The built-in potential (Diffusion Potential) in a p-n junction

The 6 V Zener diode shown in figure has zero Zener resistance and a knee current of 5 mA. The minimum value of R, so that the voltage across it does not fall below 6 V is:

Referring to the figure. The switch S is in position 1 initially and steady state conditions exist from time t = 0 to t = t₀. At t = t₀, the switch is suddenly thrown into position 2. The current I through the 10K resistor as a function of time t, from t = 0 is (in mA) _____________.

(Give the sketch showing the magnitudes of the current at t = 0, t = t₀ and t = $$\infty$$ )

The small signal capacitances of an abrupt P⁺−N junction is 1 nF at zero bias. If the built-in voltage is 1 volt, the capacitance at a reverse bias voltage of 99 volts is equal to