The document discusses tunnel diodes and their switching properties. Tunnel diodes can switch between conducting and non-conducting states at very high speeds due to their narrow depletion widths allowing quantum tunneling. They have short storage times in the nanosecond range. When used as switches, the current in a tunnel diode does not change instantaneously between on and off states, but decays exponentially with a storage delay time ts dictated by the removal of stored minority carriers. Reducing carrier lifetimes and using narrow-base diodes can decrease this switching transient time.

1. Sourav Sarkar
Assistant Professor
Department of 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 Law Referencehuman brain in 2012?

5.  As mentioned earlier, 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 was introduced 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, carrier must 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-threshold swing 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-doped pn 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) Fermi level 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. EC
EV
EF
I
V
(a)
(b)
(c)
(d)
(e)
(a) (b) (c) (d) (e)

13. I
V
Peak current
100 kA/cm2
Peak-to-Valley Ratio (PVR)

14. The tunnel diode exhibits 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 Diodes were
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 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.

17. Summary
 HFETs use heterostructures 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. Diode Transient /switching

19. The diodes are used 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 can be 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 of a switching
circuit

22. Diode current- and voltage-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 charge causing switching delay

24. Decay of stored hole charge in a pn junction

26. 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 .

27. The decay time t2 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 .