Answer:

1.The more accurate expression for collector current IC  is given by $I_C=\beta I_B+(\beta+1)I_{CBO}$

2.Here,ICBO  is the  current from collector to base when the emitter is not connected.(The 'O' stands for open )

3.ICBO is a strong function of temperature. A rise of $10{^\circ}C$C doubles the ICBO and IC will increase $(\beta+1)$ times of ICBO

4.The flow of IC produces heat within the transistor and raises the transistor temperature further and therefore further increase in ICBO

5.The effect is cumulative and in a few seconds,the IC may become large enough to burn the transistor

6.The self destruction of an unstabilized transistor (BJT) is known as thermal runaway