Ideal Diode Model.

    • The ideal small signal diode conducts current in one direction (forward-conducting) and blocks current in the other direction (reverse-blocking).
    • Signal Diodes are used in a wide variety of applications such as a switch in rectifiers, limiters, wave-shaping circuits.

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    Practical Diode Model (model 1, 2).

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    • The practical model 1 take into consideration the actual voltage drop across the diode that makes it ON. However, the model still neglects the diode internal resistance that is considered zero.
    • Signal Diodes are used in a wide variety of applications such as a switch in rectifiers, limiters, wave-shaping circuits.
    • The practical model 2 take into consideration the actual voltage drop across the diode that makes it ON.
    • Also, it takes into consideration the diode internal resistance.

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    Junction Diode Symbol and Static I-V Characteristics

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    • Is : is the saturation current.
    • VT : is a constant votage called the thermal voltage.
    • n : is a constant has a value between 1 and 2 depending on the material and physical structure of the diode.
    • If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
    • By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased.

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    There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:

    1. Zero Bias – No external voltage potential is applied to the PN-junction.
    2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN-junction width.
    3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.

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      Semiconductor Basics .

        • Semiconductors have intermediate energy gape. In their pure (intrinsic) state, semiconductors are neither good conductors nor good insulators.
        • Semiconductors do not used in their pure state because at room temperature very few electrons can jump the energy gap to the conduction band and become free electrons that causing conduction current.
        • To effectively increase the conductivity of semiconductors we have to add impurities to intrinsic semiconductors to increase the free electron in conduction band.

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        There are two types of semiconductors; N-type and P-type.

        • In N-type semiconductors, penta-valent impurity atoms (atoms with five electrons in the valence band) are added to intrinsic semiconductors. Four electrons of these five are bonding with different four atoms of semiconductors substance leaving one free electron.
        • While for P-type semiconductors, tri-valent impurity atoms (atoms with three electrons in the valence band) are added to intrinsic semiconductor. These three electrons are bonding with three semiconductor’s atoms. The fourth semiconductor adjacent atom find no valence electron to share but instead imaginary hole (same but positively charge as electron).
        • When the N and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the junction so some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.
        • However, because the electrons have moved across the junction from the N-type silicon to the P-type silicon, they leave behind positively charged donor ions (ND) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region were there are large numbers of free electrons.

         

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        • As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions (NA), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the junction is known as diffusion.
        • The significance of this built-in potential across the junction, is that it opposes both the flow of holes and electrons across the junction and is why it is called the potential barrier.
        • In practice, a PN junction is formed within a single crystal of material rather than just simply joining or fusing together two separate pieces.
        • Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be made to an external circuit.
        • Then the resulting device that has been made is called a PN junction Diode or Signal Diode.

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        • If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
        • By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased.

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        Semiconductor Basics

          • Electrons that are in orbits further from the nucleus have higher energy & are less tightly bound to the atom.
          • The force attraction between the positive charged nucleus & the negatively charged electron decreases with increasing distance from the nucleus.
          • This outermost shell is known as the valence shell & electrons in this shell are called valence electronics.

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          • When a valence electron absorb energy from a heat or light source it can actually escape from the outer shell and the resulting is a positive charged (more protons than electrons) atom is called positive ion i.e. H+. The escaped valence electron is called a free electron.
          • When a free electron loses energy and falls into the outer shell of an atom, the atom becomes negatively charged (more electrons than protons) and is called a negative ion i.e. H– .
          • Materials can be categorized into conductors, semiconductors or insulators by their ability to conduct electricity.
          • Conductors: Metals conduct electricity easily because there is no band gap since the conduction overlaps the valence band.
          • Semiconductors: The band gap is small enough that electron that absorb thermal energy can bridge the gap to the conduction band.
          • Insulators: Very large band gap between the valence & conduction bands makes it hard for electrons to bridge the gap.

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          • Two types of semi conductive materials are silicon and germanium. Both have four valence electrons.
          • The valence electrons in germanium are in 4th shell while the ones in silicon are in 3rd shell, closer to the nucleus.
          • This means that the germanium valence electrons are at higher energy levels than those in silicon. Thus require a smaller amount of additional energy to escape from the atom.
          • This property makes germanium more unstable than silicon at high temperatures, which is the main reason silicon, is the most widely used semi conductive material.

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        Diodes And Its Applications : introduction

        introduction

        • If resistors are the most basic passive component in electrical or electronic circuits, the Signal Diode as being the most basic “Active” component.
        • Unlike a resistor, a diode does not behave linearly with respect to the applied voltage as it has an exponential I-V relationship and therefore can not be described simply by using Ohm’s law as we do for resistors.
        • Diode is a two-terminal electronic device in which the applied voltage across its terminals controls the current passing through the diode.
        • It conducts if a voltage source is applied in one direction, and refuses to conduct significant current when the voltage is applied with the opposite polarity.
        • So diodes are basic unidirectional semiconductor devices that will only allow current to flow through them in one direction only, acting more like a one way electrical valve, (Forward Biased Condition).
        • Diodes are made from semiconductors materials; the intermediate class between conductors and insulators.

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        • So before we have a look at how signal or power diodes work we first need to understand the semiconductors basic construction and concept.

         

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