Semiconductor PN Junction Diode PDF

Course : PGPathshala-Biophysics Paper : Physics (Electronics) Module: Semiconductor PN junction Diode Content Writer: Dr. Sangeeta Semwal, Ministry of Electronics and Information Technology, New Delhi Quadrant I In the last module, we studied about the basics of semiconductors and also learnt that...

Course : PGPathshala-Biophysics Paper : Physics (Electronics) Module: Semiconductor PN junction Diode Content Writer: Dr. Sangeeta Semwal, Ministry of Electronics and Information Technology, New Delhi Quadrant I In the last module, we studied about the basics of semiconductors and also learnt that almost all electronic devices available in market are semiconductor based devices. The most common semiconductor device is P-N junction diode. Use of semiconductor materials to build the electronic components started with diodes. Before the invention of diode, vacuum tubes were used. Both diode and vacuum tubes devices are similar but in the case of vacuum tubes the size occupied and power consumption is much greater than the diodes. Also, the construction of vacuum tubes is a bit complex and they are difficult to maintain as compared to the semiconductor diodes. Main feature of any diode is that it conducts only in one direction (unidirectional device). The present module discusses the basics and applications of semiconductor diodes. Introduction: The major objectives of this Module are given below: OBJECTIVES: ------------------------------------------------------------------------------------------------------------- Intrinsic and Extrinsic semiconductors What is a diode? Types of diodes. Construction of a Diode. Depletion Layer Biasing Forward Biasing Reverse Biasing V-I Characteristics Zener Diode Zener and Avalanche Breakdown Diode as a Rectifier Applications of a p-n junction diode 1. Intrinsic and Extrinsic semiconductors In the last module, we studied about the conductivity of a pure (intrinsic) semiconductor and how it is affected by doping. Doping from third and fifth group elements from periodic table creates extrinsic semiconductors i.e. P-type and N-type semiconductors. The P-Type semiconductor has excess holes and is of positive charge. The N-Type semiconductor has excess electrons and is of negative charge. 2. What is a diode? The word diode can be explained as ‘Di’ means two and ‘ode’ is obtained from electrode. A diode has two terminals or electrodes (one connected to P-type and the other to the N-type). A P-N junction diode is formed either by using any semiconductor such as Si or Ge. The P and  N type regions are referred to as anode and cathode respectively. Its circuit symbol is shown in fig. In its symbol, arrowhead indicates the direction of current flow. 3. Types of Diodes Now a day’s several types of diodes are available such as PN Junction, Gunn Diode, Laser diode, Light emitting diodes, Photodiode, PIN diode, Backward diode, Schottky diodes, Step recovery diode, Tunnel diode, Varactor diode and Zener diode. 4. Construction of a Diode A diode is formed by joining two equivalently doped P-Type and N-Type semiconductor. A diode is basically constructed using a single piece of semiconductor, half of which is doped by P-type impurity and the other half by N-type. The plane dividing the two zones is called P-N Junction. 5. Depletion layer At the dividing plane, the holes in the P-Type attract electrons from the N-Type material. Hence the electron diffuses and recombines with the holes in the P-Type material. It causes a small region of the N-type near the junction to lose electrons and behave like intrinsic semiconductor. Similarly, in the P-side also a small region behaves like an intrinsic semiconductor. This thin intrinsic region is called depletion layer, as it is depleted of charge (see figure). This region offers high resistance and prevents the further diffusion of majority charge carriers. P-N junction has three main characteristics: i. A thin “Depletion Layer” on both sides of the junction. It is depleted of free charge carriers (thickness about 10-6 m) also called as “Space Charge”. ii. A potential barrier across the junction. iii. Capacitance effect due to the presence of the depletion layer. The width of the depletion region is a function of both the applied bias as well as the level of doping. In the forward bias condition, the width of the depletion region reduces with the increase in the applied voltage which eventually leads to an increase in the amount of current flow. On the other hand, if the P-N junction is reversed biased, the width of the depletion region increases with applied voltage. 