Category: semiconductor Devices

Feedback & Compensation The basic purpose of the bias network is to establish collector–base–emitter voltage and current relationships at the operating point of the circuit (the operating point is also known as the quiescent point, Q–point, no–signal point, idle point, or static point). Since, transistors rarely operate at this Q-point, the basic bias networks are generally used as a reference or starting point for design. The actual circuit configuration and especially, the bias network values are selected on the basis of dynamic circuit conditions (desired output voltage swing, expected input signal level, etc.) Once the desired operating point is established, the next function of the bias network is to stabilize the amplifier circuit at this point. The basic bias network must maintain the desired current relationships in the presence of temperature and power supply changes, and possible transistor replacement. In some cases, frequency changes and changes caused by the component again must also be offset by the bias network. This process is generally referred to as bias stabilization. Proper bias stabilization will maintain the amplifier circuit at the desired operating point (within practical limits), and will prevent thermal runaway. Stability Factor ‘S’ It is defined as the rate of change of collector current w.r.t. reverse saturation current, keeping β and VBE constant. It is expressed as $$S = frac{mathrm{d}I_c }{mathrm{d} I_c}$$ Bias Stabilization Methods The method of making the operating point independent of temperature changes or variations in transistors’ parameters is known as stabilization. There are several schemes for providing bias stabilization of solid–state amplifiers. All these schemes engage a form of negative feedback. That is any stage in transistor currents produce a corresponding voltage or current change that tends to counterbalance the initial change. There are two fundamental methods for producing negative feedback, inverse–voltage feedback and inverse–current feedback. Inverse-Voltage Feedback The following figure shows the basic inverse–voltage bias network. The emitter–base junction is forward biased by the voltage at the junction of R1 and R2. The base–collector junction is reverse biased by the differential between voltages at the collector and the base. Normally, the collector of a resistance coupled amplifier is at a voltage about one half that of the supply Resistor (R3), connected between the collector and base. Since the collector voltage is positive, a portion of this voltage is feedback to the base to support the forward bias. The normal (or Q point) forward bias on the emitter–base junction is the result of all the voltages between the emitter and the base. As the collector current increases, a larger voltage drop is produced across RL. As a result, the voltage on the collector decreases, reducing the voltage feedback to the base through R3. This reduces the emitter–base forward bias, reducing the emitter current and lowering the collector current to its normal value. As there is an initial decrease in the collector current, an opposite action takes place, and the collector current is raised to its normal (Q point) value. Any form of negative or inverse feedback in an amplifier has a tendency to oppose all changes even those produced by the signal being amplified. This inverse or negative feedback tends to reduce and stabilize gain, as well as undesired change. This principle of stabilizing gain by means of feedback is used in more or less all types of amplifiers. Inverse-Current Feedback The following figure shows a distinctive inverse–current (emitter– feedback) bias network using an NPN transistor. Current feedback is more commonly used than voltage feedback in solid–state amplifiers. This is because transistors are mainly current–operated devices, rather than voltage–operated devices. The use of an emitter–feedback resistance in any bias circuit can be summed up as follows: Base current depends upon the differential in voltage between the base and the emitter. If the differential voltage is lowered, less base current will flow. The opposite is true when the differential is increased. All current flowing through the collector. The voltage drops across the emitter resistor and is therefore not fully dependent. As the collector current increases, the emitter current and the voltage drop across the emitter resistor, will also increase. This negative feedback tends to decrease the differential between the base and the emitter, thus lowering the base current. In turn, the lower base current tends to decrease the collector current, and counterbalance the initial collector–current increases. Bias Compensation In solid state amplifiers, when the loss in signal gain is intolerable in a particular application, compensating techniques are often used to reduce the drift of operating point. In order to provide maximum bias and thermal stabilization, both compensation and stabilization methods can be employed together. The following figure shows diode compensation technique that utilized both the diode compensation and self-bias stabilization. If both diode and transistor are of the same type, then they have the same temperature coefficient across the circuit. Here, diode is forward biased. KVL for the given circuit can be expressed as − $$I_c = frac{beta [V – (V_{BE} – V_o)] + (Rb + Rc)(beta + 1)ICO}{Rb + Rc(1 + beta)}$$ It is clear from the above equation that $V_{BE}$ follows VO w.r.t. temperature and Ic will have no effect to variations in $V_{BE}$. This is an effective method to take care of the operating point of the transistor due to variation in $V_{BE}$. Temperature Compensation Device We can also use some temperature sensitive device to compensate for variations of transistor internal characteristics. Thermistor has a negative temperature coefficient, which means with the rise in temperature, its resistance decreases exponentially. The following figure shows a circuit that uses thermistor (RT) to reduce the increase in collector current due to change in $V_{BE}$, ICO, or β with temperature. When temperature increases, RT decreases and the current fed through RT into RE increases. The action voltage drop across RE is in the opposite direction to reverse bias the transistor. RT acts so as to tend to compensate the increase in IC, which increases due to the rise in temperature. Learning working make money

