Hey friends, welcome to the kohikhi.com ALL ABOUT ELECTRONICS. So, in this article and the subsequent article, we will learn about the diode. And these are the following topics that we will cover in the series of articles. And in this particular article, we will learn that what is a diode, and how the V-I characteristics of the diode will look like.

What Is Diode

we will also see the equivalent circuit for the diode. And in the subsequent article, we will learn about the device physics of this diode and we will also see the different applications of the diode. So, now the question is what is a diode. Now, we all know about the resistor. It is the most widely used type of passive circuit element. And it allows the flow of current in both directions.

Similarly, the diode is a two-terminal semiconductor device that allows the flow of current only in one direction. And if you see the symbol of the diode, it looks like this. So, here this arrow indicates the direction in which this diode allows the flow of current.

Now, the one terminal of this diode is known as the anode and the second terminal of the diode is known as the cathode. Now, whether this diode will allow the flow of current or not, it depends on the polarity of the voltage that is applied between this anode and cathode.

So, if the voltage that is applied between this anode and cathode is positive, then, in general, we can say that this Article will allow the flow of current in one direction. While the voltage that is applied between this anode and cathode is negative then it will not allow the flow of current. So, now when this diode connected to a particular circuit and if we want to analyze that circuit, then the circuit can be analyzed by knowing the voltage and current that is flowing through this diode. And that can be found using the V-I characteristicsof the diode.

Now, for the resistor, we know that it is a linear element. And the relationship between the voltage and current is linear. So, using Ohm’s law, we can easily find the voltage and current through this resistor. But unlike the case of a resistor, the diode is a non-linear element. So, the relationship between the voltage and current for the diode is non-linear. And moreover that this diode allows the flow of current only in one direction. And it almost blocks the current in the reverse direction. Now, if you look at this graph, then it looks like, the graph is symmetrical in both directions. But don’t get confused by the symmetricity,

Diode V-I characteristics

because here in this graph, both positive and the negative axis has a different scale. For example, on the positive Y-axis, we have a scale in milli-ampere. While on the negative Y-axis, we have a scale in micro-ampere. Similarly, on the positive X-axis, if you see, the voltage varies like 0.5 V, 1V, and 1.5V. While, if you observe on the negative X-axis, the voltage scale varies like -10V, -20V and-30V.

So, because of the different scales, this characteristic looks symmetrical on both axes. But if we keep the same scale on both axes then the diode characteristics will look like this. So, as you can see, in the reverse direction the current that is flowing through the diode is almost negligible. So, using this V-I characteristic of the diode, we can easily find the voltage and current that is flowing through the diode.

And using this we can easily analyze any circuit which contains this diode. But because of the non-linear characteristics of the diode curve, it is a bit difficult to find the voltage and current very easily. So, what we can do, we can approximate this diode characteristic, and, using this we can analyze the circuits.

Ideal Diode

So, first of all, what we will do, we will consider this diode as an ideal diode and for this ideal diode, we will see the V-I characteristics. And also we will see the equivalent circuit for this ideal diode.

And step by step, we will introduce a few more parameters to this ideal diode. And using this we will approximate the diode characteristic curve. So, for the ideal diode, whenever the voltage that is applied between the anode and cathode is positive, then simply it will act as a closed switch. On the other end, if the voltage that is applied between this anode and cathode is negative, in that case simply it will act as an open switch.

Ideal Diode V-I characteristics

now let’s see the V-I characteristic forthis ideal diode.

case-1

when the applied voltage is positive, on the V-I characteristic we will get the vertical line

case-2

when the applied voltage is negative then, for that case, we will get the horizontal line on the negative x-direction

So, this is the V-I characteristics for the ideal diode. So, now whenever the positive voltage is applied to this diode, then it can be said that the diode has been forward biased. And it will allow the flow of current. While on the other end, whenever the negative voltage is applied to this diode then it will not allow the flow of current. And simply we can say that it has been reversed biased.

So, we will talk more about this forward and reverse bias in the latter part of the Article. So, now let’s take one simple example, in which the diode is connected in series with voltage source and resistor. Now, here we are assuming that the diode is the ideal diode.

