In an NPN transistor $10^{10}$ electrons enter the emitter in $10^{-6} \mathrm{~s}$ and $2 \%$ electrons recombine with holes in base. The current ratios ' $\alpha$ ' and ' $\beta$ ' of a transistor are respectively (nearly)

The output of following combination is same as that of

In an n-p-n transistor, the collector current is $$28 \mathrm{~mA}$$. If $$80 \%$$ of electrons reach the collector, its base current in $$\mathrm{mA}$$ is

When the conductivity of a semiconductor is only due to the breaking of the covalent bonds, the semiconductor is called