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International Journal of Advances in Electrical and Electronics Engineering 30
ISSN: 2319-1112
ISSN:2319-1112 /V1N1:30-36 IJAEEE
Power efficiency of diode
(calculation of various resistances)Shashiketan Samal
1, Rama komargiri
2,Kotni Siddharth
3
Department of Electronics and communication Engineering1 3
Student, ISM Dhanbad2Asst. Prof. NIT Calicut
ABSTRACT-The day-to-day life is becoming much faster and hectic. So, there is a long quest between power
consumption and reducing the cost of a device. Hence, in this long run using a particular device efficiently
comes into the field. A simple diode can be much more useful to us if used at a particular voltage considering
the resistances associated with it. This problem is being solved using the simulations of PN junctions with
varying widths. The calculation of series resistance and high level injection resistance gives a brief idea aboutthe resistance associated with the diode. Variation of its nature with voltage and current can be very useful to
calculate the working voltage of a diode.
Keywords-pn junction; diode; power efficiency; series resistance; high level injection resistance; junction
resistance
I. Introduction
Indeed, the diode is the simplest element of electronics. Analyzing the diode in a circuit containing voltage source, It
has been found that the current across the diode increases with increase in applied voltage. This suggests that diode
has certain resistance attached to it. From the theory it is determined that at very high voltage a process called high
level injection occurs which adds to the resistance of diode. Hence a diode can be thought of attached with three
types of resistances:
Series resistance Junction resistance High level injection resistance.
The series resistance can be approximated to be equal on both sides of P and N junctions for equal doping
concentrations. This resistance is negligible for lower width of PN junctions.Hence an analysis has been done to calculate the high level injection resistance and the series resistance of
the diode.
There is a small difference between the voltage applied and voltage across the diode. is the voltage across the
depletion region formed in the diode and this is the value used in the Shockley equation in (1)
(1)
Where, I- diode current
- Saturation current
- Voltage across diode
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31Power efficiency of diode (calculation of various resistances)
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n- Ideality factor
-thermal voltage
A. calculating high level injection resistance
Increasing the voltage, a condition appears when the injected minority carrier densities become comparable to
majority carrier densities and a substantial portion of the applied bias drops across the region outside the depletion
region which itself shrinks. This is high level injection. The diode shows a peculiar behavior at this level in
consideration to resistance called high level injection resistance.
To calculate the high level injection resistance a PN junction of lower thickness (1 ) was used so as to neglect
the effect of series resistance. The simulations done are based on quantum mechanical model on Medici.
The doping concentrations were taken to be on both sides of junctions. The on voltage of diode has been
calculated as
(2)
Where, - cut off voltage
k- Boltzmann constant-
T- temperature-273K
q- charge of electron
- acceptor ions
- donor ions
free electrons
The cut-off voltage was found to be 0.575 volts.
The saturation nature of I-V characteristics of diode has been approximated using (1).
The value of ideality factor, n has been found to be around 2.18 in the linear region next to cut in voltage. Its growthis very rapid in the subsequent voltages.
Hence, the voltage across the diode could be calculated for a particular current value flowing in the circuit due to
the applied voltage. A brief comparison has been provided for V-I characteristics of diode using simulation through
Medici and through Shockley equation as shown in figure.1
Figure.1: Comparison of characteristics of ideal diode and practical diode through simulations on MEDICI
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ISSN:2319-1112 /V1N1:30-36 IJAEEE
Using Kirchhoffs voltage rule in a circuit shown in figure.2.we have,
Figure.2: A circuit showing application of voltage to diode
(3)
Where, - Voltage applied
- Voltage across Diode
- Diode current
- High level injection resistance
The applied voltage gets distributed in to the diode voltage and some part is lost in high level injection resistance.
Findings:
It has been observed that with increase in applied voltage the voltage across the diode increases very
sluggishly after the cut in voltage and remains to be around cut in voltage and becomes constant afterwards. This is
because a voltage barrier is formed due to the depletion region.
The variation of high level injection resistance with applied voltage and current has been studied and graphs are
plotted accordingly in figure.3.
Figure.3.(i): Variation of high level injection resistance with voltage
As this resistance is entirely due to high level injection, so there should be a minimum voltage where this
high level injection occurs. It has been justified to be the cut-in voltage as resistance values below this voltage are
absurd.
The high level injection resistance is found to increase linearly with applied voltage. As at high voltages current
saturates so the slope of voltage and high level injection resistance remains constant. The minimum value of the
resistance can be approximated due to the series resistance.
