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BIPOLAR JUNCTION TRANSISTOR (BJT) UNIT I / Microwave Engineering / Bipolar Junction Transistor 1 A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses both electrons and holes as charge carriers. Unipolar transistors, such as field-effect transistors, only use one kind of charge carrier. BJTs use two junctions between two semiconductor types, ◦ n-type ◦ p-type BJTs are manufactured in two types: ◦ NPN PNP NPN ◦ PNP Schematic Symbols are available as individual components, or fabricated in integrated circuits, often in large numbers. BJTs can be used as amplifiers or switches .This ability gives them many applications in electronic equipment such as computers, televisions, mobile phones, audio amplifiers, industrial control, and radio transmitters. UNIT I / Microwave Engineering / Bipolar Junction Transistor 2 STRUCTURE A BJT consists of three differently • The emitter is heavily doped doped semiconductor regions: the emitter region, the base region and the • The collector is lightly doped, allowing collector region. a large reverse bias voltage to be These regions are, respectively, p applied before the collector–base type, n type and p type in a PNP junction breaks down. transistor, and n type, p type and n type in an NPN transistor. • The base is physically located between Each semiconductor region is the emitter and the collector and is connected to a terminal, appropriately made from lightly doped, high- labeled: emitter (E), base (B) and resistivity material. collector (C). • The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. • This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. • Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5. • Early transistors were made from germanium but most modern BJTs are made from siliconUNIT. A I significant / Microwave Engineering minority / areBipolar also now made from gallium arsenide, especially for very high speed applications Junction Transistor 3 BIPOLAR TRANSISTOR CONSTRUCTION UNIT I / Microwave Engineering / Bipolar Junction Transistor 4 FUNCTION Charge flow in a BJT is due to diffusion of charge carriers across a junction between two regions of different charge concentrations. The regions of a BJT are called emitter, base, and collector. A discrete transistor has three leads for connection to these regions. The emitter region is heavily doped compared to the other two layers. The collector is doped much lighter than the base (collector doping is typically ten times lighter than base doping ). By design, most of the BJT collector current is due to the flow of charge carriers (electrons or holes) injected from a heavily doped emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices. UNIT I / Microwave Engineering / Bipolar Junction Transistor 5 NPN PNP NPN consists of a layer of P-doped semiconductor PNP consists of a layer of N-doped semiconductor (the "base") between two N-doped layers. between two layers of P-doped material. A small current entering the base is amplified to A small current leaving the base is amplified in the produce a large collector and emitter current. collector output. That is, a PNP transistor is "on" That is, when there is a positive potential difference when its base is pulled low relative to the emitter. measured from the base of an NPN transistor to its In a PNP transistor, the emitter–base region is emitter as well as a positive potential difference forward biased, so holes are injected into the base measured from the collector to the emitter, the as minority carriers. transistor becomes active. The base is very thin, and most of the holes cross In this "on" state, current flows from the collector the reverse-biased base–collector junction to the to the emitter of the transistor. collector. Most of the current is carried by electrons moving The arrows in the NPN and PNP transistor symbols from emitter to collector as minority carriers in the indicate the PN junction between the base and P-type base region. emitter. To allow for greater current and faster operation, When the device is in forward active or forward most bipolar transistors used today are NPN saturated mode, the arrow, placed on the emitter because electron mobility is higher than hole leg, points in the direction of the conventional UNIT I / Microwave Engineering / Bipolar mobility. current. Junction Transistor 6 WORKING PRINCIPLE OF BJT The BE junction is a forward bias and the CB is a reverse bias junction. The width of the depletion region of the CB junction is higher than the BE junction. The forward bias at the BE junction decreases the barrier potential and produces electrons to flow from the emitter to the base and the base is a thin and lightly doped it has very few holes and