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 VI 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 . 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 is a twoterminal semiconductor device that allows the flow of current only in one direction. And if you see the symbol of the , 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 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 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 VI characteristics of the .
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 is a nonlinear element. So, the relationship between the voltage and current for the is nonlinear. And moreover that this 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 VI characteristics
because here in this graph, both positive and the negative axis has a different scale. For example, on the positive Yaxis, we have a scale in milliampere. While on the negative Yaxis, we have a scale in microampere. Similarly, on the positive Xaxis, if you see, the voltage varies like 0.5 V, 1V, and 1.5V. While, if you observe on the negative Xaxis, the voltage scale varies like 10V, 20V and30V.
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 characteristics will look like this. So, as you can see, in the reverse direction the current that is flowing through the is almost negligible. So, using this VI characteristic the, we can easily find the voltage and current that are flowing through the diode.
And using this we can easily analyze any circuit which contains this diode. But because of the nonlinear characteristics of the curve, it is a bit difficult to find the voltage and current very easily. So, what we can do, we can approximate this characteristic, and, using this we can analyze the circuits.
Ideal Diode
So, first of all, what we will do, we will consider this as an ideal diode and for this ideal , we will see the VI 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 characteristic curve. So, for the ideal , 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 VI characteristics
So, this is the VI characteristic for the ideal diode. So, now whenever the positive voltage is applied to this , then it can be said that the 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 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 is connected in series with voltage source and resistor. Now, here we are assuming that the is the ideal diode.
It means that whenever the voltage that disappearing across the anode and cathode of the is positive then simply it can be represented by the close switch. And as you can see over here, the applied voltage across the 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 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. So, that can be found by finding The venin’sequivalent between these two terminals. So, simply you need to remove this 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’s equivalent voltage which appears between this anode and cathode in this particular case will be equal to 10V.
And because of that, they will be nonconducting. On the other end, if 10V is appearing between these two terminals then the will simply act as a closed switch. Now, if you see the VI characteristics of this ideal, then you can observe that even if we apply a very small positive voltage between this anode and cathode, then also this 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 VI characteristics
if you see the VCharacteristics, 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 Cutin voltage for the . Now, as I said before, this 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 curve. So, here in this approximation, we are considering this as an ideal 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 as an ideal 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 that is used is the silicon. 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 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 100ohm resistor, it will be equal to 10 V minus 0.7V divided by 100 ohms. which is equal to 0.093A.
While on the earlier case, when we have considered this as an ideal, 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 has zero resistance. And that can be also visible from the VI characteristics of this. 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 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 has some finite resistance. And in that case, if you see the VI 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 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. 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 because of its internal structure. So, we will talk more about this diode resistance in the separate Article.
Read Here : About Diode Resistance https://www.electrical4u.com/dioderesistance/
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 resistance, in that case, the resistance can be neglected. But if the Thevenin’s equivalent resistance which5 appears across the is comparable to the resistance, in that case, we also need to consider this resistance. So, for example,
in this case, let’s assume that the has a resistance of 25 ohms. Now, this resistance is comparable to the the100ohm resistor, so we need to consider this resistance. So, if we consider this diode resistance, then the current that is flowing through this 100ohm 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 resistance, if you see, the actual current that is flowing through this 100ohm 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 equivalent resistance which appears across the used to be much larger than this diode resistance. And simply we can neglect this diode resistance.
So, in this second approximation of this , we have assumed that the is nonconducting 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 into the piecewise linear characteristics. So, up to the threshold voltage, we have assumed that the is nonconducting. 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 curve, as I said before, this region is known as the forward region. Because, once the applied voltage crosses the threshold voltage, then the starts conducting. And whenever the applied voltage is less than the threshold voltage, in that case, the current that is flowing through the is almost negligible.
So, now whenever we apply the reverse voltage to this, then that region of operation is known as the reverse region of operation. And in this region, the current that is flowing through the used to be very small, typically in the microamperes. 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 they 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 are specifically made to operate in this particular breakdown region. And These types are known as the Zener diode. So, we will talk more about this Zener diode in 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 has been mentioned. So, the applied reverse voltage should be less than this breakdown voltage. Now, in the forward region of this characteristic curve, if you observe, as the voltage across the increases, the current that is flowing through the 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.
FAQ

What is a diode used for?
Main functions. the foremost common perform of a diode is to permit an electrical current to pass in one direction (called the diode’s forward direction), whereas interference it within the other way (the reverse direction). As such, the diode are often viewed as AN electronic version of a check valve.

What are the types of diodes?
Different Types of Diodes
Small Signal Diode. …
Large Signal Diode. …
Zener Diode. …
Light Emitting Diode (LED) …
Constant Current Diodes. …
Schottky Diode. …
Shockley Diode. …
Step Recovery Diodes. 
What is diode and its symbol?
The point points within the direction of standard current flow within the forward biased condition. meaning the anode is connected to the p aspect and also the cathode is connected to the n aspect. … The terminal connected to the ptype is that the anode. The terminal connected to the ntype aspect is that the cathode.

Are diodes AC or DC?
A single diode or four diodes rework 110V social unit power to DC by forming a halfway (single diode) or a fullwave (four diodes) rectifier. The diode permits solely half the AC wave form to travel through it.
YouTube Video
Semiconductor diode PDF in Hindi
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 VI characteristics of the diode. So, if you have any questions or suggestions then do let me know in the comment section below.
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