The semiconductor used for fabrication of visible LEDS must at least have a band gap of

In a common emitter amplifier, AC current gains 40 and input resistance is $2 \mathrm{k} \Omega$. The load resistance is given as $10 \mathrm{k} \Omega$. Then, the voltage gain is

The voltage gain and current gain of a transistor amplifier in common emitter configuration are respectively 150 and 50 . If the resistance in the base circuit is $850 \Omega$, then the resistance in collector circuit is

If the energy gap of a substance is 5.4 eV , then the substance is