What is a diode?

In the following circuit,$PN$-junction diodes $D_1$,$D_2$,and $D_3$ are ideal. For the following potentials of $A$ and $B$,the correct increasing order of equivalent resistance between $A$ and $B$ will be:
$(i) V_A = -10 V, V_B = -5 V$
$(ii) V_A = -5 V, V_B = -10 V$
$(iii) V_A = -4 V, V_B = -12 V$

Which one of the following represents a forward-biased diode?

In a semiconductor $p-n$ diode, the doping concentrations on $p$-side and $n$-side are $10^{15} \text{ atoms/cm}^3$ and $10^{18} \text{ atoms/cm}^3$, respectively. Which one of the following statements is true?

Diffusion current in a $p-n$ junction is greater than the drift current in magnitude when:

The length of a germanium rod is $0.925 \ cm$ and its area of cross-section is $1 \ mm^2$. If for germanium $n_i = 2.5 \times 10^{19} \ m^{-3}$,$\mu_h = 0.19 \ m^2/V\cdot s$,and $\mu_e = 0.39 \ m^2/V\cdot s$,then the resistance of the rod is: