PhD Defense: "Growth, fabrication and characterization of III-Nitride Hot Electron Transistors"

Sansaptak Dasgupta

August 19th (Friday), 3:00pm
Elings Hall (CNSI), Room 1605

A hot electron transistor (HET) is a vertical device similar in design like the heterojunction bipolar transistor (HBT) . However, unlike the HBT, the hot electron transistor is unipolar in nature. In a hot electron transistor, a large band-gap material is used as the emitter to inject carriers (electrons) at high velocities which then travel through a thin base layer and are collected in the collector contact. The base thickness in the HET is nominally low, of the order of the mean free path of the injected electrons, to minimize scattering of the injected electrons and thereby enabling large current gain in the device.

The III-nitride system offers several unique material properties advantageous to the design of the HET. Using Al(Ga)N alloys as the large band-gap material and GaN, large conduction band discontinuities are possible, enabling the efficient design of both base-emitter and base-collector barriers. The presence of polarization induced charges is another advantage and can be utilized to reduce base and emitter contact resistances. Low base and emitter resistances are critical for the reduction of parasitic delays to enable high frequency performance of the HET.

Based on the above design considerations, III-nitride HETs using GaN as the base layer were developed. High current gain (β > 120) and near unity transfer ratio (α > 0.99) were obtained for these HETs at room temperature. Insertion of current blocking layers, and emitter and base-contact regrowths were also implemented to improve the device characteristics. Finally the HET was used to experimentally determine the mean free path and relaxation time of the injected electrons. For injection energies far from the higher valley (A valley in our case) a good fit to the theoretical prediction for relaxation time in GaN, based on the LO-phonon interaction, was obtained.

Hosted by: Professor Umesh Mishra

Hosted by: Professor Umesh Mishra