CONTENTS 1. Certificate 2 Achnenindenronnt ay forward Biased PN Junction Diode 8. Breakdown Region 10. Junction Diode ideal: and Real Characteristics ‘BibliographyA diode is a specialized electronic component with two electrodes called the anode ard the cathode. Most diodes are made with semiconductor materials. stich as silicon, germanium, or selenium. If we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to-a battery source, an additional energy source now ex barrier. ists tolovercomié the potential The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other, The behavior of the PN junction with regards to the potential barrier's width prodtces an asymmetrical conducting two terminal device, better known as the PN Junction Diode. Anode > Cath (+) (-)DIODE (THEORY—> PN JUNCTION, BIASING, CHARACTERISTIC CURVES) THEORY: A PN Junction Diode is one of the simplest Semiconductor Devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode: does not behave linearly with respect to the: applied voltage as the diode has an exponential current-voltage (1-V) relationship and therefore we camrot described its operation by simply using an equation such as Olim’s law. w The characteristic curve of a junction diode is also called an IV Curve. It is typically a graph showing the current flow at differentThe curtent is typically on the y-axis, and the voltage on tre x- axis, This type of graph provides engineers with a visual record of the operating characteristics of the component, This information enables them to use the component more appropriately within @ circuit, There are many different types of diddes, and they all have different characteristi curves anit applications, Here are some diodes yor might come across: Zener, Germanium, Guin, Tunnel, and Schottky, The current that flows through itis not proportional to the applied voltage. If a suitable positive voliage (forward bias) is applied between the tivo ends of the PN junction, it can supply free electrons and holes with the extra energy they reqnire to crass the juuetion as the width of the depletion layer around the P? junction is deereBy applying a negative voltage (reverse bias) results in the free charges being pulled away from the jrnction resulting in the depletion layer width being increased. This has the effect of increasing ar decreasing the effective resistance of the junction itself allowing or blocking current flow through Ure diode, Then the depletion layer widens with an increase én the application of a reverse voltage and narrows with an increase ir the application of a forward voltage. This is due to the differences in the élvctrical properties Iting in physical changes taking place, One of the results produces rectification as sein in the PN junction on the tuo sides of the PN junction diodes static 1-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below. But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, i.e connect a voltage potential across it. Gn the voltage axis above, “Reverse Bias”refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the “Forward Bias” direction. There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are: + |.Zero Bias - No external voltage potential is applied to the PN junction diode. * 2.Reverse - 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 diode’s 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 diodes width. Zero Biased Junction Diode When a diode is connected in a Zero Bias condition, no external potential energy is applied te the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the “Forward Current” and is referenced as IF Likewise, holes generated in the N-type material (minority carriers), find this situation favorable and move across the junction in the opposite direction. This is known as the “Reverse Current” and is referenced as IR. This transfer of electrons. and holes back and forth across the PN junction is known as diffusion, as shown below.Zero Biased PN Junction Dtobe PN Junction ———— , Zero Bias The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minarity carriers (few free electrons in the P-region and few holes in the N-region} to drift across the junction, Then an “Equilibrium” or balance will be established when the majority carriers are equal and both moving in opposite directions, se that the net result is zero current flowing In the cirevit. When this occurs the junction is said tobe in a state of “Dynamic Equilibrium’. The minority carriers are constantly generated due to thermal energy so this state ‘of equillbriurn can be broken by raising the temperature of the PN junction causing an increase in the géneération of minority carriers, thereby resulting in.an increase in leakage current but an electric current cannot flow since no circuit has been connected'to the PM junction.Retime Biasefa PN Junction Biotie When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. Barter The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator, The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material. Increase in the Depletion Layer due to Reverse Bias PN-junction region T Reverse Biasing Voltage This condition represents a high resistance value to the PN junction and practically zero ‘current flows through the junction diode with an increase in bias voltage. However, a very ‘small leakage current does flow through the junction which can be measured in micro- amperes, ( |JA). One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.= 000A, tm Reverse Cumeat. (tn), Sometimes this avalanche effect has practical applications in voltage stabilizing circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial, Forward Biased Junction Diode Diode 1 Characteristics fi I hia Forward Bias bieWhen a diode is connected in a Forward Bias candition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for siticon and 0,3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow. + cae ‘Anade Forward Biased bauer + - This is because the negative voltage pushes or repels electrans towards-the junction giving them the energy te cross over and combine with the holes being pushed in the Spposite direction towards the junction by the positive voltage. This results ina aracteristics curve of zero current flowing up to this voltage paint, called the knee” on the static curves and then a high current flow through the diode with little Increase in the external voltage as shawn below. Forward Characteristics Curde for a Junction Diode The application of 3 forward_blasing valtage on the junction diode results in the depletion layer becoming very thin and narrow which represents a fow impedance path through the junction thereby allowing high currents ta flow, The paint at which this sudden increase in current takes place is represented an Forward Current teat Forward Veltone, (Vp vette |Reduction in the Depletion Layer tone to Fortoard Bias Prijunction teiogon 4 Pereigon To = Ime ery smal + idogiation tayer o— 0 or Reverse Biased, VD < 0. Either way we can model these current-voltage characteristics for both an ideal diode and for a real diode.Breakboton Region mone Diode Characteristics Breakdown wey Then something interesting happens at the knee part of the curve. This point is called the breakdown voltage. Suddenly there is an increased flow in reverse current. No matter how much reverse voltage is applied, the voltage across the diode does not change. In specification sheets some manufacturers refer to the reverse current, or leakage current, as the point near the knee, just before breakdown occurs.Useful Diode Parameters The maximum forward bias current (If) of a diode is a very useful parameter. Different diodes are capable of carrying different amount of current. If you are placing a protection diode into a circuit, you will need to know how much current your circuit will draw, and whether the diode is up to the job of carrying it. The forward voltage drop (Vf), also known as the cut-off voltage, is a very useful parameter worth knowing. This parameter is defined at a specified current and temperature in datasheets. This value tells you that the diode will continue conducting even when the voltage falls as low as that figure. What most people do not realize is that the forward voltage drop is ideally specified when the forward current is zero. This is when the curve cuts the x-axis and the current is zero. If you were a crystal radio enthusiast, you would be interested in this parameter because it determines how sensitive your crystal radio diode is, Diodes are Very sensitive to heat and their resistance can change creating, a drift. This is Why datasheets always mention the parameters taken ata specific temperature. Typically it is at 25 °C.junction Diode Ideal and Real Characteristics oy