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Biasing Modes for Diodes
Diodes have two biasing
modes:
 Reverse biasing :
diode current (ID) is
almost zero. The only
current is the reverse
saturation current (Is or
I0 )
I D  I s
Forward Biased Diode
 Forward Biasing:
diode current (ID)
is the difference
between diffusion
current (Id) and
reverse saturation
current (Is).
VT = 0.025 volts at
room temperature
(Thermal Voltage)
I D  Id  Is
I d  I se
I D  I s (e
qv D
kT
qv D
kT
I D  I s (e
vD
VT
 1)
 1)
Ideal
Current-Voltage
(I-V)
Characteristics
The p-n junction
only conducts
significant current in
the forward-bias region.
iD is an exponential
function in this region.
Essentially no
current flows in reverse
bias.
Circuit Symbol
Conventional current
direction of the Diode
(ID) is from P-type to Ntype.
polarity of voltage drop
(VD) is as shown in the
figure.
Diode Breakdown voltage
 Happens when a large
reverse voltage applied
to the diode.
 At breakdown the diode
conducts current in the
reverse direction.
 The magnitude of the
breakdown voltage (BV)
is smaller for heavily
doped diodes as
compared to more lightly
doped diodes.
Diode I-V curve
Cct. Models for semiconductor diodes
Diode Models
Large Signal
Ideal Diode
Small Signal
Offset Diode
Piecewise Linear
Large Signal Vs. Small signal Models
 In small signal model, the exact I-V char.
Curve is used to determine diode voltage and
current
 it is used to determine accurate operating
point of the diode.
 Large signal models use approximating I-V
char. Curve and thus approximate results of
diode voltage and current.
Large signal ideal diode model
 treats the diode as a simple
on-off device.
 The analysis of a circuit
containing a diode may be
greatly simplified by using the
short-circuit–open-circuit
model.
approximated by an open
circuit when vD < 0
 approximated by short circuit
when vD ≥ 0
Large signal ideal diode model
 For Ideal diode model we assume the following:
 Cutoff frequency Vγ=zero
 Reverse Biased current =zero
 Internal resistance of the diode=zero (no limiting
resistance for the forward current)
How to solve circuits with diodes ???
 Diodes are considered non-linear elements since
it has non-linear I-V char. Curve.
 If we have a diode in a circuit we need first to
determine the conduction state of the diode
(Forward or Reverse) (conducting or not)
 After the state is determined, we can replace the
diode with open or short circuit (linear elements)
 Finally we solve the resulting linear circuit using
one of the linear DC circuit analysis techniques.
Circuits with more than one diode
(1)Assume a state for each diode, either “on”
or “off” (conducting state)
(2) Assume a short circuit for diode “on” and
an open circuit for diode “off”
(3) Check to see if the result is consistent with
the assumed state for each diode (current
must flow in the forward direction for diode
“on” and the voltage across the diodes (VD)
assumed to be “off” must be positive at the
cathode – reverse bias)
(4) If the results are consistent with the
assumed states, the analysis is finished.
Otherwise return to step (1) and choose a
different combination of diode states.
How to determine the conducting
state?
Example 1: Diode in DC circuit
DC Circuit containing ideal diode
 We can use
Kirchhoff's
voltage law and
Kirchhoff's
current law.
 Thevenin and
Norton Theorems
can also be used.
Nodal analysis
Mesh analysis
Kirchhoff’s voltage law
Thevenin & Norton theorems
Example 1: Ideal Diode in DC circuit
(cont.)
 First, we assume diode is
not conduction (open
circuit)
 Then check VD value.
 If positive  assumption VD = 1.5 > 0
wrong Diode is short
 If Negative assumption Assumption wrong  Diode should be
shorted
correct  diode is open
 Then replace the diode by
open or short.
 Finally solve the linear
circuit.
Example 1: Ideal Diode in DC
circuit (another approach)
 First, we assume diode is
conducting (short circuit)
 Then check ID direction.
 If ID from P to N side 
assumption correct  Diode
is short
 If ID from N to P side
ID = 1.5 mA (from P to N side)
assumption wrong diode
Assumption correct  Diode should be
is open
 Then replace the diode by shorted
open or short.
 Finally solve the linear
circuit.
Example 2 : Diode in DC circuit
 Determine the conduction state of
the diode.
1) We assume it is open.
2) We find VD to be (-3 volts)
3) Our assumption is correct (the
diode is open.
Example 3 : Diode in DC circuit
Practice
Determine if the diode is on or off. ?
More than one diode in a circuit.
Analyze the circuit by assuming D1is off and D2 on
(1) assume
D1 off, D2 on 

(2) assume
D1 on, D2 off
i D2

 0.5mA OK!
v D1  7V
not OK!
 i D1  1 mA OK!
v D2  -3 V
OK!
More than one diode in a circuit.
* Find the diode states by using ideal-diode model. Starting by
assuming both diodes are on.
(1) assume
D3 on 
D4 on
(2) assume D3 off and D4 on
iD 3


 -1.7 mA, not OK
i D 4  6.7 mA, OK
 i D 4  5 mA, OK
v D 3  -5 V,
OK