Category: semiconductor Devices

Semiconductor Devices – Bipolar Transistors Bipolar transistors are mainly formed of two layers of semiconductor material of the opposite type, connected back to back. The type of impurity added to silicon or germanium decides the polarity when it is formed. NPN Transistor An NPN transistor is composed of two N type material separated by a thin layer of P type semiconductor material. The crystal structure and schematic symbol of the NPN transistor are shown in the above figure. There are three leads taken out from each type of material recognized as the emitter, base, and collector. In the symbol, when the arrowhead of the emitter is directed outwards from the base, it indicates that the device is of the NPN type. PNP Transistor A PNP transistor is composed of two P type material separated by a thin layer of N type semiconductor material. The crystal structure and schematic symbol of a PNP transistor is shown below. In the symbol, when the arrowhead of the emitter is directed inwards towards the base, it indicates that the device is of the PNP type. Learning working make money

Diode Characteristics There are diverse current scales for forward bias and reverse bias operations. The forward portion of the curve indicates that the diode conducts simply when the P-region is made positive and the N-region negative. The diode conducts almost no current in the high resistance direction, i.e. when the Pregion is made negative and the N-region is made positive. Now the holes and electrons are drained away from the junction, causing the barrier potential to increase. This condition is indicated by the reverse current portion of the curve. The dotted section of the curve indicates the ideal curve, which would result if it were not for avalanche breakdown. The following figure shows the static characteristic of a junction diode. DIODE IV Characteristics The forward and reverse current voltage (IV) characteristics of a diode are generally compared on a single characteristic curve. The figure depicted under the section Forward Characteristic shows that Forward Voltage and Reverse Voltage are usually plotted on the horizontal line of the graph. Forward and reverse current values are shown on the vertical axis of the graph. Forward Voltage represented to the right and Reverse Voltage to the left. The point of beginning or zero value is at the center of the graph. Forward Current lengthens above the horizontal axis with Reverse Current extending downward. The combined Forward Voltage and Forward Current values are located in the upper right part of the graph and Reverse Voltage and Reverse Current in the lower left corner. Different scales are normally used to display forward and reverse values. Forward Characteristic When a diode is forward biased it conducts current (IF) in forward direction. The value of IF is directly dependent on the amount of forward voltage. The relationship of forward voltage and forward current is called the ampere-volt, or IV characteristic of a diode. A typical diode forward IV characteristic is shown in the following figure. Following are the observations − Forward Voltage is measured across the diode and Forward Current is a measure of current through the diode. When the forward voltage across the diode equals 0V, forward current (IF) equals 0 mA. When the value starts from the starting point (0) of the graph, if VF is progressively increased in 0.1-V steps, IF begins to rise. When the value of VF is large enough to overcome the barrier potential of the P-N junction, a considerable increase in IF occurs. The point at which this occurs is often called the knee voltage VK. For germanium diodes, VK is approximately 0.3 V, and 0.7 V for silicon. If the value of IF increases much beyond VK, the forward current becomes quite large. This operation causes excessive heat to develop across the junction and can destroy a diode. To avoid this situation, a protective resistor is connected in series with the diode. This resistor limits the forward current to its maximum rated value. Normally, a currentlimiting resistor is used when diodes are operated in the forward direction. Reverse Characteristic When a diode is reverse biased, it conducts Reverse current that is usually quite small. A typical diode reverse IV characteristic is shown in the above figure. The vertical reverse current line in this graph has current values expressed in microamperes. The amount of minority current carriers that take part in conduction of reverse current is quite small. In general, this means that reverse current remains constant over a large part of reverse voltage. When the reverse voltage of a diode is increased from the start, there is a very slight change in the reverse current. At the breakdown voltage (VBR) point, current increases very rapidly. The voltage across the diode remains reasonably constant at this time. This constant-voltage characteristic leads to a number of applications of diode under reverse bias condition. The processes which are responsible for current conduction in a reverse-biased diode are called as Avalanche breakdown and Zener breakdown. Diode Specifications Like any other selection, selection of a diode for a specific application must be considered. Manufacturer generally provides this type of information. Specifications like maximum voltage and current ratings, usual operating conditions, mechanical facts, lead identification, mounting procedures, etc. Following are some of the important specifications − Maximum forward current (IFM) − The absolute maximum repetitive forward current that can pass through a diode. Maximum reverse voltage (VRM) − The absolute maximum or peak reverse bias voltage that can be applied to a diode. Reverse breakdown voltage (VBR) − The minimum steady-state reverse voltage at which breakdown will occur. Maximum forward surge current (IFM-surge) − The maximum current that can be tolerated for a short interval of time. This current value is much greater than IFM. Maximum reverse current (IR) − The absolute maximum reverse current that can be tolerated at device operating temperature. Forward voltage (VF) − Maximum forward voltage drop for a given forward current at device operating temperature. Power dissipation (PD) − The maximum power that the device can safely absorb on a continuous basis in free air at 25° C. Reverse recovery time (Trr) − The maximum time that it takes the device to switch from on to off stat. Important Terms Breakdown Voltage − It is the minimum reverse bias voltage at which PN junction breaks down with sudden rise in reverse current. Knee Voltage − It is the forward voltage at which the current through the junction starts to increase rapidly. Peak Inverse Voltage − It is the maximum reverse voltage that can be applied to the PN junction, without damaging it. Maximum Forward Rating − It is the highest instantaneous forward current that a PN junction can pass, without damaging it. Maximum Power Rating − It is the maximum power that can be dissipated from the junction, without damaging the junction. Learning working make money

