Transistores

LABORATORY WRITE-UP

TRANSISTOR CHARACTERISTICS

AUTHOR’S NAME GOES HERE

STUDENT NUMBER: 111-22-3333

TRANSISTOR CHARACTERISTICS

1. PURPOSE

The transistor ranks as one of the greatest inventions of 20th century technology. It finds application in virtually all electronic devices from radios to computers. Integrated circuits typically contain millions of transistors, formed on a single tiny chip of silicon. Two of the basic uses of a transistor, which will be explored in this experiment, are as an amplifier and as a switch.

Fig. 1 a) The pnp transistor b) Circuit symbol c) Common emitter amplifier circuit

The pnp transistor, shown in Fig. 1a) contains three distinct regions, a p-type "emitter", an n-type "base" and a p-type "collector", which together form two pn junctions. In a typical amplifier circuit, voltages are supplied so that the emitter-base junction is forward-biased and the collector-base junction is reverse-biased. This means that VCE > VBE. Fig. 1c illustrates a "common emitter" circuit, so called because the emitter is common to the input circuit on the left and the output circuit on the right.

Consider first the forward-biased emitter-base junction. The doping of the emitter is made much heavier than that of the base so that positive holes from the emitter form almost all of the current, IE, from emitter to base. The base, being lightly doped, does not have many electrons available for recombination with these holes to form neutral atoms. It is also very narrow (< 1 m) making it easy for a large fraction, , of the holes to diffuse across to the collector-base junction where the junction voltage accelerates them into the collector region to form the collector current, IC.

Thus, IC = IE .......................(1)

The remaining fraction, (1-), of holes leave the base through the external connection to form the base current, IB, where

IB = (1-)IE ...................(2)

The "current gain", , of the transistor is defined by

= IC/IB .....................(3)

= /(1-) .

For typical transistors, ~ 0.9 to 0.995, giving values of  ~ 10 to 200. Thus we have a "current amplifier", in that a small change in IB will cause a large change in IC. The "voltage gain", AV, is the ratio of the voltage drop, ICRC, across the output resistor, RC, to the voltage, VBB, of the input source:

AV = ICRC/VBB.

Applying the loop theorem to the input circuit in Fig. 1c), and assuming IE ~ IC = IB, it is easy to show that

AV = RC/(RB+rb)………………….4 .

where rb is the resistance of the emitter-base junction. By a suitable choice of resistors, an appreciable voltage amplification may be obtained.

Fig. 2 a) The AC amplifier

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