The only way to carbonate beer that i know of is either add sugar or force carb (if kegging) as you're bottling, yes you need sugar. If you just bottle and put it in the fridge you'll get cold flat beer. The basics of carbonating are that you add a little bit of pure sugar that the remaining yeast can digest easily and fast to create CO2. Since it is in the bottle and the bottles are capped, the CO2 cannot escape and it fills the head space, then is forced into the remaining liquid. Then after the yeast digest all the sugar and go dormant you have CO2 and beer. The reason you put it in the fridge is because colder temperatures force gases into liquids, so the CO2 the yeast made from the bottling sugar is forced into the liquid over a few days and you end up with carbonated beer. It's actually a pretty interesting little bit of chemistry that occurs. But in short, yes you absolutely need to add sugar at bottling or there will be no CO2 and therefore no carbonated beer.