Semiconductor Devices – Photovoltaic Cells A basic photovoltaic cell consists of a n-type and a p-type semiconductor forming a p-n junction. The upper area is extended and transparent, generally exposed to the sun. These diodes or cells are exceptional that generate a voltage when exposed to light. The cells convert light energy directly into electrical energy. The following figure shows the symbol of photovoltaic cell. Working of a Photovoltaic Cell The construction of a photovoltaic cell is similar to that of a PN junction diode. There is no current flow through the device when no light is applied. In this state, the cell will not be able to generate current. It is essential to bias the cell properly which requires a fair amount of light. As soon as light is applied, a remarkable state of PN junction diode can be observed. As a result, the electrons acquire sufficient energy and break away from the parent atoms. These newly generated electron-hole pairs in the depletion region crosses the junction. In this action, the electrons move into the N type material because of its normal positive ion concentration. Likewise holes sweep into the P type material because of its negativeion content. This causes the N type material to instantly take on a negative charge and the P material to take on a positive charge. The P-N junction then delivers a small voltage as a response. Characteristics of a Photovoltaic Cell The following figure on the left, shows one of the characteristics, a graph between reverse current (IR) and illumination (E) of a photo diode. IR is measured on the vertical axis and illumination is measured on the horizontal axis. The graph is a straight line passing through the zero position. i.e, IR = mE m = graph straight line slope The parameter m is the sensitivity of the diode. The figure on the right, shows another characteristic of the photo diode, a graph between reverse current (IR) and reverse voltage of a photo diode. It is clear from the graph that for a given reverse voltage, reverse current increases as the illumination increases on the PN junction. These cells generally supply electrical power to a load device when light is applied. If a larger voltage is required, array of these cells are used to provide the same. For this reason, photovoltaic cells are used in applications where high levels of light energy are available. Learning working make money

Types of Semiconductor When voltage is applied to semiconductor devices, electron current flows toward the positive side of the source and holes current flows towards the negative side of the source. Such a situation occurs only in a semiconductor material. Silicon and Germanium are the most common semiconductor materials. Generally, the conductivity of a semiconductor lies in between the conductivities of metals and insulators. Germanium as a Semiconductor Following are some important points about Germanium − There are four electrons in the outermost orbit of germanium. In bonds, atoms are shown with their outer electrons only. The germanium atoms will share valence electrons in a covalent bond. This is shown in the following figure. Germanium are the ones that are associated with the covalent bonding. The crystalline form of germanium is called the crystal lattices. This type of structure has the atoms arranged in the way as shown in the following figure. In such an arrangement, the electrons are in a very stable state and thus are less appropriate to be associated with conductors. In the pure form, germanium is an insulating material and is called as an intrinsic semiconductor. The following figure shows the atomic structures of Silicon and Germanium. Silicon as a Semiconductor Semiconductor devices also use silicon in the manufacturing of various electronic components. The atomic structure of silicon and germanium is shown in the above figure. The crystal lattice structure of silicon is similar to that of Germanium. Following are some of the important points about Silicon − It has four electrons in its outermost shell like germanium. In pure form, it is of no use as a semiconductor device. A desired amount of conductivity can be obtained by adding up of impurities. Adding up of impurity must be done carefully and in a controlled environment. Depending on the type of impurity added, it will create either an excess or a deficit of electrons. The following figure shows the intrinsic crystal of Silicon. Learning working make money

