| We have successfully fabricated the first heterojunction bipolar transistor in the gallium nitride and silicon carbide material system. These high-temperature, high-power transistors combine a wide bandgap GaN emitter, 3.4 eV, with a 6H-SiC base and collector, 3.0 eV. The large difference in bandgap between these two materials provides for a high emitter efficiency along with a high current gain. Furthermore, SiC is a preferred material for high-power, high temperature semiconductor devices, primarily because of its large energy-bandgap and high thermal conductivity, 5 W/(cm-{dollar}spcircrm C).{dollar}; Currently, most transistors are fabricated in the silicon material system. Unfortunately, silicon has a low energy bandgap, 1.12 eV, and low thermal conductivity, 1.46 W/(cm-{dollar}spcircrm C),{dollar} which limits its current gain to 150 and temperature operation to below {dollar}300spcircrm C.{dollar}; Device characteristics of the GaN/SiC HBT have been measured. The GaN/SiC transistor has achieved a world's record current gain of 100,000,000 at room temperature and a current gain of 100 at {dollar}535spcircrm C.{dollar} The I-V characteristics changed with increasing ambient temperature due to an increase of leakage current, caused by an increase in the intrinsic carrier density with increasing temperature. Common emitter characteristics were not achievable due to the high gain of the devices and the leakage current between base and collector. Common emitter characteristics have been obtained on devices with lower gain and devices where the gain had degraded. A preliminary measurement of the frequency response of these transistors yield a {dollar}-{dollar}3 dB frequency slightly over 800 kHz. The speed of the device is currently limited by high capacitance and high contact resistance.; The maximum applied emitter current was 100 mA for a 75 {dollar}murm m{dollar} x 75 {dollar}murm m{dollar} device providing a current density of 1777 {dollar}rm A/cmsp2.{dollar} However, the maximum current density is 3,750 {dollar}rm A/cmsp2,{dollar} which occurred by applying 60 mA to a 40 {dollar}murm m{dollar} by 40 {dollar}murm m{dollar} emitter contact. The maximum power density achieved was 35 {dollar}rm kW/cmsp2.{dollar} From Gummel plot measurements, a room temperature current gain over {dollar}10sp8{dollar} has been obtained. The devices have been measured in ambient temperatures up to {dollar}555spcircrm C.{dollar}; In addition to the experimental results, I shall also present the basic theoretical background for HBTs, the development of our process fabrication procedure, and a detailed discussion of our results. |
