tailieunhanh - Introduction to Electronics - Part 4

Bipolar Junction Transistors (BJTs) BJT là một thiết bị phi tuyến 3-thiết bị đầu cuối dựa trên các diode ngã ba. Một cơ cấu đại diện bánh sandwich một trong chất bán dẫn loại giữa các lớp của các loại đối diện. Chúng tôi lần đầu tiên kiểm tra các BJT NPN: | Bipolar Junction Transistors BJTs Introduction to Electronics 76 Bipolar Junction Transistors BJTs Introduction The BJT is a nonlinear 3-terminal device based on the junction diode. A representative structure sandwiches one semiconductor type between layers of the opposite type. We first examine the npn BJt n-type collector p-type base iB Bo n-type emitter OC i Vce VBE - ỎE Two junctions collectorbase junction CBJ emitter-base junction EBJ . Current in one p-n junction affects the current in the other p-n junction. There are four regions of operation Fig. 115. The npn BJT representative physical structure left and circuit symbol right . Operating Region EBJ CBJ Feature cutoff rev. rev. iC iE iB 0 active fwd. rev. amplifier saturation fwd. fwd. vCE nearly zero inverse rev. fwd. limited use We re most interested in the active region but will have to deal with cutoff and saturation as well. Discussion of inverse region operation is left for another time. Bipolar Junction Transistors BJTs Introduction to Electronics 77 Qualitative Description of BJT Active-Region Operation Emitter region is heavily doped . . .lots of electrons available to conduct current. Base region very lightly doped and very narrow . . .very few holes available to conduct current. Rev-biased CBJ collector positive base. Fwd-biased EBJ base positive emitter. Emitter current iE consists mostly of electrons being injected into base region because the base is lightly doped iB is small. Some of the injected electrons combine with holes in base region. Most of the electrons travel across the narrow base and are attracted to the positive collector voltage creating a collector current Fig. 116. Active-region BJT currents. The relative current magnitudes are indicated by the arrow thicknesses in the figure. Because iB is so small a small change in base current can cause a large change in collector current - this is how we get this device to amplify Bipolar Junction Transistors BJTs Introduction