Semiconductor Devices – Photo Diode A photodiode is a P-N junction diode that will conduct current when exposed to light. This diode is actually designed to operate in the reverse bias mode. It means that larger the intensity of falling light, the greater will be the reverse bias current. The following figure shows a schematic symbol and constructional detail of a photo diode. Working of a Photo Diode It is a reverse-biased diode. Reverse current increases as the intensity of incident light increases. This means that reverse current is directly proportional to the intensity of falling light. It consists of a PN junction mounted on a P-type substrate and sealed in a metallic case. The junction point is made of transparent lens and it is the window where the light is supposed to fall. As we know, when PN junction diode is reverse biased, a very small amount of reverse current flows. The reverse current is generated thermally by electron-hole pairs in the depletion region of the diode. When light falls on PN junction, it is absorbed by the junction. This will generate more electron-hole pairs. Or we can say, characteristically, the amount of reverse current increases. In other words, as the intensity of falling light increases, resistance of the PN junction diode decreases. This action makes the diode more conductive. These diodes have very fast response time These are used in high computing devices. It is also used in alarm circuits, counter circuits, etc. Learning working make money

Semiconductor Devices – Depletion Zone Initially, when a junction diode is formed, there is a unique interaction between current carriers. In N type material, the electrons move readily across the junction to fill holes in the P material. This act is commonly called diffusion. Diffusion is the result of high accumulation of carriers in one material and a lower gathering in the other. Generally, the current carriers which are near to the junction only takes part in the process of diffusion. Electrons departing the N material cause positive ions to be generated in their place. While entering the P material to fill holes, negative ions are created by these electrons. As a result, each side of the junction contains a large number of positive and negative ions. The area where these holes and electrons become depleted is generally known by the term depletion region. It is an area where there is lack of majority current carriers. Normally, a depletion region is developed when P-N junction is formed. The following figure shows the depletion region of a junction diode. Learning working make money

Construction of a Transistor Following are some manufacturing techniques used in the construction of a transistor − Diffusion Type In this method, the wafer of semiconductor is subjected to some gaseous diffusion of both N type and P type impurities to form emitter and collector junctions. First, base-collector junction is determined and photo-etched just prior to base diffusion. Later, the emitter is diffused on the base. Transistors manufactured by this technique have better noise figure and improvement in current gain is also seen. Grown Type It is formed by drawing a single crystal from melted silicon or germanium. The required concentration of impurity is added during crystal drawing operation. Epitaxial Type A very high purity and thin single-crystal layer of silicon or germanium is grown on a heavily doped substrate of the same type. This improved version of the crystal forms the collector on which the emitter and base junctions are formed. Alloy Type In this method, the base section is made of a thin slice of N type material. At the opposite sides of the slice, two small dots of Indium are attached and the complete formation is kept to a high temperature for a shorter time. The temperature would be above melting temperature of Indium and below Germanium. This technique is also known as fused construction. Electrochemically Etched Type In this method, on the opposite sides of a semiconductor wafer, depression is etched in order to reduce the width of the base region. Then a suitable metal is electroplated into the depressions area to form emitter and collector junctions. Learning working make money

Semiconductor Devices – Practical Op-Amps Inverting Amplifier The following figure shows an inverting amplifier. The input signal is amplified and inverted. This is the most widely used constant-gain amplifier circuit. Vo = -Rf.Vin /R1 Voltage gain A = (-Rf /R1) Non-Inverting Amplifier The following figure shows an op-amp circuit that works as a non-inverting amplifier or constant-gain multiplier and it has better frequency stability. The input signal is amplified but it is not inverted. Output Vo = [(R1 + Rf) / R1] V1 Voltage gain A = (R1 + Rf) / R1 Inverting Summing Amplifier The following figure shows an inverting summing amplifier. It is the most used circuit of the op-amp. The circuit shows a three-input summing amplifier, which provides a means of algebraically summing three voltages, each multiplied by a constant-gain factor. The output voltage is expressed as, Vo = [(-R4 / R1) V1][(-R4 / R2) V2][(-R4 / R3) V3] Vo = -R4(V1 / R1 + V2 / R2 + V3 / R3) If, R1 = R2 = R3 = R4 = R & Rs = R/3 Vo = -(V1 + V2 + V3) Learning working make money

