High-Field Transport Modeling for Compact Power Sources

1. High-Field Transport Modeling for Compact Power Sources K. W. Kim, J.-B. Jeon & S.M. Komirenko North Carolina State University Raleigh, NC 27695 ONR MURI Review February 12, 2002

2. Current Objectives 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.

3. Low-field runaway:possible scenarios Ekin unstable Ex Energy Injection energy stable  * runaway (unstable) At field  > bias * Electron injected with any energy will runaway saturation (stable)

4. Einj.< Ex   pz Vdr 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.

5. Einj.> Ex Vdr is higher and has a minimum. The velocity RAE at low field is detected. Due to LFRAE, the distribution function has twomaxima, i.e. there ate twodistinct groups of carriers with different velocities. Non-parabolicity helps to keep hot carriers in the  valley for more than 100 nm   pz

6. HET modeling High-field collector region RAE or LFRAE base emitter collector W* W

7. Parameter definition (E in kV/cm)

8. RAE from barrier  1.8 V  pz 18.2 V

9. RAE of termalized electrons

10. Upper valley impact Field (0<w<w*) 283 kV/cm 250 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

11. 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 Vc=20 V, Ic=4mA, Rb~209, and Cbe~5fF, ft ~ fmax can be satisfied only for sub-micron scale collector with relatively long high-field region. RC-delay needs to be optimized. • Since Tv 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?

12. 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 ft/fmax ratio require reduction of RC-delay (proper Ic management) in HET collector.

13. Future Plans In collaboration with Dr. Asbeck, develop a simulator for calculation of parameters Tef, ft, fmax. Optimize the design of HET collector to minimize the transit time and to improve ft/fmax ratio exploring RAE and LFRAE for generation of the velocity-distance profiles shown schematically: Vdr Investigate potential of multinary In-contained compounds for this purpose. Include into RAE simulator effects of hot carrier captures by upper valleys. w* w