Bipolar Junction Transistors (BJTs) PDF

ACTIVE DEVICES I /CHAPTER III : BJT’s L03/ACTIVE DEVICES I BIPOLAR JUNCTION TRANSISTOR TRANSISTORS S (BJT’S)) 1. Introduction...

ACTIVE DEVICES I /CHAPTER III : BJT’s L03/ACTIVE DEVICES I BIPOLAR JUNCTION TRANSISTOR TRANSISTORS S (BJT’S)) 1. Introduction The pn junction studied in Chapter II forms the basis of a large number of semiconductor devices. The semiconductor diode, a twotwo-terminal terminal device, is the most direct application of the pn junction. In this Chapter, Chapter, we will introduce the physics of transistor devices as intuitively as possible, resorting to an analysis of their i -v v characteristics to discover important properties and applications. applica 2. Definition Definitions a. Transistor : Transfer Transfer resistor re The term Transistor was adopted because it best describes the operation of the transistor which is the transfer of an input signal current from a low-resistance low resistance circuit to a high-- resistance circuit. The transistor is used to  Amplify current, voltage and power signal signals.  Switch witch electronic signals (such as logic gates) and electrical power (such as switched power supplies) b. Bipolar junction transistor (BJT) A BJT is formed by joining three sections of semiconductor material, each with a different doping concentration. Fig. Fig.3.1: npn and pnp bipolar junction transistors (Structures and symbols) IGEE/UMBB | Teacher:: Dr L.SADOUKI P3-1 ACTIVE DEVICES I /CHAPTER III : BJT’s L03/ACTIVE DEVICES I The three sections can be either a thin n region sandwiched between p and p layers, or a p region between n and n layers, the resulting BJTs are called pnp and npn transistors, respectively. (See Fig.3.1) Fig.3.1 The three layers of a BJT are:  The emitter layer (E) is heavily doped semiconductor, it emits current carriers.  The collector layer (C) lightly lightl doped, it collects the current carriers.  The base (B) is very thin lightly doped section which controls the flow of current carriers. The most important property of the bipolar transistor is that the small bas current controls the amount of the much larger collector current. 3. BJT’ss description Fig.3. 3.2: 2: BJT’s biasing The operation of the npn BJT (pnp BJT BJT) may be explained by considering the transistor as consisting of two back-to-back back back pn junctions. The base-emitter emitter (BE) ( ) junction (J1) acts very much like a diode when it is forward forward-biased biased; this bias allows the free electrons (holes) in emitter to go through the pn junction to arrive at the base, forming the emitter current (IE). IGEE/UMBB | Teacher:: Dr L.SADOUKI P3-2 ACTIVE DEVICES I /CHAPTER III : BJT’s L03/ACTIVE DEVICES I As the p-type base ( n-type base) is thin and lightly doped, only a small number of the electrons (holes) from the emitter are combined with the holes (electrons) in base to form the base current (IB). The base-collector (BC) junction (J2) is reverse-biased, in this case, most of the electrons (holes) coming from the emitter, will diffuse and cross the reverse biased junction to arrive at the collector to create the collector current (IC). By applying KCL, we have: IE = IB + IC 4. BJT’s configurations Depending on which of the three terminals is used as common terminal, there can be three possible configurations for the two port network formed by transistor. 1) Common base configuration (CB) 2) Common emitter configuration (CE) 3) Common collector configuration (CC) Ii Io Two-port Vi system Vo Fig.3.3: Two-port system 4.1. Common base configuration (CB) In this configuration the input stage is the Emitter-Base stage, with: input current is IE, input voltage is VEB and The output stage is the collector –base stage, with: output current is IC, output voltage is VCB. (See Fig.3.3.1) The total collector current: IC= IE+ICBO Where : ICBO is the minority carriers current of (BC) junction with IE=0 (Saturation current). ICBO can be ignored, IC  IE  The current gain: IC/IE  >, IE/IB   5. Load line analysis The load line is drawn on the collector curves between the cut-off cut off and saturation points. Intersection between the DC load line and the (IC, VCE) curve gives the quiescent point (Q(I (ICQ, VCEQ)) )) or the operating point. This load line is determined by the collector circuit: Vcc-VCE - RC IC = 0 The load line equation: Vcc Vcc- IC = IGEE/UMBB | Teacher:: Dr L.SADOUKI P3-7  ACTIVE DEVICES I /CHAPTER III : BJT’s L03/ACTIVE DEVICES I If IC=0  VCE= Vcc (Cut-off ( off region) region If VCE= 0  IC=Vcc/RC (Saturation Saturation region region) Load line Operating point ICQ VCEQ Fig.3.44: Load line analysis In order to change the operating point from one position to another, we have three possibilities: a. By changing the current IB (changing the input stage) IB0