Discuss Semiconductor Devices The electronic components exploiting the electronic properties of semiconductor materials, are termed as semiconductor devices. This tutorial discusses the functional operation of semiconductor devices, explains the operation of devices in a circuit, etc. Each topic in this tutorial is explained well using circuit diagrams for better understanding. After completing this tutorial, readers will be at a moderate level of expertise to explain the basics related to semiconductor devices. Learning working make money

Semiconductor Devices – Leakage Current An important conduction limitation of PN junction diode is leakage current. When a diode is reverse biased, the width of the depletion region increases. Generally, this condition is required to restrict the current carrier accumulation near the junction. Majority current carriers are primarily negated in the depletion region and hence the depletion region acts as an insulator. Normally, current carriers do not pass through an insulator. It is seen that in a reverse-biased diode, some current flows through the depletion region. This current is called leakage current. Leakage current is dependent on minority current carriers. As we know that the minority carriers are electrons in the P type material and holes in the N type material. The following figure shows how current carriers react when a diode is reverse biased. Following are the observations − Minority carriers of each material are pushed through the depletion zone to the junction. This action causes a very small leakage current to occur. Generally, leakage current is so small that it can be considered as negligible. Here, in case of leakage current, temperature plays an important role. The minority current carriers are mostly temperature dependent. At room temperatures of 25°C or 78°F, there is negligible amount of minority carriers present in a reverse bias diode. When the surrounding temperature rises, it causes significant increase in minority carrier creation and as a result it causes a corresponding increase in leakage current. In all reverse-biased diodes, occurrence of leakage current is normal to some extent. In Germanium and Silicon diodes, leakage current is only of few microamperes and nanoamperes, respectively. Germanium is much more susceptible to temperature than silicon. For this reason, mostly Silicon is used in modern semiconductor devices. Learning working make money

Semiconductor Devices – Junction Biasing The term bias refers to the application of DC voltage to set up certain operating conditions. Or when an external source of energy is applied to a P-N junction it is called a bias voltage or simply biasing. This method either increases or decreases the barrier potential of the junction. As a result, the reduction of the barrier potential causes current carriers to return to the depletion region. Following two bias conditions are applied w.r.t. PN junctions. Forward Biasing − An external voltage is added of the same polarity to the barrier potential, which causes an increase in the width of the depletion region. Reverse Biasing − A PN junction is biased in such a way that the application of external voltage action prevents current carriers from entering the depletion region. Forward Biasing The following figure shows a forward biased PN junction diode with external voltage applied. You can see that the positive terminal of the battery is connected to the P material and the negative terminal of the battery is connected to the N material. Following are the observations − This bias voltage repels the majority current carriers of each P and N type material. As a result, large number of holes and electrons start appearing at the junction. At the N-side of the junction, electrons move in to neutralize the positive ions in the depletion region. On the P-side material, electrons are dragged from negative ions, which cause them to become neutral again. This means that forward biasing collapses the depletion region and hence the barrier potential too. It means that when P-N junction is forward biased, it will allow a continuous current flow. The following figure shows the flow of current carriers of a forward-biased diode. A constant supply of electrons is available due to an external voltage source connected to the diode. The flow and direction of the current is shown by large arrows outside the diode in the diagram. Note that the electron flow and the current flow refers to the same thing. Following are the observations − Suppose electrons flow through a wire from the negative battery terminal to the N material. Upon entering this material, they flow immediately to the junction. Similarly, on the other side an equal number of electrons are pulled from P side and are returned to the positive battery terminal. This action creates new holes and causes them to move toward the junction. When these holes and electrons reach the junction they join together and effectively disappear. As a result, new holes and electrons emerge at the outer ends of the diode. These majority carriers are created on a continuous basis. This action continues as long as the external voltage source is applied. When diode is forward biased it can be noticed that electrons flow through the entire structure of diode. This is common in N type material, whereas in the P material holes are the moving current carriers. Notice that the hole movement in one direction must begin by electron movement in the opposite direction. Therefore, the total current flow is the addition of holes and electrons flow through a diode. Reverse Biasing The following figure shows reverse biased PN junction diode with external voltage applied. You can see that the positive terminal of the battery is connected to the N material and the negative terminal of the battery is connected to the P material. Note that in such an arrangement, battery polarity is to oppose the material polarity of the diode so that dissimilar charges attract. Hence, majority charge carriers of each material are dragged away from the junction. Reverse biasing causes the diode to be nonconductive. The following figure shows the arrangement of the majority current carriers in a reverse biased diode. Following are the observations − Due to circuit action electrons of the N material are pulled toward the positive battery terminal. Each electron that moves or departs the diode causes a positive ion to emerge in its place. As a result, this causes an equivalent increase in the width of the depletion region on the N side of the junction. The P side of the diode has a similar effect alike the N side. In this action, a number of electrons leave the negative battery terminal and enter the P type material. These electrons then straight away move in and fill a number of holes. Each occupied hole then becomes a negative ion. These ions in turn are then repelled by the negative battery terminal and driven toward the junction. Due to this, there is an increase in the width of the depletion region on the P side of the junction. The overall width of the depletion region directly depends on an external voltage source of a reverse-biased diode. In this case, the diode cannot efficiently support the current flow through the wide depletion region. As a result, the potential charge starts developing across the junction and increases until the barrier potential equals the external bias voltage. After this, the diode behaves as a nonconductor. Learning working make money

