Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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12.15 Semiconductor Devices • 491

Reverse bias

Breakdown

Voltage, V

Forward bias

Current,

I

+V 0

• V 0 +

+

• 
  


• 
  

  IR

  
  

IF

0

Figure 12.22 The current–voltage

characteristics of ap–njunction for forward

and reverse biases. The phenomenon of

breakdown is also shown.

another, according to

electron+hole→energy (12.21)

Thus for this bias, large numbers of charge carriers flow across the semiconductor

and to the junction, as evidenced by an appreciable current and a low resistivity. The

current–voltage characteristics for forward bias are shown on the right-hand half of

Figure 12.22.

For reverse bias (Figure 12.21c), both holes and electrons, as majority carriers,

are rapidly drawn away from the junction; this separation of positive and negative

charges (or polarization) leaves the junction region relatively free of mobile charge

carriers. Recombination will not occur to any appreciable extent, so that the junction

is now highly insulative. Figure 12.22 also illustrates the current–voltage behavior for

reverse bias.

The rectification process in terms of input voltage and output current is demon-

strated in Figure 12.23. Whereas voltage varies sinusoidally with time (Figure 12.23a),

maximum current flow for reverse bias voltageIRis extremely small in comparison

to that for forward biasIF(Figure 12.23b). Furthermore, correspondence betweenIF

andIRand the imposed maximum voltage (±V 0 ) is noted in Figure 12.22.

At high reverse bias voltages, sometimes on the order of several hundred volts,

large numbers of charge carriers (electrons and holes) are generated. This gives rise

to a very abrupt increase in current, a phenomenon known asbreakdown, also shown

in Figure 12.22 and discussed in more detail in Section 12.22.

The Transistor

Transistors, which are extremely important semiconducting devices in today’s mi-

croelectronic circuitry, are capable of two primary types of function. First, they can

perform the same operation as their vacuum tube precursor, the triode; that is, they

can amplify an electrical signal. In addition, they serve as switching devices in com-

puters for the processing and storage of information. The two major types are the

junction transistor junction(or bimodal)transistorand themetal-oxide-semiconductor field-effect tran-

MOSFET sistor(abbreviated asMOSFET).