Why does the width of the depletion region decrease,when the diode is forward biased?Is it because the ions have moved to p-junction,or n-junction? Please clarify.
First of all note, that ions do not move in the semiconductor. Only electrons and holes move there.
The depletion results from diffusion of the free electrons from the n-region into the p-region and the diffusion of holes from the p-region into the n-region.
In either case it is a thermodynamical process. The electrons form something like a dense gas in the n-region and the p-region is like an electron-vacuum. If the density of electrons at the pn-junction would be higher in the n-region than in the p-region then Brown's motion would drive more electrons from the n-region to the p-region than from the p-region to the n-region. Therefore, in the thermodynamical equilibrium there must be some mean density at the junction. The electrostatic field force counteracts this.
If electrons are moving from the n-domain into the p-domain and holes from the p-domain into the n-domain the n-domain becomes positively charged and the p-domain negatively. These space-charges create an electrical field which drives electrons back into the n-region and the holes back into the p-region.
The forward bias-voltage supports the field-force driving the electrons and holes back. Thus the depletion zone decreases.