Question

Which of the following Band alignment graph represents Broken-gap heterojunction? 

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

To instantly spot a Broken-gap (Type III) junction, draw a horizontal line originating from the highest valence band. If it goes cleanly over the top of the adjacent material's conduction band, the gap is "broken". InSb/InAs is a classic real-world example of this unique alignment.

Answer 1

The Correct Option is C

Step 1: Concept:
The question asks us to identify a specific type of semiconductor heterojunction band alignment from four provided energy band diagrams. Heterojunctions are classified into three types based on how the conduction ($E_c$) and valence ($E_v$) bands of the two materials align at the interface.

Step 2: Key Classifications of Heterojunctions:
- Type I (Straddling Gap): The bandgap of one material is completely contained within the bandgap of the other. The $E_c$ of material 2 is lower than material 1, and the $E_v$ of material 2 is higher than material 1.
- Type II (Staggered Gap): The bandgaps partially overlap. The $E_c$ and $E_v$ of material 2 are either both higher or both lower than those of material 1.
- Type III (Broken Gap): The bandgaps do not overlap at all. The conduction band edge ($E_c$) of one material drops energetically below the valence band edge ($E_v$) of the adjoining material.

Step 3: Step-by-step Explanation:
Let's analyze the provided graphical diagrams based on these definitions:

• Graph (1): The right material has a narrower bandgap that sits entirely within the wider bandgap of the left material. This represents a Type I (Straddling) alignment.

• Graph (2): The right material's bands are shifted upward relative to the left, but their bandgaps still share a region of overlapping energy. This represents a Type II (Staggered) alignment.

• Graph (3): The conduction band ($E_c$) of the right material is positioned at a lower energy level than the valence band ($E_v$) of the left material. There is zero energy overlap between the two bandgaps. This drastic misalignment represents a Type III (Broken-gap) heterojunction.

• Graph (4): Shows the $E_c$ aligned identically while the $E_v$ differs, which is a specific boundary case of Type I or II, but definitely not a broken gap.

Step 4: Final Answer:
Graph (3) accurately depicts the broken-gap alignment, making option (C) correct.