You might come across the terms ideal and practical diodes. As a beginner, these terms might confuse you. So, I think it is important to discuss the properties of an ideal diode and a practical diode.
Outline
- Discuss different diode approximation or diode models
- How does practical and ideal diode model differ from each other?
- Why do we need to learn the behaviour of an ideal diode?
Diode Models And IV Characteristics:
The current-voltage characteristics curve is shown in figure 7. Look at the graph, how much a practical diode deviates from ideal behaviour. An ideal diode is a piecewise linear device, while a practical diode is a non-linear device. Linear device means its current-voltage graph is a straight line. A diode is non-linear because its current-voltage graph is a non-linear curve. For voltage values less than 0.7V, the current is zero. Just above 0.7V, the current increases rapidly. Current doesn't increase proportionally to an increase in voltage.
First Approximation Or Ideal diode:
This is the simplest approximation. The graph is simple and piecewise linear. Zero resistance when forward biased, infinite resistance when reverse biased. It is analogous to a mechanical switch. Because the switch has zero resistance when close. And a switch has infinite resistance when open. The voltage across the diode is
VD = 0
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| Figure 1 Ideal diode and its equivalent |
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| Figure 2: Ideal diode IV Curve |
Second Approximation Or Practical diode Model:
The graph is piecewise linear. Look at the graph, the diode doesn't conduct until the voltage reaches 0.7.
According to this approximation, a diode is analogous to a switch in series with a barrier potential of 0.7V. Look at the figure, the diode becomes forward bias when the applied potential is at least 0.7V (close switch). Hence current increase rapidly.
For applied voltage below 0.7V, it remains reverse bias (open switch). The voltage across the diode is
VD = 0.7 V
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| Figure 3 Non-ideal diode and its equivalent |
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| Figure 4 Non-ideal diode, IV Curve |
Third Approximation Or Detailed Model:
According to this approximation, a diode is analogous to a switch, a barrier potential and a resistor (Rf), connected serially. Rf shows the internal resistance of semiconductors.
Now examine the effect of Rf. The diode turns on as the applied voltage is 0.7V or above. Thereafter, the current increases as an increase in voltage (Rf is ohmic resistance). We can apply Ohm's law and find voltage and current through it. The voltage across the diode is
VD = IDRf +0.7
Where
VD = diode voltage
ID = diode current
Rf = forward resistance of diode or bulk resistance
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| Figure 5 Third approximation of diode |
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| Figure 6 Third approximation IV Curve |
Difference between ideal and practical diode:
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| Figure 7 Practical diode Vs Ideal diode IV Curve |
You have seen all three diode approximations. The figure above is the graph obtained from a real diode. In this graph, we consider all the effects of ohmic resistance, threshold voltage, leakage current and breakdown region.
Forward Biased
Ideal Diode
- It behaves as a perfect conductor
- An ideal diode is like a closed switch. It has 0Ω resistance between anode and cathode
- No need for threshold voltage
- It has zero resistance, and hence infinite current through the diode. (I = V/R)
Real Diode
- It also acts as a conductor. Due to imperfections, it offers a small forward resistance Rf
- It is also like a closed switch, except a small forward resistance Rf
- Real diodes need a little voltage called knee voltage or threshold voltage to overcome barrier potential
- Due to small forward resistance Rf, there is a voltage drop at the diode and hence finite current through the diode
Reverse Biased
Ideal Diode:
- It behaves as a perfect insulator
- An ideal diode is like an open switch in reverse bias conditions. No current can flow from anode to cathode
- Breakdown region is not possible at any magnitude of applied voltage
Real Diode:
- It behaves as an insulator. There is a voltage limit called the breakdown voltage. The diode can not operate beyond this limit
- Real diodes deviate from ideal behaviour because of minority carriers. Due to minority carriers, it has some reverse current flows from cathode to anode. This current is called reverse saturation current or leakage current
- They do have a breakdown voltage. If the applied voltage exceeds the rated voltage, results in junction breakdown
Need of Ideal Diode Behaviour:
You have read the features of an ideal diode. An ideal diode can not possible to produce. It has infinite resistance in reverse biased conditions, and hence no current. It has zero resistance in forwarding biased conditions, and hence infinite current. Such conditions are not possible practically.
Which diode model commonly use during analysis?
Most of the time we consider the third approximation.
Why do we consider an ideal diode?
We consider the ideal diode at the first stage of our analysis and troubleshooting. It is easy to understand the circuit analysis with an ideal diode. As a beginner, it is better to start with ideal diodes, to keep circuit analysis as simple as possible.
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