6. Biasing P-N junction is said to be biased when an external voltage is applied across it. Zero Bias When a diode is zero biased, that is when not connected to any battery, almost no current passes through the diode. However if anode and cathode of the diode are connected directly, a small voltage or current that is insignificant is observed. For practical reasons this current is assumed to be zero. Reverse Bias In reverse bias the P-type region is connected to negative voltage and N-type is connected to positive terminal as shown in figure. In this condition the holes in P-type gets recombined by electrons from the battery. The electrons in N-type material are pulled out of the diode by the positive terminal of the battery. So the diode gets depleted of charge. So initially the depletion layer widens (see figure) and it occupies the entire diode. The resistance offered by the diode is very high. The current in reverse bias is only due to minority charge which is in nano amperes in silicon and micro amperes in high power Si and Ge diodes. Forward Bias In forward bias the P-Region of the diode is connected with the positive terminal of the battery and N-region is connected with the negative region. During the forward bias, the positive of the battery pumps more holes into the P-region of the diode. The negative terminal pumps electrons into the N-region. The excess of charge in P and N region apply pressure on the depletion region and make it contract. As the voltage increases the depletion layer become thinner and thinner and hence diode offers lesser and lesser resistance. Since the resistance decreases the current increases (though not proportional) to the voltage. At one particular voltage level Vf i.e. at threshold/cut-off voltage the depletion layer disappears (overwhelmed by the charge) and hence from this point onwards the diode starts to conduct very easily. From this point on the diode current increases exponentially to the voltage applied. 7. V-I characteristics of p-n junction diode The graph between applied voltage and current flowed in a p-n junction diode is called the V-I characteristics. Forward V-I characteristics of p-n junction diode If the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be in forward bias. In forward biased P-N junction diode, VF represents the forward voltage, whereas IF represents the forward current. If the external voltage applied on the silicon diode is less than 0.7 volts, the silicon diode allows only a small electric current. However, this small electric current is considered as negligible. When the external voltage applied on the silicon diode reaches 0.7 volts, the p-n junction diode starts allowing large electric current through it. At this point, a small increase in voltage increases the electric current rapidly. The forward voltage at which the silicon diode starts allowing large electric current is called cut-in voltage. The cut-in  voltage for silicon diode is approximately 0.7 volts. Reverse V-I characteristics of p-n junction diode If the negative terminal of the battery is connected to the p-type semiconductor and the positive terminal of the battery is connected to the n-type semiconductor, the diode is said to be in reverse bias. In reverse biased P-N junction diode, VR represents the reverse voltage whereas IR represents the reverse current. If the external reverse voltage applied on the P-N junction diode increases, the free electrons from the n-type semiconductor and the holes from the p-type semiconductor are moved away from the p-n junction. This increases the width of depletion region. The wide depletion region of reverse biased P-N junction diode completely blocks the majority charge carrier current. However, it allows the minority charge carrier current. The free electrons (minority carriers) in the p-type semiconductor and the holes (minority carriers) in the n-type semiconductor carry the electric current. The electric current, which is carried by the minority charge carriers in the p-n junction diode, is called reverse current. In n-type and p-type semiconductors, very small number of minority charge carriers are present. Hence, a small voltage applied on the diode pushes all the minority carriers towards the junction. Thus, further increase in the external voltage does not increase the electric current. This electric current is called reverse saturation current. In other words, the voltage or point at which the electric current reaches its maximum level and further increase in voltage does not increase the electric current is called reverse saturation current. The reverse saturation current depends on the temperature. With temperature, the generation of minority charge carriers increases. Hence, the reverse current increases with the increase in temperature. However, the reverse saturation current is independent of the external reverse voltage. Hence, the reverse saturation current remains constant with the increase in voltage. However, if the voltage applied on the diode is increased continuously, the P-N junction diode reaches to a state where junction breakdown occurs and reverse current increases rapidly. In germanium diodes, a small increase in temperature generates large number of minority charge carriers. The number of minority charge carriers generated in the germanium diodes is greater than the silicon diodes. Hence, the reverse saturation current in the germanium diodes is greater than the silicon diodes. 8. Zener diode A Zener diode is a special type of device designed to operate in the zener breakdown region. Zener diodes acts like normal p-n junction diodes under forward biased condition. When forward biased voltage is applied to the zener diode it allows large amount of electric current and blocks only a small amount of electric current.  