PhD Defense: "Exploration of GaN based Heterojunction Bipolar Transistors"

Ajay Raman

June 18th (Tuesday), 3:00pm
Engineering Science Building (ESB), Room 2001


This defense seminar presents work on exploration of GaN based Heterojunction Bipolar Transistors (HBTs). Ga-polar AlGaN/GaN HBTs grown by MOCVD with regrown emitters were fabricated. The HBTs had a high current density (> 1 kA/cm2) and a peak current gain of 30. However, the fabrication of these devices revealed a problem of effectively activating the p-type GaN base layer buried underneath the emitter layer. Ammonia Molecular Beam Epitaxy (NH3 MBE) is attractive as an alternate method for the growth of GaN HBTs and other vertical devices. Chief benefits include sharp Mg concentration profiles, absence of Ga-filled dislocations that can cause vertical leakage currents, and active p-type layers. AlGaN/GaN HBTs grown by NH3 MBE are demonstrated. To obtain good ohmic contacts to the base layer, devices were fabricated with two different processes involving regrowths – Regrown base contact (Wafer A) and Regrown emitter (Wafer B). Common Emitter (CE) operation with a peak collector current density, JC > 1 kA/cm2 was obtained from devices on both wafers. In CE operation, a current gain (β) of 1.5-3 is observed in devices on both wafers. High base resistance is found to limit the performance of the devices. Maximum β values observed from Gummel plots at a fixed Base-Collector bias of zero volts, are β ~ 15 at JC = 0.88 kA/cm2 for Wafer A and β ~ 13.5 at JC = 3 kA/cm2 for Wafer B.

In devices on both wafers, variation is observed in the plots of β vs. IC (Collector current) for devices with the same emitter dimensions and geometry but with different lateral spacing between emitter mesa and base contacts (WEB). An explanation is provided for the observed behaviour.

InGaN layers doped with Mg are a better option compared to GaN as the base layer of the HBT. Due to higher ionization efficiency of Mg in InGaN than GaN, higher hole concentrations can be obtained in p-type InGaN layers. GaN/InGaN/GaN HBTs with CE operation and good collector current densities are demonstrated. Recombination of electrons at the emitter sidewalls and extrinsic base surface due to ion etch-damage is found to lower the current gain. A photoelectrochemical (PEC) etch in KOH solution under UV illumination is found to be promising in reducing sidewall recombination and enhancing current gain.

High hole concentrations can be obtained in graded AlGaN or InGaN layers doped with Mg by the mechanism of polarization-induced doping. N-polar GaN enables design of HBTs with graded base layers that can have high hole concentrations from polarization doping as well as quasi-electric fields that propel electrons from emitter to collector enhancing current gain. N-polar HBTs were constructed using both MOCVD and NH3 MBE growth techniques. The device results are presented. Problems in the device behavior and possible solutions are discussed.

Hosted by: Professor Umesh Mishra