Doping in Semiconductors Pure Silicon or Germanium are rarely used as semiconductors. Practically usable semiconductors must have controlled quantity of impurities added to them. Addition of impurity will change the conductor ability and it acts as a semiconductor. The process of adding an impurity to an intrinsic or pure material is called doping and the impurity is called a dopant. After doping, an intrinsic material becomes an extrinsic material. Practically only after doping these materials become usable. When an impurity is added to silicon or germanium without modifying the crystal structure, an N-type material is produced. In some atoms, electrons have five electrons in their valence band such as arsenic (As) and antimony (Sb). Doping of silicon with either impurity must not change the crystal structure or the bonding process. The extra electron of impurity atom does not take part in a covalent bonding. These electrons are loosely held together by their originator atoms. The following figure shows alteration of silicon crystal with the addition of an impurity atom. Effect of Doping on N-type Material The effect of doping on an N-type material is as follows − On addition of Arsenic to pure Silicon, the crystal becomes an N-type material. Arsenic atom has additional electrons or negative charges that do not take part in the process of covalent bonding. These impurities give up or donate, one electron to the crystal and they are referred to as donor impurities. An N-type material has extra or free electrons than an intrinsic material. An N-type material is not negatively charged. Actually all of its atoms are all electrically neutral. These extra electrons do not take part in the covalent bonding process. They are free to move about through the crystal structure. An N-type extrinsic silicon crystal will go into conduction with only 0.005eV of energy applied. Only 0.7eV is required to move electrons of intrinsic crystal from the valence band into the conduction band. Normally, electrons are considered to be the majority current carriers in this type of crystal and holes are the minority current carriers. The quantity of donor material added to Silicon finds out the number of majority current carriers in its structure. The number of electrons in an N-type silicon is many times greater than the electron-hole pairs of intrinsic silicon. At room temperature, there is a firm difference in the electrical conductivity of this material. There are abundant current carriers to take part in the current flow. The flow of current is achieved mostly by electrons in this type of material. Therefore, an extrinsic material becomes a good electrical conductor. Effect of Doping on P-type Material The effect of doping on a P-type material is as follows − When Indium (In) or Gallium (Ga) is added to pure silicon, a P-type material is formed. This type of dopant material has three valence electrons. They are eagerly looking for a fourth electron. In P type material, each hole can be filled with an electron. To fill this hole area, very less energy is required by electrons from the neighboring covalent bonded groups. Silicon is typically doped with doping material in the range of 1 to 106. This means that P material will have much more holes than the electron-hole pairs of pure silicon. At room temperature, there is a very determined characteristic difference in the electrical conductivity of this material. The following figure shows how the crystal structure of Silicon is altered when doped with an acceptor element — in this case, Indium. A piece of P material is not positively charged. Its atoms are primarily all electrically neutral. There are, however, holes in the covalent structure of many atom groups. When an electron moves in and fills a hole, the hole becomes void. A new hole is created in the bonded group where the electron left. Hole movement in effect is the result of electron movement. A P-type material will go into conduction with only 0.05 eV of energy applied. The above figure shows how a P-type crystal will respond when connected to a voltage source. Note that there are larger numbers of holes than electrons. With voltage applied, the electrons are attracted to the positive battery terminal. Holes move, in a sense, toward the negative battery terminal. An electron is picked up at this point. The electron immediately fills a hole. The hole then becomes void. At the same time, an electron is pulled from the material by the positive battery terminal. Holes therefore move toward the negative terminal due to electrons shifting between different bonded groups. With energy applied, hole flow is continuous. Learning working make money