The variation of this resistance with current flowing in the circuit is shown in fig 3.(ii).
There is a parabolic increase in resistance with current flowing in the circuit.
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Figure.3.(ii): Variation of high level injection resistance with current
B. calculating series resistanceThe series resistance is contributed due to varying width of p-n junction. As the doping concentration is considered
same on both sides so resistance considered is equal. The high level injection resistance has been calculated and it
becomes easier to calculate the series resistance of a diode. Hence a PN junction of larger width (3 ) and doping
concentrations (same as before) has been used.
It has been observed that:
Increasing the doping of P and N junctions, the current saturates at a higher value. This indicates thedecreased level of resistance. This phenomenon is because of increase in number of carriers of holes and
electrons.
Decreasing the doping of junctions saturates the current much earlier. Hence the high level injectionresistance could be neglected in such cases and the resistance can be assumed completely due to the series
and junction resistance.
Hence, the doping level is chosen accordingly so as to include the effect of high level injection resistance.
The series resistance is calculated using (4). Using Kirchhoffs voltage rule in the circuit as shown in fig.2 and
accounting for high level injection resistance, so
(4)
Where - series resistance
The effect of voltage across the diode and high level injection resistance is used to calculate the series the
resistance.
Figure.1 is used to calculate the voltage across the diode for a applied voltage and 2nd
degree interpolation is
used for calculating high level injection resistance
findings:
The variation of series resistance with applied voltage has been plotted below:
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Figure.4.(i): Variation of series resistance with voltage
The series resistance is observed to be constant in the region near the bias voltages. Initially, the series
resistance is high indicating negligible current through the circuit. The resistance is found to increase linearly
after a particular voltage. This may be due to the increase in number of carriers or combination with high levelinjection resistance.
The variation of series resistance with current flowing in the circuit is found to be similar with the above
behavior as shown below:
Figure.4.(ii): Variation of series resistance with current
The resistance increase parabolically after a limiting current in the circuit. Till then the resistance varies sluggishly.
C. junction resistance
As the p and n junctions which have large concentration of holes and electrons are joined, the diffusion of carrier
takes place at the junction due to concentration gradient. In equilibrium the ionized region consisting of positive and
negative charges is called depletion. This gives rise to an electric field and thus electric potential. This built in
potential opposes the flow of majority carriers.
Hence, they are said to face a resistance which appears across the junction i.e. across the depletion region called
junction resistance. This resistance is very high which doesnt allow any current to flow in the circuit initially till the
cutoff voltage given by (2). The variation of built in potential with application of voltage is shown in fig 4.
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Figure.5: Variation of built in potential with applied voltage
The junction potential reaches a constant value swiftly and hence junction resistance is assumed to be of constant
high value.
II. inference
Hence, every diode is associated with three types of resistance- high level injection resistance and series
resistance depending upon the voltages applied. At lower voltages, apart from junction resistance series
resistance dominates which is nearly constant. So, current increases rapidly. As the applied voltage increases,
series resistance and high level injection resistance start increasing linearly although junction resistance is
constant and hence the current saturates at a particular value as justified by the simulations.
Hence while using diode for practical applications in life; special considerations need to be taken for
resistance of diode and voltage being applied to it. To derive maximum power output from the diode the
external resistance should be made equal to the variable diode resistance.
For maximum power output, only increase in voltage wont serve the purpose because after a particular voltagevalue (as observed) the increase in power output is very minimal as shown in the fig.6
Figure.6: Variation of output power with applied voltage
After 2.1 Volts, increase in power output is very less hence this diode should be appropriately used at that
voltage for its aptness.
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III. Conclusion
A diode can be used meticulously and properly if we change the junctions from PN to P+ N. and hence, for its
maximum power output it should be operated at a particular voltage taking resistance (High level injection and
series) into considerations.
IV. ACKNOWLEDGMENT
I would like to express my deep sense of gratitudeto our Professor Dr. Rama Komaragiri, Department of ECE, NIT
Calicut for providing us such a great opportunity to work under him.We would like to thank Dr. P.S. Sathidevi,
HOD, ECED, NITCalicut for her support and providing us the facilities.
V. REFERENCES
[1] Semiconductor optoelectronic Devices Pallab Bhattacharya.[2] Robertboylestad, Louis nashelsky Electronic Devices and Circuit Theory