It means that whenever the voltage that disappearing across the anode and cathode of the diode is positive then simply it can be represented by the close switch. And as you can see over here, the applied voltage across the diode is 10V. So, simply it can be represented by the close switch. And the current that is flowing through the the100ohm resistor will be equal to 10V divided by 100 ohms. That is equal to 0.1A.

While on the other end, if we apply the -10V in this circuit, then the applied voltage across the diode will be -10V. And it will not allow any flow of current. So, simply it will act as an open switch. And the current that is flowing through the circuit will be equal to 0A. Now,

some of you might have a question that how to find the voltage across this diode. So, that can be found by finding The venin’sequivalent between these two terminals. So, simply you need to remove this diode from the circuit and you need to find the Thevenin’s equivalent voltage between these two terminals. So, if we remove this diode, then the Thevenin’ equivalent voltage which appears between this anode and cathode in this particular case will be equal to -10V.

And because of that, the diode will be nonconducting. On the other end, if 10V is appearing between these two terminals then the diode will simply act as a closed switch. Now, if you see the V-I characteristics of this ideal diode, then you can observe that even if we apply a very small positive voltage between this anode and cathode, then also this diode will start conducting.

So, let’s say, even if we apply positive 0.1V, between this anode and cathode then also this ideal diode should start conducting. But if you see the actual diode, it will start conducting, only after the applied voltage crosses some threshold voltage.

Diode V-I characteristics

if you see the V-Characteristics, then it will look like this. So, up to a certain threshold voltage, the diode will not allow any flow of current. And whenever the applied voltage crosses this threshold voltage, then only, the diode will allow the flow of current. So, this voltage is known as the threshold voltage or the Cut-in voltage for the diode. Now, as I said before, this diode is a semiconductor device. And usually, it is made of either silicon or germanium. So, for silicon, this threshold voltage used to be in the range of 0.6 to 0.7 V. While for germanium usually, it is around0.3V.

So, actually, the diode will start conducting once the applied voltage crosses this threshold voltage. And before that, it will not allow any flow of current. So, this is the first approximation for the diode curve. So, here in this approximation, we are considering this diode as an ideal diode but it will only allow the flow of current once the applied voltage crosses this threshold voltage.

And the equivalent circuit will look like this. So, in the case of reversed bias, it will simply act as an open switch. But whenever the applied voltage crosses this threshold voltage then only it will allow the flow of current. And simply it will act as a closed switch.

so, now let’s take the same example that we have taken earlier. So, in this approximation, we are considering this diode as an ideal diode except for the fact that it will allow the flow of current only after that applied voltage crosses the threshold voltage. And here we are assuming that the diode that is used is the silicon diode. So, it has a threshold voltage of 0.7V.

So, once the applied voltage crosses this0.7V barrier, then only it will allow the flow of current. Now, here as the applied voltage is 10V, So simply this diode will allow the flow of current. And the equivalent circuit will look like this. So, now if you find the current that is flowing this 100-ohm resistor, it will be equal to 10 V minus 0.7V divided by 100 ohms.that is equal to 0.093A.

While on the earlier case, when we have considered this diode as an ideal diode, at that time, we found the value of this current as 0.1A.So, using this approximation, we can find more accurate values of voltage and current in the circuit. Now, in the same approximation, suppose if we apply the -10V, then, in that case, the circuit will simply act as an open switch. And it will not allow any flow of current. So, the current that is flowing through this resistor will be equal to 0A.

Now, in this first approximation also, we have considered that the diode has zero resistance. And that can be also visible from the V-I characteristics of this diode. Because if you observe over here, once the applied voltage crosses this threshold voltage then the voltage that appears across the diode will remain constant.

And the current will increase. It means that the diode offers zero resistance. But in the actual case, every device has some finite resistance. which will limit the current that is flowing through that particular device. Now, let’s consider the second approximation, where we are also considering that the diode has some finite resistance. And in that case, if you see the V-I characteristics, then it will look like this. So, up to the threshold voltage, the diode will offer an infinite resistance, or simply it will act as an open switch. And then after it will provide some finite resistance. And that resistance can be found from the slope of this curve.

