WHAT IS A DIODE?
It is a semiconductor device that has two terminals, an anode (+) and a cathode (-) and only allow the flow of electricity in one direction. Due to this, the diode has the same characteristics as a switch.
The ideal diode is a component that has zero resistance to the passage of current in a certain direction, and infinite resistance in the opposite direction. In the following example we can notice that by having the polarized diode correctly it acts as a closed switch, in the opposite way, by having the polarized diode incorrectly it acts as an open switch, which causes the circuit not to be completed.
Note: Some diodes are polarized inversely, for example the zener diode.
Diodes are generally identified by a reference:
- In the American system the reference consists of the prefix “1N” followed by the serial number. In this case the "N" means that it is a semiconductor, the "1" indicates the number of PN junctions and the serial number "XXXX" the exact characteristics or specifications of the device.
- In the European or continental system the two-letter prefix is used. In this case we will use the BY254 as an example, where the “B” indicates the material (silicon) and the “Y” type (rectifier).
Note: Many manufacturers use their own references.
COMPOSITION OF A DIODE
To be able to talk about composition of a diode, we must first know the difference between a material type “P” and “N”.
Material type P
A P-type semiconductor is obtained by carrying out a doping process, adding atoms to the semiconductor to increase the number of charge carriers (in this case positive charges or gaps).
Material type N
A semiconductor type N is obtained by carrying out a doping process, adding atoms to the semiconductor to increase the number of charge carriers (in this case, negative charges or electrons).
The semiconductor diode consists mainly of a PN junction, adding a terminal connection to each of the metal contacts of its ends and a capsule that houses the whole, leaving outside the terminals corresponding to the anode (zone P) and the cathode (zone N).
POLARIZATION OF A DIODE
There are two types of polarization for a diode, direct and reverse.
The anode is connected to the positive terminal of the battery and the cathode to the negative terminal. One of the characteristics of direct polarization is that the diode conducts with a voltage drop of 0.6 to 0.7 V. The applied voltage exceeds the contact potential and reduces the depletion region. The anode, in effect, becomes a source of holes and the cathode becomes a source of electrons, so that gaps and electrons are continuously generated in the joint. The current increases exponentially as the applied voltage tends to the value of the contact potential (0.6 to 0.7 V for silicon). This effect is described quantitatively with the diode equation:
Where ID is the current through the junction, Io is the reverse saturation current, q is the charge of an electron (1.60 x 10-19 C), k is the Boltzman constant (1.381 x10-23 J/K), VD is the direct bias voltage across the junction and T is the absolute temperature of the junction in kelvin.
The anode is connected to the negative terminal of the battery and the cathode to the positive terminal. One of the characteristics of reverse polarization is that, the value of the internal resistance of the diode is very high and consequently acts as an open switch. The anode is connected to the n-type silicon and the cathode to the p-type silicon, the depletion region increases, which inhibits the diffusion of electrons and therefore the current. Although a reverse saturation current flows (Io), it is extremely small (on the order of 10-9 to 10-15 A).
TYPES OF DIODES
There are different types of diodes, which may differ in their physical appearance, impurities, use of electrons and some that have particular electrical characteristics used for a special application in a circuit.
These are some of the commonly used diodes:
Like all electronic components, diodes have properties that differentiate them from other semiconductors. It is necessary to know these data sheets and design needs so require. In these notes the most important characteristics will be presented from the practical point of view.
Nominal voltage values:
- VF = Direct voltage at the ends of the conducting diode.
- VR = Reverse voltage at the ends of the reverse polarized diode.
- VRSM = Reverse voltage of non-repetitive peak.
- VRRM = Reverse Peak Reverse Voltage.
- VRWM = Operating peak reverse voltage.
Nominal current values:
- IF = Direct current.
- IR = Reverse current.
- IFAV = Average value of the waveform of the current during a period.
- IFRMS = Effective current in a conduction state. It is the maximum effective current that the diode is capable of supporting.
- IFSM = Peak direct current (initial) non-repetitive.
- AV = Average
- RMS = Root Mean Square (root of the root mean)
Nominal temperature values:
- Tstg = Indicates the maximum and minimum values of the storage temperature.
- Tj = Maximum value of the temperature supported by the semiconductor junction.
DIODE CHARACTERISTIC CURVE
The current-voltage characteristic curve for the ideal diode is shown in the figure in red. This model implies that the diode is fully activated for any voltage greater than or equal to 0. In addition, it is assumed that the reverse saturation current is 0 when it has reverse polarization. A good initial approximation for the real diode is given by the blue line, since they replicate the actual voltage drop from 0.6 to 0.7 V, measured through the silicon diode when it has direct polarization.
“An ideal diode has zero resistance when it is directly polarized and infinite resistance when it is reverse polarized”
A real diode requires approximately 0.7 V of direct polarization to allow a significant current flow. When a real diode is reversely biased, it can withstand a reverse voltage up to limit known as breakdown voltage, where the diode will fail as the reverse current raises precipitously.
CAUTION WHEN USING A DIODE
- Breaking voltage: Applying a large enough negative voltage to the diode could yield and allow the current to flow in the reverse direction, for common diodes the breaking voltage ranges from -50V to -100V (Some diodes are specially designed to work in the region of breaking off).