UNIT 3 Transistors JFET

Transcription

1 UNIT 3 Transistors JFET Mosfet Definition of BJT A bipolar junction transistor is a three terminal semiconductor device consisting of two p-n junctions which is able to amplify or magnify a signal. It is a current controlled device. The three terminals of the BJT are the base, the collector and the emitter. A signal of small amplitude if applied to the base is available in the amplified form at the collector of the transistor. This is the amplification provided by the BJT. Note that it does require an external source of DC power supply to carry out the amplification process. The basic diagrams of the two types of bipolar junction transistors mentioned above are given below. From the above figure, we can see that every BJT has three parts named emitter, base and collector. J E and J C represent junction of emitter and junction of collector respectively. Now initially it is sufficient for us to know that emitter based junction is forward biased and collector base junctions is reverse biased. The next topic will describe the two types of this transistors. N-P-N Bipolar Junction Transistor As started before in n-p-n bipolar transistor one p - type semiconductor resides between two n- type semiconductors the diagram below a n-p-n transistor is shown Now I E, I C

2 is emitter current and collect current respectively and V EB and V CB are emitter base voltage and collector base voltage respectively. According to convention if for the emitter, base and collector current I E, I B and I C current goes into the transistor the sign of the current is taken as positive and if current goes out from the transistor then the sign is taken as negative. We can tabulate the different currents and voltages inside the n-p-n transistor. Transistor type I E I B I C V EB V CB V CE n - p n P-N-P Bipolar Junction Transistor Similarly for p - n - p bipolar junction transistor a n-type semiconductors is sandwiched between two p-type semiconductors. The diagram of a p - n - p transistor is shown below For p-n-p transistors, current enters into the transistor through the emitter terminal. Like any bipolar junction transistor, the emitter-base junction is forward biased and the collector-base junction is reverse biased. We can tabulate the emitter, base and collector current, as well as the emitter base, collector base and collector emitter voltage for p-n-p transistors also. Transistor type I E I B I C V EB V CB V CE p - n p Working Principle of BJT Figure shows an n-p-n transistor biased in the active region (See transistor biasing), the BE junction is forward biased whereas the CB junction is reversed biased. The width of the depletion region of the BE junction is small as compared to that of the CB junction. The forward bias at the BE junction reduces the barrier potential and causes the electrons to flow from the emitter to base. As the base is thin and lightly doped it consists of very few holes so some of the electrons from the emitter (about 2%) recombine with the holes present in the base region and flow out of the base terminal. This constitutes the base current, it flows due to recombination of electrons and holes (Note that the direction of conventional current flow is opposite to that of flow of electrons). The remaining large number of electrons will cross the reverse biased collector junction to constitute the collector current. Thus by KCL, The base current is very small as compared to emitter and collector current. Here, the

3 majority charge carriers are electrons. The operation of a p-n-p transistor is same as of the n-p-n, the only difference is that the majority charge carriers are holes instead of electrons. Only a small part current flows due to majority carriers and most of the current flows due to minority charge carriers in a BJT. Hence, they are called as minority carrier devices. Equivalent Circuit of BJT A p-n junction is represented by a diode. As a transistor has two p-n junctions, it is equivalent to two diodes connected back to back. This is called as the two diode analogy of the BJT. Bipolar Junction Transistors Characteristics The three parts of a BJT are collector, emitter and base. Before knowing about the bipolar junction transistor characteristics, we have to know about the modes of operation for this type of transistors. The modes are 1. Common Base (CB) mode 2. Common Emitter (CE) mode 3. Common Collector (CC) mode All three types of modes are shown below Operation of BJT Bipolar junction transistors has two junctions base emitter junction, base collector junction. Accordingly there are four different regions of operation in which either of the two junctions are forward biased reverse biased or both. But the BJT can be effectively operated in there different modes according to the external bias voltage applied at each junction. i.e. Transistor in active region, saturation and cutoff. The other region of operation of BJT is called as inverse active region.the operation of transistor in active mode is explained below. Transistor in active region Active region is one in which Base emitter junction is forward biased and Base Collector junction will be reverse biased in a transistor. In NPN transistor when you bias it in active region the currents flowing through it will be as follows The currents flowing through the three terminals of BJT are