Semiconductor Devices – Junction Diodes A crystal structure made of P and N materials is generally known as junction diode. It is generally regarded as a two-terminal device. As shown in the following diagram one terminal is attached to P-type material and the other to N-type material. The common bond point where these materials are connected is called a junction. A junction diode allows current carriers to flow in one direction and obstruct the flow of current in the reverse direction. The following figure shows the crystal structure of a junction diode. Take a look at the location of the P type and N type materials with respect to the junction. The structure of crystal is continuous from one end to the other. The junction acts only as a separating point that represents the end of one material and the beginning of the other. Such structure allows electrons to move thoroughly in the entire structure. The following diagram shows two portions of semiconductor substance before they are shaped into a P-N junction. As specified, each part of material has majority and minority current carriers. The quantity of carrier symbols shown in each material indicates the minority or majority function. As we know electrons are the majority carriers in the N type material and holes are the minority carriers. In P type material, holes are the majority carriers and electrons are in the minority. Learning working make money

Semiconductor Devices – Zener Diode It is a specific type of semiconductor diode, which is made to operate in the reverse breakdown region. The following figure depicts the crystal structure and the symbol of a Zener diode. It is mostly similar to that of a conventional diode. However, small modification is done to distinguish it from a symbol of a regular diode. The bent line indicates letter ‘Z’ of the Zener. The most significant difference in Zener diodes and regular PN junction diodes is in the mode which they are used in circuits. These diodes are normally operated only in the reverse bias direction, which implies that the anode must be connected to the negative side of the voltage source and the cathode to the positive. If a regular diode is used in the same way as Zener diode, it will be destroyed due to excessive current. This property makes the Zener diode less significant. The following illustration shows a regulator with a Zener diode. The Zener diode is connected in reverse bias direction across unregulated DC supply source. It is heavily doped so that the reverse breakdown voltage is reduced. This results in a very thin depletion layer. Due to this, the Zener diode has sharp reverse breakdown voltage Vz. As per the circuit action, breakdown occurs sharply with a sudden increase in current as shown in the following figure. Voltage Vz remains constant with an increase in current. Due to this property, Zener diode is widely used in voltage regulation. It provides almost constant output voltage irrespective of the change in current through the Zener. Thus, the load voltage remains at a constant value. We can see that at a particular reverse voltage known as knee voltage, current increases sharply with constant voltage. Due to this property, Zener diodes are widely used in voltage stabilization. Learning working make money

Semiconductor Devices – Barrier Potential N-type and P-type material are considered as electrically neutral before they are joined together at a common junction. However, after joining diffusion takes place instantaneously, as electrons cross the junction to fill holes causing negative ions to emerge in the P material, this action causes the nearby area of the junction to take on a negative charge. Electrons departing the N material causes it to generate positive ions. All this process, in turn, causes the N side of the junction to take on a net positive charge. This particular charge creation tends to force the remaining electrons and holes away from the junction. This action makes it somewhat hard for other charge carriers to diffuse across the junction. As a result, the charge is built up or barrier potential emerges across the junction. As shown in the following figure. The resultant barrier potential has a small battery connected across the P-N junction. In the given figure observe the polarity of this potential barrier with respect to P and N material. This voltage or potential will exist when the crystal is not connected to an external source of energy. The barrier potential of germanium is approximately 0.3 V, and of silicon is 0.7 V. These values cannot be measured directly and appears across the space charge region of the junction. In order to produce current conduction, the barrier potential of a P-N junction must be overcome by an external voltage source. Learning working make money

Conductivity & Mobility As discussed earlier, there may be one or more free electrons per atom which moves all the way through the interior of the metal under the influence of an applied field. The following figure shows charge distribution within a metal. It is known as the electron-gas description of a metal. The hashed region represents the nucleus with a positive charge. The blue dots represent the valence electrons in the outer shell of an atom. Basically, these electrons do not belong to any specific atom and as a result, they have lost their individual identity and roam freely atom to atom. When the electrons are in an uninterrupted motion, the direction of transportation is changed at each collision with the heavy ions. This is based on electron-gas theory of a metal. The average distance between collisions is called the mean free path. The electrons, passing through a unit area, in the metal in the opposite direction in a given time, on a random basis, makes the average current zero. Learning working make money

