B737 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 175; Flight Crew Total 244; Flight Crew Type 244
Communication Breakdown; Confusion; Situational Awareness; Human-Machine Interface
ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT; Inflight Event / Encounter Unstabilized Approach
Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors; Procedure
While on vectors to final; we were cleared for the visual. Vector was going to put us almost on top of FAF for ILS XXR at an excessive angle and higher than desired altitude. Last assigned altitude prior to 'Cleared for the visual' was at 9;000 ft. First Officer called for 7;000 ft. Upon reaching 7;000 ft; the Controller announced 'Altitude alert; below minimum vectoring altitude; you are below Class Bravo; minimum vectoring altitude is 7;500 ft. ' We climbed to 7;500 ft due to caution.If we would have stayed at 9;000 ft on the vector; we would have been 2;000 ft high at the Final Approach Fix. Aircraft would have captured Localizer but would have been too high to capture glide slope. We had the aircraft configured with flaps 1; speed 210 with level change selected. After caution we quickly climbed 500; which resulted in a higher than desired altitude as we crossed FAF. Attached pictures show approximate position in red.
Pilot reported being alerted by ATC after First Officer descended 500 feet below minimum approach altitude and below Class Bravo airspace. First Officer climbed 500 feet and continued the approach.