The Technician Skill tree is for players who lean toward the technological side of the heist - things like drilling into the safe or setting up defenses. These skills focus on fortification. Good skills to unlocked include...
...Demolition Man. This gives you additional Trip Mines. The Trip Mine is unlocked when opening this skill tree, and it's the best perimeter defense you have until you unlock the Sentry Guy.
...Blast Radius. If you've yet to unlock the Sentry Gun, this is the skill to get. It increases the blast radius of your Trip Mines.
...Shaped Charge. This allows you to convert Trip Mines into C4. C4 is the quickest (and loudest) way to breach a door or bust a safe.
...Nerves of Steel. With this, you take less damage when interacting with things. As the hands-on member of the team, you don't want to be gunned down while trying to pick a lock.
...Sentry Gun. This unlocks the Sentry Gun (duh). This is a fantastic upgrade from the Trip Mines, and serves as a fantastic perimeter defense tool.
...Sentry Targeting Package and Sentry Combat Upgrade. These improve the Sentry Gun's accuracy and increases its ammunition, respectively. Both are essential in fortifying your perimeter defenses.
...Sentry Tower Defense. This will give you a second Sentry Gun, literally doubling your defenses.