Derby 2-3 Southampton (4-4 on agg)
Derby win 4-3 on penalties
Idiakez blasts his effort high over the bar to send Derby to Wembley
Inigo Idiakez's penalty miss sent Derby into the Championship play-off final after a pulsating semi-final against Southampton ended in a shoot-out.
Grzegorz Rasiak's late goal gave Saints a 3-2 lead and took the game into extra time with the scores 4-4 on aggregate.
Darren Moore's powerful header gave Derby an early lead, only for Jhon Viafara to level within a minute.
In a match of swinging fortunes Viafara thumped in another and Leon Best scored in his own net before Rasiak struck.
The Rams will now face either West Brom or Wolves in the Wembley final on Monday 28 May.
Saints, who lost the first leg at home 2-1, travelled with a makeshift defence which was tested immediately by their hosts in the opening salvos.
And it was soon breached by Moore's header from a Matt Oakley corner to give Derby the ideal start on three minutes.
However, while Pride Park was still celebrating, Moore's heavy header back to Bywater forced the keeper into a weak clearing header of his own which was sidefooted in by Viafara from 30 yards.
That explosive start set the tempo for a cut-and-thrust match and there was the promise of more goals as visiting keeper Kelvin Davis looked far from convincing.
But Southampton's defence, which included midfielder Rudi Skacel at left-back, fit-again Chris Makin and the returning Martin Cranie, stood firm.
And as the visitors grew into the game they were rewarded with a second goal from Viafara on 54 minutes.
Derby's Dean Leacock misjudged a long ball forward which Best expertly controlled before swivelling to release Viafara on the right, the Colombian rounding off the move with a rasping, rising shot beyond Bywater.
Rasiak celebrates his late goal which sent the tie into extra time
Best wasted a glorious chance at the right end, before his ungainly attempted clearance of Seth Johnson's corner at the wrong end skimmed off his shin and beyond Davis.
Lively Saints substitute Djamel Belmadi had a shot deflected wide by Moore, and just as Derby fans began to dream of Wembley former Rams striker Rasiak lashed in, after a forceful run from Best, to send the match into extra time.
The hosts forced the early pace in the ensuing 30 minutes with Saints skipper Chris Baird clearing off his line and Steve Howard and Giles Barnes heading wide of the target.
While Best shot weakly as time ticked down to a shoot-out, Rasiak saw an audacious low effort arc wide and Belmadi had another effort blocked by Moore.
Saints had never won at Pride Park before, but their efforts counted for nothing in the shoot-out.
Best fired their first spot-kick wide, and with Dave Jones, Howard, Barnes and Jay McEveley all scoring for Derby, the miss by Idiakez, another County old boy, decided the tie 4-3 on penalties.
Derby boss Billy Davies on going to Wembley:
"It will be an unbelievable day out and we will go with great belief we can go all the way.
"It was a great game of football, real end-to-end stuff, and in added time I thought we really performed very well.
"We were the only team who looked like winning it in extra time, but in the end we needed four well-taken penalties."
Southampton manager George Burley:
"Losing on penalties is always a tough situation but we have got to take the disappointment on the chin.
"I was very, very pleased with the way we played, I do not think I could have expected any more from my players.
"We worked our socks off and both the goals we conceded came from dead-ball situations, so you could argue Derby failed to score from open play."
Derby: Bywater, Mears, Moore, Leacock, McEveley, Fagan (Currie 105), Oakley, Seth Johnson (Jones 86), Pearson, Macken (Barnes 62), Howard.
Subs Not Used: Camp, Edworthy.
Booked: Seth Johnson.
Goals: Moore 3, Best 66 og.
Southampton: Davis, Makin (Belmadi 71), Baird, Cranie, Skacel, Viafara, Pele, Guthrie (Idiakez 83), Surman, Best, Saganowski (Rasiak 74).
Subs Not Used: Bialkowski, Wright-Phillips.
Goals: Viafara 4, 54, Rasiak 89.
Agg: 4-4 - Derby win 4-3 on penalties
Ref: A D'Urso (Essex).