Question

When maximum reverse-biasing voltage is applied between the collector and base terminals of the transistor and emitter is open circuited, breakdown occurs due to
1. Avalanche breakdown 
2. Avalanche multiplication 
3. Punch-through 
4. Reach- through 

Answer 1

Correct Answer - Option 2 : Avalanche multiplication 

Avalanche multiplication:

• The maximum reverse biasing voltage which may be applied before breakdown between the collector and base terminals of the transistor, under the condition of the emitter lead be open-circuited, is represented by the symbol BV_CBO.
• This breakdown voltage is a characteristic of the transistor alone. Breakdown may occur because of the avalanche multiplication of the current I_CO that crosses the collector junction.
• As a result of this multiplication, the current becomes MI_CO in which M is the factor by which the original current I_CO is multiplied by the avalanche effect.
• At a high enough voltage, namely, BV_CBO, the multiplication factor M becomes nominally infinite, and the region of breakdown is then attained.
• Here the current rises abruptly, and large changes in current accompany small changes in applied voltage.
• The avalanche multiplication factor depends on the voltage V_CB between the collector and base.

Reach-through:

• The mechanism by which a transistor’s usefulness may be terminated as the collector voltage is increased is called punch-through, or reach-through, and results from the increased width of the collector junction transistor region with increased collector junction voltage (the early effect).
• Punch-through differs from avalanche breakdown in that it takes place at a fixed voltage between collector and base and is not dependent on circuit configuration.
• In a particular transistor, the voltage limit is determined by punch-through or breakdown, whichever occurs at the lower voltage