GaNHBT: Toward an RF device

This paper reviews efforts to develop growth and fabrication technology for the GaN HBT. Conventional devices are grown by plasma assisted MBE on MOCV D GaN templates on sapphire, HBTs were fabricated on LEO material identifyi ng threading dislocations as the primary source of collector-emitter leakag e which was reduced by four orders of magnitude for devices on nondislocate d material. Base doping studies show that the mechanism of this leakage is localized punch-through caused by compensation near the dislocation, High c ontact and lateral resistance in the base cause large parasitic common emit ter offset voltages (from 1 to 5 V) in GaN HBTs, The effect of this voltage drop in common emitter characteristics is discussed. The combination of th is voltage drop and the emitter collector leakage make Gummel and common ba se characteristics unreliable,without verification with common emitter char acteristics. The selectively regrown emitter bipolar transistor is presente d with a de current gain of 6 and early voltage greater than 400 V, The tra nsistor operated to voltages over 70 V, This device design reduces base con tact resistance, and circumvented difficulties associated with the emitter mesa etch process. The Mg memory effect in MOCVD grown GaN HBTs is discusse d, and MBE grown device layers are shown to produce sharp doping profiles. The low current gain of these devices, (3-6) is discussed, and an HBT,vith a compositionally graded base is presented, as well as simulations predicti ng further current gain improvements with base grading.