Chapter 4 - Active components

Abstract

This chapter highlights the common two- and three-terminal devices: diodes, thyristors, triacs, transistors, FETs, and IGBTs. The diode is a two-terminal device whose function is to pass current in one direction. A conventional diode is formed from the junction of p type and n-type silicon. The ideal device has a “brick-wall” V–I characteristic: the practical silicon diode has an exponential characteristic, which approximates to the brick wall, if viewed on a large enough scale. An ideal diode blocks all current flow in the reverse direction and a practical diode does not. There are two main reverse characteristics: reverse leakage current IR and reverse breakdown voltage VBR. The diode equation holds good in the reverse direction until VBR is approached; in the low-voltage region IR is almost equal to IS. VBR is that voltage at which the reverse-biased junction can no longer withstand the applied electric field. At this point, avalanche breakdown occurs and a current limited mainly by the external source impedance flows. If the device maximum power dissipation is exceeded, the junction is destroyed. Diodes operated conventionally, as opposed to zener diodes, are always run at reverse voltages lower than VBR.