bei48482_FM

The Solid State 367

Two mechanisms contribute to the sharp rise in current. One, called avalanche mul-

tiplication,occurs when an electron near the junction is sufficiently accelerated by the

electric field to ionize atoms it collides with, thereby creating fresh electron-hole pairs.

The new electrons in their turn continue the process to produce a flood of charge

carriers in the diode.

The other mechanism, called Zener breakdown,involves the tunneling of valence-

band electrons on the pside of the junction to the conduction band on the nside even

though these electrons do not have enough energy to first enter the conduction band

on the pside. (Such tunneling is in the opposite direction to that occurring in a tunnel

diode.) Zener breakdown can occur in heavily doped diodes at voltages of 6 V or less.

In lightly doped diodes the necessary voltage is higher, and avalanche multiplication

is then the chief process involved.

Junction Transistor

A transistoris a semiconductor device that can amplify a weak signal into a strong

one when appropriately connected. Figure 10.36 shows an n-p-njunction transistor,

which consists of a thin p-type region called the basethat is sandwiched between two

n-type regions called the emitterand the collector.(A p-n-ptransistor behaves in a

similar manner, except that the current then is carried by holes rather than by elec-

trons.) The energy-band structure of an n-p-ntransistor is given in Fig. 10.37.

The transistor is given a forward bias across the emitter-base junction and a reverse

bias across the base-collector junction. The emitter is more heavily doped than the

base, so nearly all the current across the emitter-base junction consists of electrons

moving from left to right. Because the base is very thin (1 m or so) and the concen-

tration of holes there is low, most of the electrons entering the base diffuse through it

to the base-collector junction where the high positive potential attracts them into the

collector. Changes in the input-circuit current are thus mirrored by changes in the

output-circuit current, which is only a few percent smaller.

The ability of the transistor of Fig. 10.36 to produce amplification comes about because

the reverse bias across the base-collector junction permits a much higher voltage in the

output circuit than that in the input circuit. Since electric power (current)(voltage),

the power of the output signal can greatly exceed the power of the input signal.

Field-Effect Transistor

Although its advent revolutionized electronics, the low input impedance of the

junction transistor is a handicap in certain applications. In addition, it is difficult

Input

signal

Emitter Base Collector

• 
  
  
  
  • – +
  
  
  
  

Output

load

nnp

Figure 10.36A simple junction-transistor amplifier.

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