Assertion (A) : Putting p-type semiconductor slabs directly in physical contact can not form `p-n` junction
Reason ( R) : The roughness at contact will be much more than inter atomic crystal spacing `(= 2 Å to 3 Å)` and continuous flow of charge carriers is not possible.

Answer

To analyze the assertion and reason provided in the question, we will break down the concepts involved step by step.

### Step 1: Understanding P-type and N-type Semiconductors
P-type semiconductors are materials that have been doped with elements that create "holes" or positive charge carriers, while N-type semiconductors are doped with elements that provide extra electrons, creating negative charge carriers.

**Hint:** Remember that P-type has more holes (positive carriers) and N-type has more electrons (negative carriers).

### Step 2: Concept of P-N Junction
A P-N junction is formed when P-type and N-type semiconductors are brought into contact. This junction is crucial for the operation of many semiconductor devices, including diodes and transistors, as it allows for the movement of charge carriers across the junction.

**Hint:** Recall that a P-N junction allows for the flow of charge carriers, which is essential for device functionality.

### Step 3: Analyzing the Assertion
The assertion states that putting P-type semiconductor slabs directly in physical contact cannot form a P-N junction. This is true because a P-N junction requires the presence of both P-type and N-type materials. If only P-type materials are in contact, there will be no N-type material to create the junction.

**Hint:** Consider what materials are needed to form a junction and what happens when only one type is present.

### Step 4: Analyzing the Reason
The reason given is that the roughness at the contact will be much more than the interatomic crystal spacing (which is about 2 Å to 3 Å). This roughness can prevent a continuous flow of charge carriers, which is necessary for the formation of a P-N junction. If the surfaces are too rough, they cannot effectively interact at the atomic level to allow charge carrier movement.

**Hint:** Think about how surface smoothness affects atomic interactions and charge flow.

### Step 5: Conclusion
Both the assertion and the reason are true. The assertion is correct because a P-N junction cannot be formed with only P-type materials. The reason is also valid, as rough surfaces can hinder the necessary interactions for charge carrier flow.

**Hint:** Evaluate the relationship between the assertion and reason to determine if they support each other.

### Final Answer
Both the assertion (A) and the reason (R) are true, and the reason correctly explains the assertion.