Explanation

(b) In a p-n junction diode, electrons in conduction band on $n$-type side travel across the junction and leave the positively ionized impurity atoms unneutralised. Consequently, there is positively charged region adjacent to the junction in $n$-type material. On $p$-type side the electrons which have traversed the boundary recombine with positive holes in the valence
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band and form a layer of unneutralised negatively ionised trivalent impurity atoms making a negatively charged region as shown in figure. The region around the junction is called charge depletion region or space charge region. Hence, within the depletion region, $p$-side is negative and $n$-side is positive.