Due to the difficulties that many students face when studying diodes, we chose to start studying them based on the principle of the old diodes, type electronic tubes, also known by their name Vacuum Diodes.

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As soon as the American engineer Thomas Edson invented the incandescent light bulb, late 19th century, based on the principle of a filament, usually manufactured with the element chemist Tungsten (today), powered by an electrical voltage and encapsulated in a glass bulb at low pressure, several experiences have emerged that have led to a technological development quite revolutionary for the time.

At the beginning of the 20th century, scientists were already aware of chemical elements that when they were subjected to high temperatures, the electrons gained energy kinetic enough to free themselves from their atoms and create a cloud of charge eletric on the surface of the metal. It was enough placed another electrode, close of the first, to establish an electric current between them. And so, was born the electronic valve.

In the Figure 61-01 we show a schematic representation of how the distribution is of the electrodes inside an electronic tube. Note that the cathode has a cylindrical shape, completely enveloping the filament. When we apply electrical voltage to the filament, it becomes red and reaches a temperature around the 1000°C. This high filament temperature causes the cathode to indirectly heat up and release electrons. The filament generally operates at a voltage of 6.3 VAC. There are tubes that work with voltages different from this. This type of electronic tube, with the cathode electrode incorporated, are called heating tubes indirect. This is due to the fact that the first tubes do not have a cathode. Who was responsible for emitting electrons was the filament itself. The inconvenience was that the filament, should be fed by direct current, as it should not voltage variations on the electrode. This type of valve was called the direct heating. It fell into disuse quickly.

The cathode in turn is grounded, assuming a negative charge with respect to the anode. In the technical environment, the anode is best known as plate. The plate is the electrode that receives the voltage positive relative to the cathode. This positive voltage generates an electric field between the two electrodes. However, we know that an electric charge suffers a force when it is under the effect of an electric field. And we know its magnitude, which is expressed by the relation below:

This force accelerates the electron toward the plate. Thus, an electric current is established between the cathode and plate. But we know that the charge of the electron is constant, or q = 1.6 x 10^-19 C. So the only way to increase the force exerted on the electrons is to increase the value of the electric field. And this is very easy to arrange. Just increase the electrical voltage of the plate.Therefore, one of the ways to control the electric current through the diode is by varying the plate voltage. In the Figure 61-02 is a graph showing the dependence of the current on the diode in function of the plate voltage.

On the other hand, it is evident that when we apply a negative voltage cathode, NO there is an electric current flow in the diode, since the force acts to keep the electrons at the cathode. We conclude that:

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The semiconductor diode, initially, was an alternative to the vacuum diode, since this needed good power to function properly. In turn, the semiconductor diode, as it had no filament, practically did not consume power.

Semiconductors are chemical substances that have a crystalline nature and have properties between conductors and non-conductors.

In the Figure 61-03 we show the symbol used to represent a diode.

Basically, there are two types of semiconductors that interest us: those of the type "N" and those of type "P". Semiconductors of type "N" are those with an excess of electrons. Those of type "P", in turn, are those that have a lack of electrons.

In order to construct a diode, there is a need to closely associate a certain amount of type P material with a certain amount of type N material. We call this association of P-N junction. If this junction is subjected to an electric field with proper orientation, this P-N junction will allow the electric current to pass through. If the orientation of the electric field is inverted, the passage of the electric current through the junction will completely cease. And this behavior characterizes a DIODE.

In the Figure 61-04 we can see the characteristic curve that a semiconductor diode presents, when submitted to a polarization. The situation of direct polarization, that is, when allows the passage of the electric current, is represented in the first quadrant of the graph. Note that for a current I_D generic (in conduction) we have a voltage of 0.7 volts on the diode. In the figure the called knee appears, which is the point where the diode starts to conducting. Under normal conditions this voltage is of the order of 0.5 volts. Below this voltage we can say that there is no current flowing through the diode.

On the other hand, in the third quadrant, the situation of reverse polarization is represented, where the diode does not allow the passage of the electric current. By increasing the reverse voltage, the diode reaches the so-called rupture voltage , characterized by a violent increase of the electric current in contrary sentid, damaging the device.

In the two figures shown above, we represent the two states in which a diode can be: in conduction or in cut. Notice that in the Figure 61-05 where we represent the diode in conduction, that is, with the positive polarity in the anode, we haveV_D = 0.7 volts. In this situation we have the value of I_D ≠ 0.

On the other hand, in the Figure 61-06 we have the situation where the diode is in cut. The polarity negative of V is in the anode. So we have I_D = 0.

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We can say that a diode, qualitatively, has a high resistance for reverse polarization and a low resistance for direct polarization. That is, it can behave as a key (as an approximation of the model). Therefore, with direct biasing is a switched-on switch, while reverse biasing is a switched-off switch.

When we lower the potential barrier of a diode, it enters the so-called conduction zone. In this zone, it behaves almost like a short circuit, allowing the passage of electric current. The equation below, relates the electric current and the voltage on the diode.

The definition of the variables involved in the equation are:

The inverse saturation current I_o, for small signal diodes used in low power circuits, and which have a junction area P - N very small, is of the order of 10^-15A. However, it is highly temperature dependent, typically doubling with each increase of 10° C.

On the other hand, the thermal voltage, V_T, is given by the equation below:

In this case the variables involved in the equation are:

For a temperature of 20°C, the value of the thermal voltage, after calculation, is approximately 25.2 mV. We will adopt, as a general rule for solving problems, the value of V_T = 25 mV at room temperature.

In the case of the directly polarized diode, the current I_D is much higher than current I_o, and therefore we can approximate the eq. 61-02, equation of the current in the diode, making the following simplification:

If for a given condition we have   I₁ = I_o e ^{V₁/ η V_T} and for a second condition, we have the relation I₂ = I_o e ^{V₂/ η V_T}, so if we divide the second equation by the first, we get I₂ / I₁ = e ^{( V₂ - V₁)/ η V_T}. Now, if in this equation we apply the natural logarithm function on the two members, we eliminate the exponential function and get the logarithm. If, on the other hand, we replace the natural logarithm with the decimal logarithm, we must multiply by the factor 2.3. Thus, we obtain the relation as shown in the equation below.

Let's suppose that I₂ = 10 I₁. When we apply the logarithm function to this relation, we realize that the difference between V₂ and V₁ increased by a factor of   2.3 η V_T. This means that every decade, the voltage in the diode rises from approximately 60 mV if η = 1 and of 120 mV if η = 2. Normally, one does not know the value of η. So it is customary, in projects, to adopt the value of   2.3 η V_T = 0.1 V / decade.

It should be noted that due to the sudden increase in current at the moment that exceeds the knee of the characteristic curve, there is a great variation of values in the electric current passing through the diode, however, the voltage drop on it is restricted to a narrow band between 0.6 and 0.8 volts. In general, in the calculations, we adopted the value of V_D = 0.7 volts as standard.

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Because the voltage and the electric current in the diode depend on V_T and I_o, and as these are a function of temperature, it is logical that the curve I - V of the diode will also depend on the temperature. Considering a given constant current, the voltage across the diode decreases from approximately -2.2 mV / °C. That is, its variation has a negative characteristic. The higher the temperature, the lower the voltage on the diode. In power amplifier designs, this feature should be taken very seriously. However, this is an interesting feature in the construction of sensors, such as temperature monitoring. We can easily an electronic design that allows temperature measurement using a diode as an information collector.