Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB High-Field Transport Modeling for Compact Power Sources K. W. Kim, J.-B. Jeon & S.M. Komirenko North Carolina State University Raleigh, NC ONR MURI Review February 12, 2002

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Implement the feature of arbitrary momentum of the injected carrier into the RAE simulator. Improve the RAE simulator by account for the non- parabolicity effects. Obtain the distribution function for low-field runaway regime and investigate its features. Start preliminary optimization of the parameters of HET collector specified by Dr. Asbeck. Current Objectives

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Low-field runaway:possible scenarios Energy  stable unstable Injection energy bias ** At field  > ** Electron injected with any energy will runaway saturation (stable) runaway (unstable) E kin Ex

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB E inj. < Ex pzpz   V dr is 14 % higher than that for carriers accelerated from the bottom of CB For 0<w<0.05  m; Due to LFRAE, the distribution function is broader => velocity is, in general, higher until the distance required for acceleration of hot carriers to the upper valley.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB E inj. > Ex V dr is higher and has a minimum. The velocity RAE at low field is detected. Due to LFRAE, the distribution function has two maxima, i.e. there ate two distinct groups of carriers with different velocities. Non-parabolicity helps to keep hot carriers in the  valley for more than 100 nm   pzpz

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB HET modeling emitter base collector High-field collector region RAE or LFRAE W*W* W

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Parameter definition (E in kV/cm)

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB RAE from barrier 1.8 V 18.2 V   pzpz

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB RAE of termalized electrons

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Upper valley impact 250 Field (0<w<w*) 283 kV/cm Capture to the upper band in a RAE regime and current reduction has to be taken into account, especially for the fields about 300 kV/cm

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Conclusions Using collector design parameters provided recently by Dr. Asbeck it is preliminary estimated that cut-off frequency in a sub-THz frequency range can be achieved in a nanoscale-range collector, however, keeping V c =20 V, Ic=4mA, R b ~209, and C be ~5fF, f t ~ f max can be satisfied only for sub-micron scale collector with relatively long high-field region. RC-delay needs to be optimized. Since T v in the low-field region is weighted, the LFRAE can be utilized for improvement of this parameter for 0<w<w*. Because high non-parabolicity is a favorable condition for an increase of w*, it is reasonable to investigate the potential of In-contained multinary compounds for the collector fabrication. The unique 2-beam character of LFRAE transport has to be considered for the novel nanoscale device applications: tunable high-frequency generation? utilization of threshold character of high-velocity beam for signaling?

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Accomplishments The RAE simulator is updated to include into consideration arbitrary momentum of the injected carriers. Carrier distribution in the LFRAE obtained for the first time. The LFRAE manifested two-beam behavior with different carrier velocity in each beam, non-termalized asymptotic, and a minimum on the dependence of average velocity on distance. Non-parabolicity of the conduction band is taken into account. Optimization of the HET collector parameters initiated. Computations suggest that improvement of f t /f max ratio require reduction of RC-delay (proper I c management) in HET collector.

Compact Power Supplies Based on Heterojunction Switching in Wide Band Gap Semiconductors NC STATE UNIVERSITY UCSB Future Plans In collaboration with Dr. Asbeck, develop a simulator for calculation of parameters T ef, f t, f max. Optimize the design of HET collector to minimize the transit time and to improve f t /f max ratio exploring RAE and LFRAE for generation of the velocity-distance profiles shown schematically: V dr w w* Investigate potential of multinary In- contained compounds for this purpose. Include into RAE simulator effects of hot carrier captures by upper valleys.