Handbook for Sound Engineers

332 Chapter 12

Punch-Through. Punch-through is the widening of the
space charge between the collector element and the base
of a transistor. As the potential VCB is increased from a
low to a high value, the collector-base space charge is
widened. This widening effect of the space charge
narrows the effective width of the base. If the diode
space charge does not avalanche before the space charge
spreads to the emitter section, a phenomenon termed
punch-through is encountered, as shown in Fig. 12-29.

The effect is the base disappears as the collector-base

space-charge layer contacts the emitter, creating rela-

tively low resistance between the emitter and the

collector. This causes a sharp rise in the current. The

transistor action then ceases. Because there is no

voltage breakdown in the transistor, it will start func-

tioning again if the voltage is lowered to a value below

where punch-through occurs.

When a transistor is operated in the punch-through

region, its functioning is not normal, and heat is gener-

ated internally that can cause permanent damage to the

transistor.

Breakdown Voltage. Breakdown voltage is that voltage

value between two given elements in a transistor at

which the crystal structure changes and current begins

to increase rapidly. Breakdown voltage may be

measured with the third electrode open, shorted, or

biased in either the forward or reverse direction. A

group of collector characteristics for different values of

base bias are shown in Fig. 12-30. The collector-

to-emitter breakdown voltage increases as the

base-to-emitter bias is decreased from the normal

forward values through zero to reverse. As the resis-

tance in the base-to-emitter circuit decreases, the

collector characteristics develop two breakdown points.

After the initial breakdown, the collector-to-emitter

voltage decreases with an increasing collector current,

until another breakdown occurs at the lower voltage.

Breakdown can be very destructive in power transis-

tors. A breakdown mechanism, termed second break-

down, is an electrical and thermal process in which

Figure 12-28. Internal capacitance of a transistor.

Figure 12-29. Spreading of the space charge between the
emitter and the collector, which creates punch-through.

CCB

CBE

90

60

30

15

6

Capacitance 1 2 5 10 20 50 100

CCB

—pF

90

60

30

15

6

Capacitance 0.1 0.2 0.5 1 2 5 10

C

CB

—pF

Emitter current IE—mA

Collector volts VCB —V

A. Capacitance between terminals.

CCE

IE = 1mA

VCB = 6 V

C. Variation of CCB with emitter current.

B. Variation of CCB with collector voltage.

VEE VCB

E C

B

Space charge

Figure 12-30. Typical collector characteristic curves

showing locations of various breakdown voltages.

Ib >>> 0

Ib > 0

Ib >> 0

Ib = 0

Rbe = 10 7

Vbe = 0

Vbe = 0.5

Rb = 10 7

BVCEO

BVCER

BVCES

BVCEX

Collector-to-emitter voltage

Collector Current