I-V characteristic is available,
select two points well above the
(mA)
knee of the curve
50
V
rB 
I
10
0.8V  0.75V 0.05V
VF (V)    1.2
VB
50mA  10mA 40mA
0.80
0.75

Department of EE 4