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![erf(x) is known as error function and an approximate
solution for storage time can be obtained as
]1ln[
F
p0
F
F
p0
s
R
s
R
I
I
t
II
It
erf
The recovery time t>ts is the time required for the junction
its steady state reverse condition. The reminder of the ex
is being removed and the space charge width is increasin
Bias value .](https://image.slidesharecdn.com/tunneldiode-180523161053/85/Tunnel-diode-26-320.jpg)
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Tunnel diode
- 1. Sourav Sarkar Assistant Professor Departmentof ECE Tunnel Diode
- 2. Why Tunneling Devices? Advantage of this quantum effect device Works at room temperature High switching speed Low power consumption Differing operating principles Quantization Quantum tunneling Negative Differential Resistance (NDR) http://www.cse.unsw.edu.au/~cs4211/proj ects/presentations/james- pp.ppt#266,9,Resonant Tunnelling Diodes
- 3. RTD Concepts: Tunneling Tunneling Quantum mechanical phenomenon Wave-particle duality Calculate tunneling probability with Schrödinger’s equation Complex barrier shapes by WKB approximation method Requires finite barrier height and thin barrier width
- 4. Obligatory Moore’s LawReferencehuman brain in 2012?
- 5. As mentionedearlier, heavily-doped, abrupt junctions are needed Can be obtained using several different methods ◦ Ion implantation ◦ Rapid thermal diffusion ◦ Molecular beam epitaxy ◦ Laser diffusion
- 6. It wasintroduced by Leo Esaki in 1958. Heavily-doped p-n junction ◦ Impurity concentration is 1 part in 10^3 as compared to 1 part in 10^8 in p-n junction diode Width of the depletion layer is very small (about 100 A). It is generally made up of Ge and GaAs. It shows tunneling phenomenon. Circuit symbol of tunnel diode is : EV
- 7. Classically, carriermust have energy at least equal to potential-barrier height to cross the junction . But according to Quantum mechanics there is finite probability that it can penetrate through the barrier for a thin width. This phenomenon is called tunneling and hence the Esaki Diode is know as Tunnel Diode.
- 8. Lower sub-thresholdswing can allow for lower operating voltages to be used Negative differential resistance (NDR) properties can be exploited to create simpler designs for bi-stable circuits, differential comparators, oscillators, etc. Leads to chips that consume less power
- 9. Simplest tunneling device Heavily-dopedpn junction • Leads to overlap of conduction and valence bands Carriers are able to tunnel inter-band Tunneling goes exponentially with tunneling distance • Requires junction to be abrupt EC EVEF
- 10. (a) Fermilevel is constant across the junction • Net tunneling current zero applied voltage is zero • Voltage applied: tunneling occurs Under what conditions? (b) Maximum tunneling current (c) Tunneling current ceases • No filled states opposite of unoccupied states (d) Normal diffusion and excess current dominates High doping Large capacitance Difficult device growth
- 12.
- 13. I V Peak current 100 kA/cm2 Peak-to-ValleyRatio (PVR)
- 14. The tunnel diodeexhibits negative resistance. It will actually conduct well with low forward bias. With further increases in bias it reaches the negative resistance range where current will actually go down. This is achieved by heavily-doped p and n materials that create a very thin depletion region which permits electrons to “tunnel” thru the barrier region. Germanium or Gallium Tank circuits oscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.
- 15. Tunnel Diodeswere discovered by Esaki in 1958 • Studied heavily (degenerately) doped germanium p-n junctions • Depletion layer width is narrow • Found NDR over part of forward characteristics
- 16. Tunnel Diodes Tank circuitsoscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.
- 17. Summary HFETs useheterostructures to separate doped region from the electron channel Adds flexibility and design based on doping and material variations in various layers that controls the flow and distribution of charge carriers Scattering effects are reduced especially at low temperatures resulting in high mobilities, high gm, and high fT operating at low power Typical materials being used include GaAs/AlGaAs InGaAs/AlGaAs and AlGaN/GaN
- 18.
- 19. The diodes areused as switches in many applications. Of prime concern is the speed at which the pn junction diode can be made to switch from “off” to “on” state and vice versa.
- 20. 20 • Diodes canbe used as switching devices • Need to change from conducting to non-conducting at high speed • Storage time or turn-off transients should be small • Add recombination centers to reduce minority carrier lifetimes For example adding 1015cm–3 gold (Au) to Si reduces hole lifetime to 0.01 s from 1 s! • Use narrow-base diodes Amount of charge stored in the neutral region of the diode will be small.
- 21. Idealized representation ofa switching circuit
- 22. Diode current- andvoltage-time transients F F F aF F R V R vV I R R R R V II Note: the current does not change to I0 (reverse saturation current), I0, instantaneously. ts is the storage time or storage delay time.
- 23. Stored minority chargecausing switching delay
- 24. Decay of storedhole charge in a pn junction
- 26. erf(x) is knownas error function and an approximate solution for storage time can be obtained as ]1ln[ F p0 F F p0 s R s R I I t II It erf The recovery time t>ts is the time required for the junction its steady state reverse condition. The reminder of the ex is being removed and the space charge width is increasin Bias value .
- 27. The decay timet2 is determined as )(1.01 )exp( R p0 2 p0 2 p0 r FI I t t t erf Turn On transient: The turn on transient occurs when the diode is switched from its off state into the forward bias on state .