UNIT:III SEMICONDUCTOR DIODES What Are Semiconductors? Semiconductors are substances that conduct electricity under certain conditions i.e. they require a medium for the conduction of electricity. They have partial properties of both conductors and non-conductors. Semiconductors are pure materials but sometime impurities are added to increase their degree of conductivity. Types The basic types of semiconductors are Intrinsic semiconductors. Extrinsic semiconductors. Intrinsic semiconductors They are pure semiconductors that contain no impurities. When temperature increases, the conduction property of the intrinsic type also increases. This is because, at high temperatures, electrons are excited to higher energy levels and create holes. These holes are positively charged and flow in the direction opposite to that of electrons thus causing electricity. In an intrinsic semiconductor, the number of holes and electrons are equal. ie; n=p. Extrinsic Semiconductors When impurities are added to intrinsic semiconductors, extrinsic semiconductors are formed. The process of adding impurities to the conductor is called doping. Doping agents give it different electrical properties than the intrinsic semiconductors. Doping In the process of doping, impure atoms are added to the intrinsic semiconductors. These impure atoms are nothing but atoms of elements that differ from the actual semiconductor element. They change the electron and hole concentrations acting as a donor or acceptor. Types of extrinsic semiconductor N-type semiconductors P-type semiconductors N-type semiconductors Extrinsic semiconductors with a larger electron concentration than hole concentration are known as n-type semiconductors. The phrase 'n-type' comes from the negative charge of the electron. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. N-type semiconductors are created by doping an intrinsic semiconductor with donor impurities. In an n-type semiconductor, the Fermi energy level is greater than the that of the intrinsic semiconductor and lies closer to the conduction band than the valance band. P-type semiconductors As opposed to n-type semiconductors, p-type semiconductors have a larger hole concentration than electron concentration. The phrase 'p-type' refers to the positive charge of the hole. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities. P-type semiconductors have Fermi energy levels below the intrinsic Fermi energy level. The Fermi energy level lies closer to the valence band than the conduction band in a p-type semiconductor. DIODE Theory of PN junction diode A diode is a two-terminal device. Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property. The directionality of current flow most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current to pass in one direction (called the forward biased condition) and to block it in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve. Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner. The most common diodes are made from semiconductor materials such as silicon or germanium. Figure 6: Various semiconductor diodes. Theory of PN junction diode Cond Currentvoltage characteristics: A semiconductor diode's currentvoltage characteristic, or VI curve, is related to the transport of carriers through the so-called depletion layer or depletion region that exists at the p-n junction between differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N- doped region diffuse into the P-doped region where there is a large population of holes with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor on the N-side and negatively charged acceptor on the P-side. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator. However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a "built-in" potential across the depletion zone. Theory of PN junction diode Cond The depletion zone continues to act as an insulator, preventing any significant electric current flow. This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P- doped region is positive with respect to the N- doped region and the diode is said to be "turned on" as it has a forward bias. A diodes IV characteristic can be approximated by four regions of operation (see the figure at right). Figure 7: V-I characteristics of a P-N junction diode Theory of PN junction diode Cond At very large reverse bias, beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current that usually damages the device permanently. The avalanche diode is deliberately designed for use in the avalanche region. The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the A range). The third region is forward but small bias, where only a small forward current is conducted. As the potential difference is increased above an arbitrarily defined "cut-in voltage" or "on-voltage", the diode current becomes appreciable,and the diode presents a very low resistance. The currentvoltage curve is exponential. In a normal silicon diode at rated currents, the arbitrary "cut-in" voltage is defined as 0.6 to 0.7 volts. Depletion Region When a p-n junction is formed, some of the free electrons in the n-region diffuse across the junction and combine with holes to form negative ions. In so doing they leave behind positive ions at the donor impurity sites. Forward-bias Forward-bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below. In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed toward the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed toward the junction, the distance between them decreases. This lowers the barrier in potential. With increasing forward-bias voltage, the depletion zone eventually becomes thin enough that the zone's electric field cannot counteract charge carrier motion across the pn junction, as a consequence reducing electrical resistance. The electrons that cross the pn junction into the P-type material (or holes that cross into the N-type material) will diffuse in the near-neutral region. Therefore, the amount of minority diffusion in the near-neutral zones determines the amount of current that may flow through the diode. Reverse-bias Connecting the P-type region to the negative terminal of the battery and the N-type region to the positive terminal, produces the reverse- bias effect. The connections are illustrated in the following diagram: Reverse-bias usually refers to how a diode is used in a circuit. If a diode is reverse- biased, the voltage at the cathode is higher than that at the anode. Therefore, no current will flow until the diode breaks down. Connecting the P-type region to the negative terminal of the battery and the N-type region to the positive terminal corresponds to reverse bias. The connections are illustrated in the following diagram: Because the p-type material is now connected to the negative terminal of the power supply, the 'holes' in the P-type material are pulled away from the junction, causing the width of the depletion zone to increase. Likewise, because the N-type region is connected to the positive terminal, the electrons will also be pulled away from the junction. Therefore, the depletion region widens, and does so increasingly with increasing reverse-bias voltage. This increases the voltage barrier causing a high resistance to the flow of charge carriers, thus allowing minimal electric current to cross the pn junction. The increase in resistance of the pn junction results in the junction behaving as an insulator. APPLICATIONS Full-wave bridge rectifier Energy band structure of open circuited PN junction Energy band structure of open circuited PN junction Cond Figure 8: Energy band structure of a P-N junction diode Energy band structure of open circuited PN junction Cond Diode current equation Diode current equation Cond Space charge Space charge is a concept in which excess electric charge is treated as being a continuum of charge distributed over a region of space rather than distinct point-like charges. This model typically applies when charge carriers have been emitted from some region of a solid. Space charge usually only occurs in dielectric media because in a conductive medium the charge tends to be rapidly neutralized or screened. The sign of the space charge can be either negative or positive. When a metal object is placed in a vacuum and is heated to incandescence, the energy is sufficient to cause electrons to "boil" away from the surface atoms and surround the metal object in a cloud of free electrons. This is called thermionic emission. Diffusion capacitance Diffusion Capacitance is the capacitance due to transport of charge carriers between two terminals of a device. In a semiconductor device with a current flowing through it at a particular moment there is necessarily some charge in the process of transit through the device. If the applied voltage changes to a different value and the current changes to a different value, a different amount of charge will be in transit in the new circumstances. The change in the amount of transiting charge divided by the change in the voltage causing it is the diffusion capacitance. the corresponding diffusion capacitance:C diff. Is Effect of temperature on PN junction diodes Effect of temperature on PN junction diodes Cond Breakdown in PN junction diodes Breakdown in PN junction diodes Cond PN diode Applications PN diode Applications Cond Zener diode Zener diode Cond