GPWS. TERRAIN CLOSURE DUE TO INAPPROPRIATE SELECTION OF ALT KNOB; THINKING IT WAS THE VERT SPD KNOB. CONSEQUENTLY THE ALT SELECTED WAS TOO LOW FOR TERRAIN CLRNC.

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TOWARD THE TERMINATION OF A 5 HR EMPTY FERRY FLT FROM BOSTON TO PHOENIX IN AN AIRBUS A320; WE RECEIVED CLRNC TO 10000 FT MSL FROM THE PHX APCH CTLR. WITHIN 35 NM FROM PHX; WE ACCEPTED THE VISUAL APCH TO RWY 26R. BEING VERY FAMILIAR WITH THE ARPT AND ITS ENVIRONMENTS; I SET THE ALT LIMIT IN THE FLT CTL UNIT TO 6000 FT; INTENDING TO LEVEL AT 6000 FT TO BEGIN FLAP EXTENSION AND SPD REDUCTION. I KNEW THIS ALT WOULD PROVIDE SUITABLE TERRAIN CLRNC IN THE AREA; AS THE MINIMUM VECTOR ALT WAS 4700 FT. DURING THE DSCNT IN 'OPEN DSCNT/IDLE' MODE; OUR DSCNT RATE SEEMED EXCESSIVE (UNCOMFORTABLE) AND I ELECTED TO SWITCH TO THE VERT SPD MODE TO EFFECT THE REMAINDER OF THE DSCNT TO 6000 FT. I DID THIS; AND VERIFIED ON THE FLT MODE ANNUNCIATOR THAT INDEED WE WERE IN VERT SPD MODE. BEING NEW TO THIS ACFT; I HAVE BEEN ESPECIALLY CAREFUL TO DOUBLE-CHK MY ACTIONS AS IT IS VERY DIFFERENT FROM THE BOEING SERIES (757) THAT I PREVIOUSLY FLEW. HOWEVER; THIS DID NOT PREVENT MY ERROR SOMEWHERE ALONG THE DSCNT. APPARENTLY; DURING THE ATTEMPT TO ADJUST THE VERT SPD SELECTOR KNOB TO MODULATE OUR DSCNT; (WHICH AT THIS TIME WE WERE EXTENDING FLAPS AND PREPARING TO TURN FINAL); I MUST HAVE GRABBED THE ALT SELECTOR KNOB LOCATED ON THE FLT CTL UNIT JUST TO THE L OF THE VERT SPD KNOB. AS I DESIRED A GREATER DSCNT RATE; I TURNED IT DOWN WHILE LOOKING OUTSIDE FOR THE RWY ALIGNMENT/TFC. NOT REALIZING THAT I DID NOT EFFECT A CHANGE IN DSCNT RATE; BUT RATHER A LOWER ALT IN THE DSCNT; I AGAIN REACHED TO THE VERT SPD KNOB AND SELECTED A GREATER DSCNT RATE. IN RETROSPECT; I SELECTED A HIGH DSCNT RATE (2ND TRY) TO A LOW ALT OF 3800 FT MSL (1ST TRY). ALTHOUGH WE WERE LOW; I HAD THE RWY IN SIGHT AND WAS FLYING 'HEAD OUTSIDE;' AS WAS MY FO. WE RECEIVED A GPWS TERRAIN CLOSURE RATE WARNING JUST AS ATC ADVISED WE WERE TOO LOW IN A HIGHER TERRAIN SECTOR. WE EFFECTED AN IMMEDIATE CLB FROM APPROX 3900 FT TO 4500 FT WHEN THE WARNING CANCELED AND TERRAIN SLOPE TOWARD THE ARPT INCREASED. NORMAL LNDG WAS COMPLETED. ALTHOUGH IT'S EASY TO MAKE EXCUSES FOR ONE'S OBVIOUS MISTAKES; I BELIEVE A COMBINATION OF DIFFERENT OPERATIONAL TECHNIQUES/PROCS BTWN THE B757/A320 AND THE LATE NIGHT VISUAL APCH ACCEPTANCE CERTAINLY CONTRIBUTED TO MY ERRORS. IT HAS HOWEVER; DRIVEN HOME THE IMPORTANCE IN THE A320 TO NOT ONLY CHK FLT MODE ANNUNCIATOR INDICATIONS FOR MODE; BUT ALSO TO CHK WHAT YOUR OWN HAND IS TURNING ON THE FLT CTL UNIT; ESPECIALLY IN VFR; HEAD-UP AND OUTSIDE FLT CONDITIONS. I HOPE OTHERS WILL BENEFIT FROM THIS INCIDENT.