4 Emitter current : A forward current flows from emitter into base consisting of electrons and hole current flowing from base to emitter. Base current : A recombination current flows from the base which in the external circuit appears as base current supplied by power supply which is exactly equal to the rate at which charge carriers (holes) are lost in base due to recombination. This current will be small as base is lightly doped and numbers of charge carriers are less. Also a reverse saturation electron current flows base to collector as base collector junction is reverse biased. Collector current : The collector current consists of two components a) Reverse saturation current through reverse biased base collector junction. The base collector junction can be thought of as reverse biased diode. Then the current through the base collector junction from the diode current equation is given as I rev,c = I co (1-exp (V bc /V t )) in case of PNP transistor as reverse current flows from the base to collector and V bc is negative for a reverse biased PN junction. For a NPN transistor I rev,c = I co *(1-exp(V cb /V t )) as reverse current flows from the collector to base and V cb is negative for a reverse biased PN junction. where I co is reverse saturation current,v t is voltage equivalent of temperature = k*t/e = 26 mv at 300 Deg C, k is Boltzmann s constant =1.38*10-23 Joule/Kelvin, T is absolute temperature in kelvin,e is electronic charge = 1.6*10-19 C. b) The emitter current left after recombination base current flows into collector. The fraction of emitter current is quantified in terms of a parameter termed as alpha.alpha(α) is the large signal current gain which is defined as ratio of collector current increment from cut off to emitter current increment from cut off. In cut off I E = 0 amps and I c = I co. α = (I c -I co ) / (I E -0) Large signal current gain of common base transistor α = (I c -I co ) / (I E -0). Alpha typically varies from 0.9 to Summing up, the collector current is given as I c = -α*i E + I co (1-e V c/v t ) If we neglect reverse saturation current I co then beta can be represented in terms of alpha, β= α / (1- α)and α = β/(1+β). substituting the value of α in terms of β in the equation for collector current and assuming the reverse current is ~I co I c = -β*i E /(1+β) I co since I C +I B +I E =0 we will get I c = β*(i C +I B )/(1+β) + I co rearranging the terms I c = (β*i B ) + I co *(1+β) By neglecting the reverse saturation current term there exists a linear relationship between collector current and base current in a common emitter transistor described by a parameter β also called as large signal current gain in common emitter configuration as in common emitter configuration input current is I b and output current is I c.. large signal current gain in common emitter configuration (β) = I c /I b

5 Phototransistors Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. Phototransistors are used in almost all electronic devices that depend on light including smoke detectors, laser-ranging finding devices, and optical remote controls. They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes. This category includes photodarlingtons. Composition of Phototransistors Bipolar transistors are the most commonly used transistors. Phototransistors are typically bipolar NPN devices and are made of three lead components: 1. The base is the lead responsible for activating the transistor. It is the gate controller device for the larger electrical supply. 2. The collector is the positive lead and the larger electrical supply. 3. The emitter is the negative lead and the outlet for the larger electrical supply. While ordinary transistors have photosensitive effects when exposed to light, phototransistors are optimized for use with light. Phototransistors have larger base and collector areas than ordinary transistors. They generally have a clear or opaque casing to enhance light exposure. Most phototransistors are made of a single material although some others may be composed of multiple materials. Early phototransistors had a homo-junction structure made of germanium or silicon (see photo below, left). Modern phototransistors may be composed of multiple material junctions of type III-V materials like gallium and arsenide (see photo below, right). The physical structure can be optimized to allow higher levels of light exposure by using an offset configuration. Image Credit: Radio-Electronics.com Phototransistors are made of semi-conductive materials. Although germanium has more desirable electrical properties, silicon is more commonly used because of its reliability and low cost. How Phototransistors Work A typical transistor consists of a collector, emitter, and base sections. The collector is biased positively with respect to the emitter and the base-collector junction is reverse biased. A phototransistor remains inactive until light falls onto the base. Light activates the phototransistor,

