Modullr-3 (Sub: Semiconductor Physics, 3rd Sem. ECE/ETC) The Bipolar Transistor: Introduction, Modes of operation; Minority Carrier distribution, Collector current, Base current, current gain, Base width Modulation by collector current, Breakdown mechanism, Equivalent Circuit Models – Ebers -Moll Model. Contents ➢ Bipolar Junction Transistor ➢ Simplified structure ➢ Modes of operation : physical operation of n-p-n and p-n-p ➢ transistors in the active region, ➢ Minority Carrier distribution :Current-voltage characteristics of BJT ➢ Base width modulation ➢ BJT as an amplifier and as a switch. ➢ Breakdown mechanism, Equivalent Circuit Models (5.1 to 5.7 of Sedra and Smith)
Bipolar Junction Transistor Introduction: The transistor is a main building block of all modern electronic systems.It is a three terminal device whose output current, voltage and/ or power are controlled by its input current. In digital computer electronics , the transistor is used as a high speed electronic switch that is capable of switching between two operating states(open and closed) at a rate of several billions of times per second. There are two types of transistors namely 1. Bipolar Junction transistor (BJT) 2. Field effect transistor (FET) Bipolar Junction transistor (BJT) is commonly known as Junction transistor or simply transistor. Two types of BJT’s are 1. NPN 2. PNP Key notes: • • • A three terminal device namely- Emitter (E), Base (B), Collector (C). Can be considered as two diodes connected back-to-back. 2 types of BJT; o npn: Two n regions separated by a p region
o pnp: Two p regions separated by an n region • • • • • • BIPOLAR: refers to the use of both holes & electrons as current carriers in the transistor structure. The transistor consists of two pn junctions. emitter–base junction (EBJ) and the collector–base junction (CBJ). Base-emitter junction: the pn junction joining the base region & the emitter region Base-collector junction: the pn junction joining the base region & the collector region. Features of the three terminalso EMITTER REGION: heavily doped, wider than base but narrower than Collector o BASE REGION: lightly doped, & very narrow area o COLLECTOR REGION: moderately doped, wide area • Device symbols- • Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of the BJT are obtained, as shown in Table below- • • The active mode is the one used if the transistor is to operate as an amplifier. Switching applications (e.g., logic circuits) utilize both the cutoff mode and the saturation mode. In the cutoff mode no current flows because both junctions are reverse biased. •
FIGURE 4.1 SIMPLIFIED npn and pnp BJT Physical Operation- • • • • • • • • • • Figure 4.2 Operation of a npn BJT The forward bias on the emitter–base junction will cause current to flow across this junction. Current will consist of two components: electrons injected from the emitter into the base, and holes injected from the base into the emitter. With a heavily doped emitter and a lightly doped base; that is, the device is designed to have a high density of electrons in the emitter and a low density of holes in the base. The current that flows across the emitter–base junction will constitute the emitter current 𝑖𝐸 , as indicated in Fig. 4.2 The direction of 𝑖𝐸 , is “out of” the emitter lead, which, is in the direction of the positivecharge flow (hole current) and opposite to the direction of the negative-charge flow (electron current). The emitter current 𝑖𝐸 , being equal to the sum of these two components. However, since the electron component is much larger than the hole component, the emitter current will be dominated by the electron component. When electrons get injected from the emitter into the base Electrons become minority carriers in the p-type base region. As the base is very thin, the steady state the excess minority carrier (electron) concentration in the base will be, as indicated by the solid straight line in Fig. 4.3.