[‚kwän·təm ¦wel in‚jek·shən ¦tranz·it ‚tīm ′dī‚ōd] (electronics) An active microwave diode that employs resonant tunneling through a gallium arsenide quantum well located between two aluminum gallium arsenide barriers to inject electrons into an undoped gallium arsenide drift region. Abbreviated QWITT diode.