Corporate jet Captain reported receiving a low altitude alert from SDL Tower on their visual approach to Runway 21.

Narrative

During our flight to SDL; I was the Captain and pilot monitoring. My first officer was the pilot flying this leg. We were approaching SDL from the northwest when ATC issued us the visual approach out of 6000 ft considerably close to the airport. The PF elected to ask for 3000 ft to be set in the altitude preselector in order to prepare for descent to pattern altitude. During this time he was also setting up for his maneuvering from present position to the right base leg for SDL RWY 21 that we were instructed to make. During this time I was busy helping configure the aircraft; run checklists; answer the radio calls and scan for traffic. I had neglected to monitor how quickly the PF was coming down to 3000 ft. The PF had apparently elected to descend rapidly to 3000 ft instead of gradually losing altitude during the visual approach as I had anticipated. During this time; he had maneuvered visually to be close to the terrain off of the approach end of RWY 21. Before I was able to give him corrective commands; SDL TWR issued a low altitude alert for our aircraft. This prompted the PF to gain a couple hundred of feet of altitude until we were visually further from the mountains and the terrain below us lowered some on approach to SDL. Due to this low position; he had elected to fly a slightly low approach angle to SDL 21. This approach was lower than the 4 degree VGSI for RWY 21; however it did not create a directly unsafe situation due to our visual avoidance with all obstacles. After informing him that he was low and needed to correct upwards; he was able to comply with my instruction and effect a safe landing in SDL.