GaN HBT: 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 MOCVD GaN templates on sapphire. HBTs were fabricated on LEO material identifying threading dislocations as the primary source of collector-emitter leakage which was reduced by four orders of magnitude for devices on nondislocated material. Base doping studies show that the mechanism of this leakage is localized punch-through caused by compensation near the dislocation. High contact and lateral resistance in the base cause large parasitic common emitter offset voltages (from 1 to 5 V) in GaN HBTs. The effect of this voltage drop on common emitter characteristics is discussed. The combination of this voltage drop and the emitter collector leakage make Gummel and common base characteristics unreliable without verification with common emitter characteristics. The selectively regrown emitter bipolar transistor is presented with a DC current gain of 6 and early voltage greater than 400 V. The transistor operated to voltages over 70 V. This device design reduces base contact resistance, and circumvented difficulties associated with the emitter mesa etch process. The Mg memory effect in MOCVD grown GaN HBTs is discussed, and MBE grown device layers are shown to produce sharp doping profiles. The low current gain of these devices is discussed, and an HBT with a compositionally graded base is presented, as well as simulations predicting further current gain improvements with base grading.