Conduction in Solid Materials The number of electrons in the outer ring of an atom is still the reason for the difference between conductors and insulators. As we know, solid materials are primarily used in electrical devices to accomplish electron conduction. These materials can be separated into conductors, semiconductors, and insulators. However, conductors, semiconductors, and insulators are differentiated by energy-level diagrams. The amount of energy needed to cause an electron to leave its valence band and go into conduction will be accounted here. The diagram is a composite of all atoms within the material. Energy-level diagrams of insulators, semiconductors, and conductors are shown in the following figure. Valence Band The bottom portion is the valence band. It represents the energy levels closest to the nucleus of the atom and the energy levels in the valance band hold the correct number of electron necessary to balance the positive charge of the nucleus. Thus, this band is called the filled band. In the valence band, electrons are tightly bound to the nucleus. Moving upward in the energy level, the electrons are more lightly bound in each succeeding level toward the nucleus. It is not easy to disturb the electrons in the energy levels closer to the nucleus, as their movement requires larger energies and each electron orbit has a distinct energy level. Conduction Band The top or outermost band in the diagram is called the conduction band. If an electron has an energy level, which lies within this band, and is comparatively free to move around in the crystal, then it conducts electric current. In semiconductor electronics, we are concerned mostly in the valence and conduction bands. Following are some basic information about it − The valence band of each atom shows the energy levels of the valence electrons in the outer shell. A definite amount of energy must be added to the valence electrons to cause them to go into the conduction band. Forbidden Gap The valence and conduction bands are separated by a gap, wherever exists, called forbidden gap. To cross the forbidden gap a definite amount of energy is needed. If it is insufficient, electrons are not released for conduction. Electrons will remain in the valence band till they receive additional energy to cross the forbidden gap. The conduction status of a particular material can be indicated by the width of the forbidden gap. In atomic theory, the width of the gap is expressed in electron volts (eV). An electron volt is defined as the amount of energy gained or lost when an electron is subjected to a potential difference of 1 V. The atoms of each element have a dissimilar energy-level value that allows conduction. Note that the forbidden region of an insulator is relatively wide. To cause an insulator to go into conduction will require a very large amount of energy. For example, Thyrite. If insulators are operated at high temperatures, the increased heat energy causes the electrons of the valence band to move into the conduction band. As it is clear from the energy band diagram, the forbidden gap of a semiconductor is much smaller than that of an insulator. For example, silicon needs to gain 0.7 eV of energy to go into the conduction band. At room temperature, the addition of heat energy may be sufficient to cause conduction in a semiconductor. This particular characteristic is of great importance in solid-state electronic devices. In case of a conductor, the conduction band and the valence band partly overlaps one another. In a sense, there is no forbidden gap. Therefore, the electrons of valence band are able to release to become free electrons. Normally at normal room temperature little electrical conduction takes place within the conductor. Learning working make money

Semiconductor Devices – Differentiator A differentiator circuit is shown in the following figure. The differentiator provides a useful operation, the resulting relation for the circuit being Vo(t) = RC(dv1(t)/dt Following are some important parameters of Operational amplifier − Open Loop Voltage Gain (AVOL) The open loop voltage gain of an operational amplifier is its differential gain under conditions where no negative feedback is used. AVOL ranges from 74 db to 100 db. AVOL = [Vo/(V1 – V2)] Output Offset Voltage (VOO) The output offset voltage of an operational amplifier is its output voltage when its differential input voltage is zero. Common Mode Rejection (CMR) If both the inputs are at the same potential, causing the differential input zero, and if the output is zero, the operational amplifier is said to have a good common mode rejection. Common Mode Gain (AC) Common mode gain of an operational amplifier is the ratio of the common mode output voltage to the common mode input voltage. Differential Gain (AD) The differential gain of an operational amplifier is the ratio of the output to the differential input. Ad = [Vo / (V1) – V2] Common Mode Rejection Ratio (CMRR) CMRR of an operational amplifier is defined as the ratio of the closed loop differential gain to the common mode gain. CMRR = Ad/AC Slew Rate (SR) Slew rate is the rate of output voltage change caused by a step input voltage. An ideal slew rate is infinite, which means that the operational amplifier output should change instantly in response to an input step voltage. We have already discussed some applications of op-amp such as differentiator, integrator, summing amplifier, etc. Some other common applications of operational amplifiers are − Logarithmic amplifier Gyrator (Inductance simulator) DC & AC voltage follower Analog to Digital converter Digital to Analog converter Power supplies for over voltage protection Polarity indicator Voltage follower Active filters Learning working make money