6 allowing the formation of hole-electron pairs and the flow of current across the collector or emitter. As the current spreads it is concentrated and converted into voltage. A phototransistor usually does not have a base connection (see diagram below). The base is left disconnected because the light is used to enable the current to flow through the phototransistor. JUNCTION FIELD EFFECT TRANSISTOR Junction field effect transistor or JFET is one of the simplest transistors from the structural point of view. It is a voltage controlled semiconductor device. In this, the current is carried by only one type of carriers. So, it is a unipolar device. It has a very high input electrical resistance. JFET consists of a doped Si or GaAs bar. There are ohmic contacts, the two ends of the bar and semiconductor junction on its two sides. If the semiconductor bar is n-type, the two sides of the bar is heavily doped with p - type impurities and this is known as n - channel JFET. On the other hand if the semiconductor bar is p- type, the two sides of the bar is heavily doped with n - type impurities and this is known as p- channel JFET. When a voltage is applied between the two ends, a current which is carried by the majority carriers of the bar flows along the length of the bar. There are several terminals in JFET. The terminal through which the majority carrier enter the bar and the terminal through which they leave are known as source (s) and drain (D) respectively. The heavily doped region on the two sides is known as the gate (G). In junction field effect transistor, the junction is a reverse biased. As a result, depletion regions form, which extend to the bar. By changing gate to source voltage, the depletion width can be controlled. So, the effective cross section area decreased with increasing reverse bias. So, the drain current is a function of the gate to the source voltage: Now days JFET is obsolete. Its applicants are limited to circuit design. Where it can be used an amplifier and as a switch both. N-Channel JFET

7 A semiconductor bar of n-type material is taken & ohmic contacts are made on either ends of the bar. Terminals are brought out from these ohmic contacts and named as drain & source as shown in the figure below. On the other two sides of the n-type semiconductor bar, heavily doped p- type regions are formed to create a p-n junction. Both these p-type regions are connected together via ohmic contacts and the gate terminal is brought out as seen below. Figure below shows the n-channel and p-channel JFET with symbols. The arrow on the gate indicates the direction of the current. Current flows through the length of the n-type bar (channel) due to majority charge carries which in this case are electrons. When a voltage is applied between the two ends, a current which is carried by the majority carriers electrons flows along the length of a bar. The majority carriers enter the bar through the source terminal and leave through the drain terminal. The heavily doped regions of the n-type bar are known as the gates. The gate source junctions is reverse is biased as a result depletion regions from which extend to the bar by changing gate to source voltage effective cross sectional area decreases with the function of the gate to source voltage. P-Channel JEFT

8 p-channel JFET consists of a p-type silicon or GaAs. Two sides of the bar is heavily doped with n-type impurities. When a voltage is applied between the two ends, a current which is carried by the majority carrier holes flow along the length of a bar. The gate source junction is reverse biased as a result depletion regions form, which extend to the bar by changing gate to extend to source voltage the depletion width can be controlled. The effective cross sectional area decreased with increasing reverse bias, so the drain current is the function of the gate to source voltage. Operation of Junction Field Effect Transistor or JFET Operation with gate to source voltage = 0 If an n-channel JFET is biased as explained above and the gate to source voltage is kept zero, due to the positive drain to source voltage few electrons which are available for conduction in the n-type material will start flowing from the narrow passage (channel) from source to drain. This current is called as drain current. As the channel has some finite resistance it will cause some voltage drop across the channel. Hence the depletion region of the p-n junction starts increasing and penetrates more into the n-type material as it is lightly doped. Due to this the width of the channel available for conduction is reduced. The penetration of the depletion region into the n- type region depends on the reverse bias voltage. Maximum drain current I D(MAX) will flow through the device when the channel is widest i.e. when V GS is zero.

