PN Junction Diode PDF

# PN Junction Diode P type and N type semiconductors, taken separately, are of very limited use. If we join a piece of P type material to a piece of N type material such that the crystal structure remains continuous at the boundary, a PN JUNCTION is formed. ### PN Junction Diode Functions * It...

# PN Junction Diode P type and N type semiconductors, taken separately, are of very limited use. If we join a piece of P type material to a piece of N type material such that the crystal structure remains continuous at the boundary, a PN JUNCTION is formed. ### PN Junction Diode Functions * It can function as: * Rectifier * Amplifier * Switching * And other operations in electronic circuits. ### PN Junction Formation * A PN junction cannot be produced by simply pushing two pieces together or by welding. This is because it gives rise to discontinuities across the crystal structure. * Special fabrication techniques are adopted to form a PN junction ### What is a PN Junction? A PN junction is a device formed by joining p-type (doped with B. Al) with n-type (doped with P. As. Sb) semiconductors and separated by a thin junction. It is called PN Junction diode or junction diode. ### Electronic Symbol The triangle in the electronic symbol indicates the direction of current. - P type - N type A depletion layer forms an insulator between the two sides. ### Diode Orientation & Carrier Types * In a PN junction diode, N is at right and P is at left. * **Majority carriers**: * N region - electrons * P region - holes ### Formation of Depletion Layer **NO external connections**: The excess electrons in the N region cross the junction and combine with the excess holes in the P region. * N region loses its electrons, becoming positively charged. * P region accepts the electrons, becoming negatively charged. At one point, the migratory action is stopped. * An additional electrons from the N region are repelled by the net negative charge of the p region. * Similarly, an additional holes from the P region are repelled by the net positive charge of the n region. **Net result**: A creation of a thin layer on each side of the junction, which is depleted (emptied) of mobile charge carriers. This is known as DEPLETION LAYER. The thickness is of the order of $10^4$ meter. * The depletion layer contains no free and mobile charge carriers but only fixed and immobile ions. * Its width depends upon the doping level. * Heavy doped - thin depletion layer * Lightly doped - thick depletion layer ### Potential Barrier * The electrons in the N region have to climb the potential hill in order to reach the P region. * Electrons trying to cross from the N region to P region experience a retarding field of the battery and therefore repelled. Similarly for holes from P region. * Potential thus produced are called **potential barrier**. * Ge..0.3 V Si..0.7V ### Diode Biasing: Forward Bias Mode A battery is connected to the diode. * **Forward bias mode**: * Positive terminal connected to p-region and negative terminal connected to n region. * It forces the majority charge carriers to move across the junction, decreasing the width of the depletion layer. * Once the junction is crossed, a number of electrons and the holes will recombine. * For each hole in the P section that combines with an electron from the N section, a covalent bond breaks and an electron is liberated which enters the positive terminal. * Thus creating an electron hole pair. * Current in the N region is carried by electrons. * Current in the P region is carried by holes. ### Diode Biasing: Reverse Bias Mode * If the + of the battery is connected to the n-type and the - terminal to the p-type, * The free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows. * Thus a reverse biased pn junction **does not conduct current**. * Only the minority carriers cross the junction, constituting **very low reverse saturation current**. * This current is of the order of micro ampere. ### Voltage-Current (V-I) Characteristics of PN Junction Diode * The curve drawn between voltage across the junction along X axis and current through the circuits along the Y axis. * They describe the d.c behavior of the diode. * **When it is in forward bias, no current flows until the barrier voltage (0.3 v for Ge) is overcome.** * **Then the curve has a linear rise and the current increases, with the increase in forward voltage like an ordinary conductor.** * **Above 3 v, the majority carriers passing the junction gain sufficient energy to knock out the valence electrons and raise them to the conduction band. Therefore , the forward current increases sharply.** * **With reverse bias: ** * The potential barrier at the junction is increased. * Junction resistance increases and prevents current flow. * The minority carriers are accelerated by the reverse voltage, resulting in a very small current (REVERSE CURRENT) in the order of micro amperes. * **When reverse voltage is increased beyond a value, called breakdown voltage ,the reverse current increases sharply and the diode shows almost zero resistance. It is known as avalanche breakdown.** * **Reverse voltage above 25 v destroys the junction permanently.** ### Working Of a PN junction - PN junction diode acts as a rectifier as seen in the IV characteristic - Certain current flows in forward bias mode - Negligible current flows in reverse bias mode until zener or avalanche breakdown happens - Refer [https://nanohub.org/resources/68](https://nanohub.org/resources/68) for a detailed disscusion on operation of PN junction. ### Rectification * Thus the P N junction diode allows the electrons flow only when P is positive. * This property is used for the conversion of AC into DC, which is called rectification. ### Diode as a Switch * When the diode is forward bias, the switch is CLOSED. * When it is reverse biased, it is OPEN ### Advantages of PN Junction Diode * No filament is necessary * Occupies lesser space * Long life ### Applications of PN Junction Diode * ...as rectifiers to convert AC into DC. * As a switch in computer circuits. * As detectors in radios to detect audio signals. * As LED to emit different colours.