AVERAGE CURRENT 29 Avalanche breakdown in a particular diode might occur The exact value of the avalanche impedance for alternating with just a few volts of reverse bias, or it might take hundreds of current is the value of resistor that would be necessary to allow volts. Rectifier diodes do not undergo avalanche breakdown the same flow of reverse current. The average ZA for alternating until the voltage becomes quite large (see AVALANCHE current differs from the instantaneous value, which fluctuates VOLTAGE). A special type of diode, called the zener diode (see as the voltage rises and falls. Avalanche is usually undesirable ZENER DIODE), is designed to have a fairly low, and very pre- in alternating-current applications. cise, avalanche breakdown voltage. Such diodes are used as voltage regulators. Zener diodes are sometimes called avalanche diodes because of their low avalanche voltages. AVALANCHE TRANSISTOR The illustration shows graphically the current flow through An avalanche transistor is an npn or pnp transistor that is de- a diode as a function of reverse voltage. The current is so small, signed to operate with a high level of reverse bias at the emitter- for small reverse voltages, that it might be considered to be zero base junction. Normally, transistors are forward-biased at the for practical purposes. When the reverse voltage becomes suffi- emitter-base junction except when cutoff conditions are de- cient to cause avalanche breakdown, however, the current rises sired when there is no signal input. rapidly. Avalanche transistors are seen in some switching applica- tions. The emitter-base junction is reverse-biased almost to the point where avalanche breakdown occurs. A small additional AVALANCHE IMPEDANCE reverse voltage, supplied by the input signal, triggers avalanche When a diode has sufficient reverse bias to cause avalanche breakdown of the junction and resultant conduction. There- breakdown, the device appears to display a finite resistance. fore, the entire transistor conducts, switching a large amount of This resistance fluctuates with the amount of reverse voltage, current in a very short time. The extremely sharp "knee" of the and is called the avalanche resistance or avalanche impedance. It reverse-voltage-vs.-current-curve facilitates this switching ca- is given by: pability (see the illustrations in AVALANCHE BREAKDO WN and AVALANCHE IMPEDANCE). A small amount of input ZA = ER/IR voltage can thus cause the switching of large values of current. for direct current, where ZA is the avalanche impedance, ER is the reverse voltage, and IR is the reverse current. Units are ohms, volts, and amperes, respectively. AVALANCHE VOLTAGE When ER is smaller than the avalanche voltage (see AVA- The avalanche voltage of a p-n semiconductor junction is the LANCHE VOLTAGE), the value of ZA is extremely large, be- amount of reverse voltage that is required to cause avalanche cause the current is small, as shown in the figure. When ER breakdown (see AVALANCHE BREAKDO WN). Normally, the exceeds the avalanche voltage, the value of ZA drops sharply. n-type semiconductor is negative with respect to the p-type in As ER is increased further and further, the value of ZA continues forward bias, and the n-type semiconductor is positive, with re- to decrease. spect to the p-type in reverse bias. The magnitude of ZA for alternating current is practically in- In some diodes, the avalanche voltage is very low, as small finite in the reverse direction, as long as the peak ac voltage as 6 volts. In other diodes, it might be hundreds of volts. When never exceeds the avalanche voltage. When the peak ac reverse the avalanche voltage is reached, the current abruptly rises voltage rises to a value greater than the avalanche voltage, the from near zero to a value that depends on the reverse bias (see impedance drops. The maximum voltage that a semiconductor the illustrations in AVALANCHE BREAKDO WN and AVA- diode can tolerate, for rectification purposes, without ava- LANCHE IMPEDANCE). lanche breakdown is called the peak inverse voltage (see PEAK Diodes with high avalanche-voltage ratings are used as rec- INVERSE VOLTAGE). tifiers in dc power supplies. The avalanche voltage for rectifier diodes is called the peak inverse voltage or peak reverse voltage 20 — ZA = (see PEAK INVERSE VOLTAGE). In the design of a dc power 5K supply, diodes with sufficiently high peak-inverse-voltage rat- ings must be chosen so that avalanche breakdown does not occur. Some diodes are deliberately designed so that an effect sim- ilar to avalanche breakdown occurs at a relatively low, and well defined, voltage. These diodes are called Zener diodes (see ZENER DIODE). They are used as voltage-regulating devices in low-voltage dc power supplies. See also DC POWER SUPPLY. ZA AVERAGE CURRENT High ZA = 10M When the current flowing through a conductor is not constant, the average current is determined as the mathematical mean value of the instantaneous current at all points during one com- plete cycle. Most ammeters register average current. Some spe- 50 100 cial devices register peak current. Reverse voltage Consider, for example, a class-B amplifier that has no col- AVALANCHE IMPEDANCE: The avalanche impedance, ZA, de- lector current in the absence of an input signal. Then, under no- pends on reverse voltage. signal conditions, the average current at the collector is zero. .