What are the Diode Models?

In this article I have discussed in detail what the diode ideal and practical model are. How do we treat the diode as a circuit element? Which diode model is best while analyzing or designing a circuit? Basically, a diode is a pn junction device, and we can analyze any circuit in three possible ways when that circuit contains a diode in it.

There are three models of the diode:

Ideal model
Practical model
Complete diode model

What is the Ideal Diode Model?

This is the most simple approach where we treat the diode as an ideal element. By ideal we mean we will ignore the barrier potential of the diode that is 0.7 V in the case of silicon and 0.3 v in the case of germanium when it is forward biased. So, when a diode is forward biased, we will say an infinite amount of current is flowing with zero voltage drop across its terminals. When it is reverse biased, we will say it has infinite resistance and hence zero leakage current across its terminals.

The ideal diode model is not a very accurate model and can be modeled as a simple switch.

In this model we have ignored barrier potential, forward dynamic resistance, and reverse leakage resistance. This model is helpful for troubleshooting or while checking if the diode is working or not properly. We get the following equations in this case.

V_F=0V

undefined

I_R=0A

V_R=V_BIAS

What is the Practical Diode Model?

In this model, barrier potential is included. When a diode is forward biased, it is considered as a closed switch with a voltage source connected in series. This voltage source is actually a barrier potential that must be exceeded by the external battery in order to cause the conduction. When a diode is reverse biased, it is equivalent to an open switch, just like in the case of ideal diode model. The barrier potential has no effect on reverse bias.

The calculations can be done using the following formulas:

VF = 0.7 V

I_R=0A

V_R=V_BIAS

What is the Complete Diode Model?

As its name indicates, all the parameters that we neglected in the ideal model are now considered. So when a diode is forward biased, a voltage source of 0.7 V and a small dynamic resistance connected in series are considered with external biased voltages. While in the reverse bias condition, it is again an open switch but with a large reverse resistance connected in parallel with a switch. This resistance provides a path to the leakage current that is always present. This is the most accurate approximation of the diode, as it considers all parameters of it during analysis.

The graph for the complete diode model appears in Figure (c). Since the barrier potential and the forward dynamic resistance are included, the diode is expected to have a voltage across it when it is forward biased. This voltage (VF) comprises barrier potential plus the small voltage drop across the dynamic resistance. The curve slops as the voltage drop across the dynamic resistance increases because of the increase of current. For the complete model of a silicon diode, the equations we can apply are:

The reverse leakage current is shown on the left side of the graph. Since the breakdown operation is not a normal operation for most of the diodes, that is why it is not shown in the characteristic curves.

Example of Practical Diode Model

Find the voltages at each point with respect to ground in the following circuit using the practical diode model.

diode models example — diode model example

Solution

As mentioned above in the complete model we assume the forward voltage drops across the diode (V_F=0.3V in case of Ge and 0.7V in case of Si)

Applying KVL, we get

-25+0.7+I_FK+0.7+8=0

-17+1.4+ I_FK=0

15.6=I_FK

——à I_F=15.6mA

This is the forward current flowing in the circuit

Now, let’s calculate the voltages at each point

At point A

V_A=25V as at this point the battery or source is connected and there is no voltage drop.

At point b

V_B=25V-0.7V=24.3V

At point C

Voltage is equal to

25V-0.7V-(15.6mA)(1K)=25V-0.7V-15.6V=8.7V

Now at the last point D

V_D=8V as it is directly connected with 8V battery terminal

5. Determine whether each silicon diode in Figure 2–92 is forward-biased or reverse-biased.
6. Determine the voltage across each diode in Figure 2–92, assuming the practical model.
7. Determine the voltage across each diode in Figure 2–92, assuming an ideal diode.
8. Determine the voltage across each diode in Figure 2–92, using the complete diode model with rd=10Ω and r_R=100M Ω

Solution:

Part 1;

part 2:

Which Diode Model is Better for a Circuit? Ideal, Practical or Complete?

If we are generally solving a numerical problem then it is preferable to use an ideal diode model where the voltages dropped across a diode are not of significance importance. For example when you are designing an inverter or a switch, it is better just to consider the 0 voltage drops when it is ON and V-bias voltages when it is OFF. While working in your college laboratory it is better to use a practical model of diode. Where you will consider 0.7V voltage drop across a diode. A complete diode model is used when you are analyzing a simple circuit or when a circuit is not much complex. Since it would be easier to consider the effect of dynamic resistance and reverse resistance for better understanding.

watch here for a video lecture.

Applications of Ideal, Practical, and Complete Diode Models

Diode models are used in electronic circuit analysis to simplify calculations and predict the behavior of real diodes. Each model is suitable for different applications depending on the required accuracy.

Summary table

Feature	Ideal model	Practical model	Complete model
Assumed forward drop	0 V	Constant drop	Constant drop plus resistive drop
Calculation complexity	Very low	Low	Moderate
Typical use	Basic circuit analysis	Everyday circuit design	More accurate current and loss estimates
Accuracy	Lowest for real-device behavior	Better	Better still within the model’s assumptions

Ideal, Complete and practical Diode Models with Solved Examples