BJT: a) Why is the collector-base junction normally reverse biased? b) Why is the emitter doped at a higher concentration than the base? c) What happens if the base is doped at a higher concentration than the emitter? d) Why is the base width small compared to the diffusion length of minority carriers in the base? e) As the collector to base reverse bias voltage increases, it is well known that the common emitter current gain changes. Does it go up or down as the reverse bias voltage increases? What is the reason for this? f) In normal mode, what is the majority current type across the base region?
MOSFET: a) Why does an n-channel MOSFET have a p-type substrate? b) What polarity of gate voltage with respect to the substrate is required to obtain conduction between drain and source? What happens if the opposite voltage polarity is applied to the gate? c) After the gate has induced a channel, explain how pinch-off occurs as the drain voltage increases. How is the current affected by this?