So, this resistance is also known as bulk resistance or body resistance. So, this resistance is the resistance that is offered by the semiconductor device out of which this diode has been made. So, now let’s see the equivalent circuit for this second approximation. So, whenever the diode is reversed biased in that case, simply it will offer infinite resistance. Or we can say that it will act as an open switch.

But whenever the applied voltage crosses this threshold voltage, in that case now it will offer some finite resistance of Rb. That is the bulk resistance of this diode. So, this is the equivalent circuit in the case of the second approximation. Now, apart from this bulk resistance, the diode has one more resistance. And this resistance is present in the diode because of its internal structure. So, we will talk more about this diode resistance in the separate Article.

No, typically the value of this resistance used to be few ohms. So, in the circuit, if the Thevenin’s equivalent resistance which appears across the diode is much more than the diode resistance, in that case, the diode resistance can be neglected. But if the Thevenin’s equivalent resistance which5 appears across the diode is comparable to the diode resistance, in that case, we also need to consider this diode resistance. So, for example,

in this case, let’s assume that the diode has a resistance of 25 ohms. Now, this resistance is comparable to the the100-ohm resistor, so we need to consider this diode resistance. So, if we consider this diode resistance, then the current that is flowing through this 100-ohm resistor will be equal to 10V minus0.7V divided by 125 ohms. And if you see the current, it will be equal to 0.074A.

So, because of this diode resistance, if you see, the actual current that is flowing through this 100-ohm resistor will change. So, using this second approximation, we can find a more accurate value of the current and voltage that is flowing through the circuit. But in most of the circuits, the Thevenin’s sequivalent resistance which appears across the diode used to be much larger than this diode resistance. And simply we can neglect this diode resistance.

So, in this second approximation of this diode, we have assumed that the diode is non-conducting till the point this applied voltage crosses the threshold voltage. And then after it offers some finite resistance. So, if you see this type of character then it is known as the piecewise linear characteristics.

Because of what we have done, we have segmented the actual characteristics of this diode into the piecewise linear characteristics. So, up to the threshold voltage, we have assumed that the diode is non-conducting. And after the threshold voltage, it will offer some finite resistance. OK, so now let’s talk about the actual curve of this diode. So, in this diode curve, as I said before, this region is known as the forward region. Because, once the applied voltage crosses the threshold voltage, then the diode starts conducting. And whenever the applied voltage is less than the threshold voltage, in that case, the current that is flowing through the diode is almost negligible.

So, now whenever we apply the reverse voltage to this diode, then that region of operation is known as the reverse region of operation. And in this region, the current that is flowing through the diode used to be very small, typically in the micro-amperes. And this current is known as the reverse saturation current of the diode.

So, as you can see from the graph, even if we increase this reverse voltage across the diode, then also the current will only increase by a marginal amount. But we cannot increase this reverse voltage indefinitely. Because if we go beyond a certain voltage then the diode will come into the breakdown region. So, this region of operation is known as the breakdown region, and this region of operation should be avoided. Now, some diodes are specifically made to operate in this particular breakdown region. And this types of diodes are known as the Zener diode. So, we will talk more about this Zener diodein the separate Article.

But for any normal signal diodes, this region of operation should be avoided. So, if you see the datasheet, you will find that the maximum breakdown voltage for the diode has been mentioned. So, the applied reverse voltage should be less than this breakdown voltage. Now, in the forward region of this diode characteristic curve, if you observe, as the voltage across the diode increases, the current that is flowing through the diode will increase exponentially. So, in datasheets, you will also find one more parameter that is known as the maximum allowable forward current. So the diode current should be less than this maximum allowable forward current.

So, here are the few parameters that you will find in the datasheet of any diode. So, now in the next article, we will talk more about the diode resistance. So, I hope in this Article, you understood what is a diode, and the V-I characteristics of the diode. So, if you have any questions or suggestion then do let me know in the comment section below.