9 Operation with negative gate to source voltage As a negative voltage is applied to the gate to source p-n junction the depletion region increases and penetration of the depletion region into the n-type channel further increases. If the negative gate to source voltage is further increased the depletion region spreads more and more inside the n-type bar. Due to this less and less number of charge carries (electrons) can pass through the channel and the drain current reduces. Hence, with increase in negative gate to source voltage drain current reduces. At a certain value of this voltage the depletion region from both the ends will increase and touch each other and the drain current will become zero. This gate to source voltage at which drain current is cutoff is called as V GS(OFF). As seen the V GS controls I D. Hence, JFET is a voltage controlled device. The relationship between I D and V GS is given by Shockley s equation Where, V P is the pinch off voltage which is the value of drain to source V DS at which drain current reaches its constant saturation value. Any further increase in V DS does not affect I D. JEFT Characteristics or Junction Field Effect Transistor Characteristics In this characteristics we can find three regions, 1. The linear or the ohmic region: Here the drain to source voltage is small and drain current in nearly proportional to the drain to source voltage. When a positive drain to source voltage is applied, this voltage increases from zero to a small value, the depletion region width remain very small and under this condition the semi conductor bar behaves just like a resistor. So,

10 drain current increases almost linearly with drain to source voltage. 2. ii) The saturation of the active region: Here the drain current is almost constant and it is not dependent on the drain to source voltage actually. When the drain to source voltage continuous to increase the channel resistance increases and at some point, the depletion regions meet near the drain to pinch off the channel. Beyond that pinch off voltage, the drain, current attains saturation. 3. iii) The breakdown voltage: Here the drain current increases rapidly with a small increase of the drain to source voltage. Actually for large value of drain to source voltage, a breakdown of the gate junction takes place which results a sharp increase of the drain current. Transfer characteristics. The graphical characteristics plot of the saturation drain current against the gate to source voltage is known as the transfer characteristics of JFET. It can be obtained from static characteristics very easily. The transfer characteristics of an n- channel is shown below MOSFET MOSFET stands for metal oxide semiconductor field effect transistor. It is capable of voltage gain and signal power gain. The MOSFET is the core of integrated circuit designed as thousands of these can be fabricated in a single chip because of its very small size. Every modern electronic

11 system consists of VLST technology and without MOSFET, large scale integration is impossible. It is a four terminals device. The drain and source terminals are connected to the heavily doped regions. The gate terminal is connected top on the oxide layer and the substrate or body terminal is connected to the intrinsic semiconductor. MOSFET has four terminals(stated above), they are gate, source drain and substrate or body. MOS capacity present in the device is the main part. The conduction and valance bands are position relative to the Fermi level at the surface is a function of MOS capacitor voltage. The metal of the gate terminal and the sc acts the parallel and the oxide layer acts as insulator of the state MOS capacitor. Between the drain and source terminal inversion layer is formed and due to the flow of carriers in it, the current flows in MOSFET the inversion layer is properties are controlled by gate voltage. Thus it is a voltage controlled device. Two basic types of MOSFET are n channel and p channel MOSFETs. In n channel MOSFET is current is due to the flow of electrons in inversion layer and in p channel current is due to the flow of holes. Another type of characteristics of clarification can be made of those are enhancement type and depletion type MOSFETs. In enhancement mode, these are normally off and turned on by applying gate voltage. The opposite phenomenon happens in depletion type MOSFETs. Working Principle of MOSFET The working principle of MOSFET depends up on the MOS capacitor. The MOS capacitor is the main part. The semiconductor surface at below the oxide layer and between the drain and source terminal can be inverted from p-type to n-type by applying a positive or negative gate voltages respectively. When we apply positive gate voltage the holes present beneath the oxide layer experience repulsive force and the holes are pushed downward with the substrate. The depletion region is populated by the bound negative charges, which are associated with the acceptor atoms. The positive voltage also attracts electrons from the n+ source and drain regions in to the channel. The electron reach channel is formed. Now, if a voltage is applied between the source and the drain, current flows freely between the source and drain gate voltage controls the electrons concentration the channel. Instead of positive if apply negative voltage a hole channel will be formed beneath the oxide layer.

