The $I-V$ characteristics of three types of diodes at the room temperature, made of semiconductors $X, Y$ and $Z$, are shown in the figure. Assume that the diodes are uniformly doped and identical in all respects except their materials. If $E_{gX}$,$E_{gY}$ and $E_{gZ}$ are the band gaps of $X, Y$ and $Z$, respectively, then
- $E_{gX}>E_{gY}>E_{gZ}$
- $E_{gX}=E_{gY}=E_{gZ}$
- $E_{gX}<E_{gY}<E_{gZ}$
- no relationship among these band gaps exists.