Lecture 15: Resonant Tunneling Diode. Operation Bi-stable switch Oscillator Fundamental Physics Application Example

Transcription

1 Lecture 5: Resonant Tunneling Diode Oeration Bi-stable switch Oscillator Fundamental Physics Alication Examle --8 Lecture 5, High Seed Deices

2 Resonant Tunneling Diode An RTD consists of two large bandga material forming a quantum well with quasi-bound states (in -direction): Energy and k consered during tunneling negatie differential resistance E E E --8 Lecture 5, High Seed Deices

3 Current (ka/cm ) Resonant Tunneling Diode Exerimental Data (T=3K) Collector oltage () Plateau is an artifact from oscillations in the bias network mortant DC figure of merits: Peak current density ( ) Peak oltage ( ) alley current density ( ) alley oltage ( ) Peak-to-alley ratio: / Thermionic emmision alley current is larger than in a real deice: deal deice, T=K Field-assisted tunneling Transort through nd subband Elastic/inelastic scattering --8 Lecture 5, High Seed Deices 3

4 Current (ka/cm ) Alication: Bistable Switch The two stable bias-oints: 8, Associated with the double barrier is a caacitance: 6 4 f, f RL ( ) s R L RTD ( ) C t b t w t b s t t b w Collector oltage () Assume RTD biased at = small increase in s forces new biasing oint f Charge stored oer C will increase: Q C f --8 Lecture 5, High Seed Deices 4

5 Current (ka/cm ) Seed ndex 8, s Q C f 6 4 f, f RL ( ) RL R L RTD ( ) This charge has to be sulied through R L delay in Collector oltage () Area inbetween cures gies current to charge C with Q f C d C f Switching time: C f f / R L ( C ) RTD ( ) d Fast switching requires a large! Seed ndex /PR --8 Lecture 5, High Seed Deices 5

6 AlSb/nAs/AlSb Switch Transition.7 S switching of ~.7 Slew-rate of 3m/S t b =.8nm t w =7.5 nm nm drift region after RTD ~.5 S E. Öbay, EDL S --8 Lecture 5, High Seed Deices 6

7 Current (ka/cm ) Alication: Negatie resistance oscillator The RTD has negatie differential resistance (NDR): r di d NDR L i(t) R s f R s (series resistance in diode) ~, the equiialent circuit is a simle RLC: i t i e g d C t j t LC 8 Some inductance in the biasing circuit. DC measurment aerages oer oscillations NDR C -g d f g d is negatie, the oscillations amlitude grows instead of decays An RTD biased in the NDR region will oscillate between and Collector oltage () --8 Lecture 5, High Seed Deices 7

8 High Frequency Oscillators f osc LC The oscillation frequency is essentially set by the effecti LC-network Can oscillate as long as Re(Z in )< g d =di/d R s Z in f max, osc C g R d s g d jwc -g d g d For a high f max, we want small R s, small C and large g d Highest RTD f max ~ 85 GH, which is the highest fundamental mode oscillator made as of today Pout w Low outut ower limits alications --8 Lecture 5, High Seed Deices 8

9 Calculation of current Assume fully coherent transort: Transmission robability qn qn q q k x k f q F, s F, s, E, Ek T k f E Ek k T k dk k f qmkt E, Ek T k T( E )ln E E F s 3 kxk y, k, ex D-D-D-system 3D-D-3D f, l de E f,l E f,l -q sd Where T(E, sd ) is the transmission robability of the quantum system E k m T(E, sd ) E T(E ) q ds --8 Lecture 5, High Seed Deices 9

10 Transmission Probability E= E= Match waefunction and its deriatie at all interfaces: m a a * =a a m * a T E A Assuming lane waes Ae Ce Ee k ik ik Be De Fe... e m ik * m E * ik ik E... imag K Should use comlex bandstructure.. E E C real k Sole for A... (e.g. Using the transfer matrix method) --8 Lecture 5, High Seed Deices E

11 Transmission Probability Transmission Probability b..4.6 Energy (e) At resonance for a symmetric system T= Finite lifetime in well gies rise to a broadening of the state G t =FWHM Around the transmission eak, T(E ) is essentially Lorenian. T G n E --8 G n G / 4 E n T * m b R n / E E n T is the transmission robability through a single barrier Lecture 5, High Seed Deices qmkt T( E )ln ex 3 E f, l E de f we want + to be large T(E ) should be wide in energy! Build RTD with thin barriers and a thin well!

12 Quantum well energy (e) Peak Current Density(kA/cm ) Current (ka/cm ) Current Density (ka/cm ) Design examle: AlGaAs/GaAs/nGaAs RTD b Al.8 Ga. As Strong deendency on barrier thickness 4nm 5 4 Calculated Exonential Fit = (-.94x) 3 E Barrier Thickness (nm) GaAs n x Ga -x As An indium-rich notch lowers st subband, and increases nd higher eak to alley ratio 8 6 T=3K n (%) in notch n (%) in notch 4 Lecture 5, High Seed Deices Collector oltage ()

13 Alication Examle: Ultra Wideband Source Startu Decay ery quick turn on/off due to the high slew-rate --8 Lecture 5, High Seed Deices 3

14 Alication Examle: Ultra Wideband Source Gated Resonant Tunnel diode nductor --8 Lecture 5, High Seed Deices 4

15 Gulses/s On-Off Keying at 6 GH Potential alications: UWB communication systems (Gbit/s oer short ranges) Pulse-based radar systems TH sectroscoy (short ulse large bandwidth) --8 Lecture 5, High Seed Deices 5

16 RTD Conclusions Large research effort at uniersities and industry during 99- At that time: f t,f max of HEMT/HBTs were ~ GH, RTD had f max ~ 7 GH! RTD uniolar two-terminal comonent, with added functionality due to the NDR Simle design gies small caacitance fast switching, high oscillation frequency Today f t, f max of transistors are aroaching TH, limited usage for RTDs Unique can lead to simler circuit solutions, e.g. oscillators as comared with traditional transistor technology. --8 Lecture 5, High Seed Deices 6