12 Now, the controlling of source to gate voltage is responsible for the conduction of current between source and the drain. If the gate voltage exceeds a given value, called the three voltage only then the conduction begins. The current equation of MOSFET in triode region is Where, u n = Mobility of the electrons C ox = Capacitance of the oxide layer W = Width of the gate area L = Length of the channel V GS = Gate to Source voltage V TH = Threshold voltage V DS = Drain to Source voltage. P-Channel MOSFET MOSFET which has p - channel region between source any gate is known as p - channel MOSFET. It is a four terminal devices, the terminals are gate, drain, source and substrate or body. The drain and source are heavily doped p+ region and the substrate is in n-type. The current flows due to the flow of positively charged holes that s why it is known as p-channel MOSFET. When we apply negative gate voltage, the electrons present beneath the oxide layer, experiences repulsive force and they are pushed downward in to the substrate, the depletion region is populated by the bound positive charges which are associated with the donor atoms. The negative gate voltage also attracts holes from p+ source and drain region in to the channel region. Thus hole which channel is formed now if a voltage between the source and the drain is applied current flows. The gate voltage controls the hole concentration of the channel. The diagram of p- channel enhancement and depletion MOSFET are given below. N-Channel MOSFET

13 MOSFET having n-channel region between source and drain is known as n-channel MOSFET. It is a four terminal device, the terminals are gate, drain and source and substrate or body. The drain and source are heavily doped n+ region and the substrate is p-type. The current flows due to flow of the negatively charged electrons, that s why it is known as n- channel MOSFET. When we apply the positive gate voltage the holes present beneath the oxide layer experiences repulsive force and the holes are pushed downwards in to the bound negative charges which are associated with the acceptor atoms. The positive gate voltage also attracts electrons from n+ source and drain region in to the channel thus an electron reach channel is formed, now if a voltage is applied between the source and drain. The gate voltage controls the electron concentration in the channel n-channel MOSFET is preferred over p-channel MOSFET as the mobility of electrons are higher than holes. The diagrams of enhancements mode and depletion mode are given below. Difference between Depletion MOSFET vs Enhancement MOSFET Depletion MOSFET

14 Figure-1 depicts construction of depletion type MOSFET. It also mentions circuit symbol of N- channel MOSFET of depletion type. Due to its construction if offers very high input resistance (about to ). Significant current flows for given V DS at V GS of 0 volt. When gate(i.e. one plate of capacitor) is made positive, the channel((i.e. the other plate of capacitor) will have positive charge induced in it. This will result into depletion of majority carriers(i.e. electrons) and hence reduction in conductivity. Hence the curve similarto JFET is obtained as shown in the figure-2. As shown in the symbol here gate is insulated from the channel. For P-channel type MOSFET symbol, arrow will be reversed. Figure-2 depicts drain characteristics and transfer curve of depletion type of MOSFET(Nchannel). Enhancement MOSFET

15 Figure-3 depicts construction of enhancement type MOSFET. It also mentions circuit symbol of N-channel MOSFET of enhancement type. Here continuous channel does not exist from source to drain. Hence no current flows at zero gate voltage. Symbol depicts broken channel between 'S' to 'D' terminals. When positive voltage is applied to the gate, it will induce a channel by flowing minority carriers(i.e. electrons) from P-type bulk into the concentrated layer. Figure-4 depicts drain characteristics and transfer curve of enhancement type of MOSFET(Nchannel). As shown in the figure-4 minimum threshold voltage is needed for the flow of drain current to start.

16 This type of FET is ideal for switching application. This is due to the fact that no gate voltage is needed to keep the device in 'off' state. Moreover the device can be powered ON with the application of same polarity as drain terminal. Following are the important comparison features between Depletion and Enhancement MOSFET types: Enhancement MOSFET does not conduct at 0 volt, as there is no channel in this type to conduct. Depletion MOSFET conducts at 0 volt. Moreover when positive cut-off gate voltage is applied to depletion MOSFET, hence it is less preferred. The depletion MOSFET does not have any kind of leakage currents such as gate oxide and sub threshold type. Depletion MOSFET logic operations are opposite to enhancement type of MOSFETs. Diffusion current(i.e. sub-threshold leakage current) exists in enhancement MOSFET while depletion MOSFET do not have any diffusion current. E&OA