Zener diode is heavily doped than the normal p-n junction diode. Hence, it has very thin depletion region. A zener diodes allow more electric current than the normal p-n junction diodes. Zener diode allows electric current in forward direction like a normal diode but also allows electric current in the reverse direction if the applied reverse voltage is greater than the zener voltage. Zener diode is always connected in reverse direction because it is specifically designed to work in reverse direction. 9. Avalanche and Zener Breakdown If reverse biased voltage applied to the P-N junction diode is highly increased, a sudden rise in current occurs. At this point, a small increase in voltage rapidly increases the electric current. This sudden rise in electric current causes a junction breakdown called zener or avalanche breakdown. The voltage at which zener breakdown occurs is called zener voltage and the sudden increase in current is called zener current. A normal P-N junction diode does not operate in breakdown region because the excess current permanently damages the diode. Normal p-n junction diodes are not designed to operate in reverse breakdown region. 10. Breakdown in Zener diode There are two types of reverse breakdown regions in a zener diode: Avalanche breakdown and Zener breakdown. Avalanche breakdown The avalanche breakdown occurs in both normal diodes and zener diodes at high reverse voltage. When high reverse voltage is applied to the P-N junction diode, the free electrons (minority carriers) gain large amount of energy and are accelerated. The free electrons moving at high speed collide with the atoms and knock off more electrons. These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated. As a result, electric current in the diode increases rapidly. This sudden increase in electric current may permanently destroy the normal diode. However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region. Zener breakdown The zener breakdown occurs in heavily doped p-n junction diodes because of its narrow depletion region. When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field. When reverse biased voltage applied to the diode reaches close to zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence band. The valence electrons which gains sufficient energy from the strong electric field of depletion region will breaks bonding with the parent atom. The valance electrons which break bonding with parent atom will become free electrons. These free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current. Difference between Avalanche and Zener Breakdown  Zener breakdown occurs at low reverse voltage whereas avalanche breakdown occurs at high reverse voltage.   Breakdown region is the normal operating region for a zener diode.  Zener breakdown occurs in zener diodes with zener voltage (Vz) less than 6V whereas Avalanche breakdown occurs in zener diodes with zener voltage (Vz) greater than 6V. 11. Ideal diode The ideal diode or perfect diode is a two terminal device, which completely allows the electric current without any loss under forward bias and completely blocks the electric current with infinite loss under reverse bias. 12. Applications of a P-N junction Diode P-N junction diode is used in various applications such as:  Rectification  In clamping circuits for DC restoration.  In clipping circuits for wave shaping.  In voltage multipliers.  As switch in digital logic circuits used in computers.  In demodulation circuits.  Laser diodes used in optical communications.  Light Emitting Diodes (LEDs), used in digital displays.  In voltage regulators. 13. Diode as a Rectifier Alternating current (AC) is a current which flows in both directions. In some applications we need DC power supply. DC supply from batteries is not economical and requires an alternative to get DC power. One good way is to convert AC power to DC power. This process is known as rectification. A P-N junction diode allows electric current in only forward bias condition and blocks electric current in reverse bias condition. As a diode allow to pass current only in a one direction (unidirectional device). This unique property of the diode allows it to acts like a rectifier. Rectification is of two types half wave rectification and full wave rectification. 14. Summary P-N junction diode is the basis of a large number of semiconducting devices besides being an important circuit in itself. Present module focussed on the construction, functioning and principles of a semiconductor diode. Characteristics of P-N junction under forward and reverse bias conditions and flow of current are also discussed. Concepts of Zener and Avalanche breakdown are also depicted to understand the functioning of Zener diode. Also, it highlighted that a diode can be used as a rectifier. It is essential to study the diodes for the understanding of semiconductor based devices such as transistors and its applications, which will be discussed in the next modules.