Why can't we take one slab of p-type semiconductor and physically join it to another slab of n-type semiconductor to get a p-n junction?

The \(V\text-I\) characteristic of a silicon diode is shown in the figure. The resistance of the diode at \(I_D = 15~\text{mA}\)$$ is: 

 
1. \(20~\Omega\)
2. \(30~\Omega\)
3. \(15~\Omega\)
4. \(10~\Omega\)

The \((V\text-I)\) characteristic of a silicon diode is shown in the figure. The resistance of the diode at \(V_D=-10\) V is:${\mathrm{}}_{}$

     
1. \(1\times10^7~\Omega~\)
2. \(2\times10^7~\Omega~\)
3. \(3\times10^7~\Omega~\)
4. \(4\times10^7~\Omega~\)$\mathrm{}$