Download Electronics-I Lecture-3

 A discrete diode is a simple PN Junction , enclosed in a
case and connections for the interfacing.
 The positive side(P-Type part) of the diode is called
anode(A) and negative side(N-Type part) , cathode(K).
 A diode can have three models (a representation of a
circuit with alternative circuit components whose
combined behavior closely resembles the actual device)
1. Ideal Model(A forward biased diode is replaced by a
short circuit and reverse biased with an open circuit)
2. Practical Model(A forward biased diode is replaced by
a battery of 0.7 V for Si diode and 0.3 V for a Ge diode
and reverse biased with an open circuit)
 For this course we will only be using practical diode
models.
 An ideal diode acts like a switch.
 When it is forward biased it allows current through it
while voltage across it remains zero. This translates to
Zero resistance or a Short circuit.
 When it is reverse biased it allows no current through
it while the voltage across it increases linearly with
applied voltage. This corresponds to Infinite
resistance or an Open circuit.
 An ideal diode is a model to which practical devices
are compared to for performance.
 A practical diode can’t have zero volts across it when
it is conducting. It requires a voltage of 0.7 V in case
of Si and 0.3 V for Gi diode.
 In practical models the short circuits of ideal diode
models are replaced by batteries of 0.7V and 0.3V for
Si and Ge diodes respectively.
 In real diodes, the forward biased diode voltage does
not remain fixed when applied potential is increased.
It increases with the applied voltage a little.
 This increase in potential is due to
1. Semiconductor body resistance
2. Contact resistance
 Consequently, as the applied voltage is increased, the
resultant increase in current increases the voltage
across the device in accordance with of Ohm’s law
(V=IR)
 Similarly, when a reverse voltage is applied across the
real diode, the current across it is not zero. There is
always some current flowing due to minority charge
carriers and is called the reverse leakage current.
 For real diodes, the current equation is given by,
 ID=IS(ekvd/T-1)
 Where Is= reverse saturation current
 k=11,600/ when =1 for Ge and high current values in Si
 =2 for small values of current in Si
 Vd= voltage across diode
 T=Tc+273
 In essence there are couple ways to solve circuits
containing diodes.
A.
B.
Using Load lines and characteristics curve
Using diode equivalent t models only
 The easiest way to solve diode circuits is by using the
second technique and it follows these steps
1. Check whether the diode is forward biased or
reverse biased in a particular circuit
A.
B.
C.
Replace diodes with hypothetical resistors
Find the direction of current through these resistors
If conventional direction of current is positive(in the
direction of arrow) and the applied voltage is greater than
diode forward biased voltages, the diode is forward biased
else the diode is reverse biased.
2. If the diode is forward biased replace it with a
battery of 0.7V for Si and 0.3V for Ge diode. If it is
reverse biased open circuit it.
3. Solve the resultant circuit using circuit analysis
techniques.
 Example-1
Example-4
 Example-2
Example-5
Repeat example- 1 with diode reversed.
 Example-3

Example-6
 Example-1
 Example-2
 Example-3
Example-4