Semiconductor Devices – Quick Guide Semiconductor Devices – Introduction It is widely seen that the distance of a nucleus from the electron of a particular atom is not equal. Normally, electrons rotate in a well-defined orbit. A particular number of electrons can only hold by outer shell or orbit. The electrical conductivity of an atom is influenced mainly by the electrons of the outer shell. These electrons have a great deal to do with the electrical conductivity. Conductors and Insulators Electrical conduction is the result of irregular or uncontrolled movement of electrons. These movements cause certain atoms to be good electrical conductors. A material with such type of atoms has many free electrons in its outer shell or orbit. Comparatively, an insulating material has a relatively small number of free electrons. Consequently, the outer shell electrons of insulators tend to hold their place firmly and hardly allow any current to flow through it. Therefore, in an insulating material, very little electrical conductivity takes place. Semiconductors In between conductors and insulators, there is a third classification of atoms (material) known as semiconductors. Generally, the conductivity of a semiconductor lies in between the conductivities of metals and insulators. However, at absolute zero temperature, the semiconductor also acts like a perfect insulator. Silicon and germanium are the most familiar semiconductor elements. Copper oxide, cadmium-sulfide, and gallium arsenide are some other semiconductor compounds that are frequently used. These kinds of material are generally classified as type IVB elements. Such atoms have four valence electrons. If they can give up four valence electrons, stability can be accomplished. It can also be achieved by accepting four electrons. Stability of an Atom The concept of stability of an atom is an important factor in the status of semiconductor materials. The maximum number of electrons in the valence band is 8. When there are exactly 8 electrons in the valence band, it can be said that the atom is stable. In a stable atom, the bonding of valence electrons is very rigid. These types of atoms are excellent insulators. In such atoms, free electrons are not available for electrical conductivity. Examples of stabilized elements are gases such as Argon, Xenon, Neon, and Krypton. Due to their property, these gases cannot be mixed with other material and are generally known as inert gases. If the number of valence electrons in the outer shell is less than 8, then the atom is said to be unstable i.e., the atoms having fewer than 8 valence electrons are unstable. They always try to borrow or donate electrons from the neighboring atoms to become stable. Atoms in the outer shell with 5, 6, or 7 valence electrons tend to borrow electrons from other atoms to seek stability, while atoms with one, two, or three valence electrons tend to release these electrons to other nearby atoms. Atomic Combinations Anything that has weight is matter. As per the theory of atom, all matter, whether it is solid, liquid, or gas is composed of atoms. An atom contains a central part called nucleus, which holds the neutrons and the protons. Normally, protons are positively charged particles and neutrons are neutrally charged particles. Electrons which are negatively charged particles are arranged in orbits around the nucleus in a way similar to the array of planets around the Sun. The following figure shows the composition of an atom. Atoms of different elements are found to have different number of protons, neutrons, and electrons. To distinguish one atom from another or to classify the various atoms, a number which indicates the number of protons in the nucleus of a given atom, is assigned to the atoms of each identified element. This number is known as the atomic number of the element. The atomic numbers for some of the elements which are associated with the study of semiconductors are given in the following table. Element Symbol Atomic Number Silicon Si 14 Germanium Ge 32 Arsenic As 33 Antimony Sb 51 Indium In 49 Gallium Ga 31 Boron B 5 Normally, an atom has an equal number of protons and planetary electrons to maintain its net charge at zero. Atoms frequently combine to form stabilized molecules or compounds through their available valence electrons. The process of combining of free valence electrons is generally called bonding. Following are the different kinds of bonding that takes place in atom combinations. Ionic bonding Covalent bonding Metallic bonding Let us now discuss in detail about these atomic bondings. Ionic Bonding Each atom is seeking stability when the atoms bond together to form molecules. When the valence band contains 8 electrons, it is said to be a stabilized condition. When the valence electrons of one atom combine with those of another atom to become stable, it is called ionic bonding. If an atom has more than 4 valence electrons in the outer shell it is seeking additional electrons. Such atom is often called an acceptor. If any atom holds less than 4 valence electrons in the outer shell, they try to move out from these electrons. These atoms are known as donors. In ionic bonding, donor and acceptor atoms frequently combine together and the combination becomes stabilized. Common salt is a common example of ionic bonding. The following figures illustrate an example of independent atoms and ionic bonding. It can be seen in the above figure that the sodium (Na) atom donates its 1 valence electron to the chloride (Cl) atom which has 7 valence electrons. The chloride atom immediately becomes overbalanced negatively when it obtains the extra electron and this causes the atom to become a negative ion. While on the other hand, the sodium atom loses its valence electron and the sodium atom then becomes a positive ion. As we know unlike charges attract, the sodium and chloride atoms are bound together by an electrostatic force. Covalent Bonding When the valence electrons of neighboring atoms are shared with other atoms, covalent bonding takes place. In covalent bonding, ions are not formed. This is