less amount of electrons from the emitter about 2% it recombine in the base region with holes and from the base terminal it will flow out. This initiates the base current flow due to combination of electrons and holes. The left over large number of electrons will pass the reverse bias collector junction to initiate the collector current. By using KCL we can observe the mathematical equation IE = IB + IC The base current is very less as compared to emitter and collector current IE ~ IC UNIT I / Microwave Engineering / Bipolar Junction Transistor 7 WORKING PRINCIPLE OF BJT Here the operation of PNP transistor is same as the NPN transistor the only difference is only holes instead of electrons. The diagram shows the PNP transistor of the active mode region. UNIT I / Microwave Engineering / Bipolar Junction Transistor 8 REGIONS OF OPERATIONS Junction Applied Junction bias Mode type voltages B-E B-C Forward- E < B < C Forward Reverse active E < B > C Forward Forward Saturation NPN E > B < C Reverse Reverse Cut-off Reverse- E > B > C Reverse Forward active Reverse- E < B < C Reverse Forward active E < B > C Reverse Reverse Cut-off PNP E > B < C Forward Forward Saturation Forward-active: Base higher than emitter, collector higher than base (in this mode the collector current is proportional toForward base current- by ). E > B > C Forward Reverse Saturation: active Base higher than emitter, but collector is not higher than base. Cut-off: Base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current go through from collector to emitter. UNIT I / Microwave Engineering / Bipolar Junction Transistor 9 Reverse-active: Base lower than emitter, collector lower than base: reverse conventional current goes through transistor. 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 Common Base (CB) mode Common Emitter (CE) mode Common Collector (CC) mode UNIT I / Microwave Engineering / Bipolar Junction Transistor 10 INPUTCOMMON CHARACTERISTICS BASE CHARACTERISTICSOUTPUT CHARACTERISTICS UNIT I / Microwave Engineering / Bipolar Junction Transistor 11 INPUTCOMMON CHARACTERISTICS EMITTER CHARACTERISTICSOUTPUT CHARACTERISTICS UNIT I / Microwave Engineering / Bipolar Junction Transistor 12 COMMON COLLECTOR CHARACTERISTICS UNIT I / Microwave Engineering / Bipolar Junction Transistor 13 EXAMPLES OF BIPOLAR JUNCTION TRANSISTORS •Grown-junction transistor – first bipolar junction transistor made. •Alloy-junction transistor – emitter and collector alloy beads fused to base. •Micro-alloy transistor (MAT) – high speed type of alloy junction transistor. •Micro-alloy diffused transistor (MADT) – high speed type of alloy junction transistor, speedier than MAT, a diffused-base transistor. •Post-alloy diffused transistor (PADT) – high speed type of alloy junction transistor, speedier than MAT, a diffused-base transistor. •Tetrode transistor – high speed variant of grown-junction transistor or alloy junction transistor with two connections to base. •Surface-barrier transistor – high-speed metal barrier junction transistor. •Drift-field transistor – high speed bipolar junction transistor. •Spacistor •Diffusion transistor – modern type bipolar junction transistor. •Diffused-base transistor – first implementation of diffusion transistor. •Mesa transistor •Planar transistor – the bipolar junction transistor that made mass-produced monolithic integrated circuits possible. •Epitaxial transistor– a bipolar junction transistor made using vapor phase deposition. See epitaxy. Allows very precise control of doping levels UNIT I / Microwave Engineering / Bipolar and gradients. Junction Transistor 14 ADVANTAGES OF BJT DISADVANTAGES OF BJT The bipolar junction transistor The bipolar junction transistor (BJT) has a large gain (BJT) more noise produced. bandwidth. The BJT are more effect by The BJT shows better performance at high frequency. radiation. The BJT has a better voltage BJT has a low thermal stability. gain. The switching frequency of BJT The BJT can be operated in low is low. or high power applications. It has a very complex base The BJT has high current control. So it may lead to density. confusion and requires a skillful There is low forward voltage handling. drop. UNIT I / Microwave Engineering / Bipolar Junction Transistor 15 APPLICATIONS OF BJT The bipolar junction transistor (BJT) is used in logic circuits. The BJT is used as an oscillator. It is used as an amplifier. It is used as a multivibrator. For wave shaping it is used in clipping circuits. Used as a detector or demodulator. It is also used as modulator. Used in timer and time delay circuits. It is used in electronics switch. It is used in switching circuits. UNIT I / Microwave Engineering / Bipolar Junction Transistor 16 .
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