The document discusses several types of semiconductor devices used for microwave generation and amplification, including MESFETs, IMPATT diodes, and TRAPATT diodes. MESFETs are metal-semiconductor field-effect transistors that operate similarly to MOSFETs but lack an insulating layer. IMPATT and TRAPATT diodes generate microwaves by exploiting carrier transit time effects during avalanche breakdown. IMPATT diodes use impact ionization in a reverse-biased p-n junction, while TRAPATT diodes trap the generated plasma carriers to achieve higher efficiencies than IMPATT diodes.

1. MESFETS
AVALANCHE TRANSIT TIME DEVICES
(IMPATT DIODE ,TRAPATT DIODE)

2. 2
MESFET: Metal Epitaxial Semiconductor Field Effect Transistor
metal (e.g. TiAu)ohmic ohmic
source
gate
drain
Schottky
diode
depletion region
Insulating substrate
n (heavy)
• Gate looks like Schottky diode:
• Don’t forward bias

3. • From a digital circuit design perspective, it is
increasingly difficult to use MESFETs as the basis
for digital integrated circuits as the scale of
integration goes up, compared to CMOS silicon
based fabrication
• The absence of an insulator under the gate
implies that the MESFET gate should, in transistor
mode, be biased such that the metal
semiconductor diode is not forward biased.

5. 5
MESFET:
metal (e.g. TiAu)
ohmic ohmic
source
gate
drain
Schottky
diode
Insulating substrate
n (heavy)
b(x)
a
 dep bi GS CS
d
X V V ( x)
eN

  
2
depletion regionXdep(x)
D J (area) e n( x ) E( x ) W b( x )       I
J e n E   
CS
CS SD
V ( x) @x
V ( x) V @x L
 
 
0 0
When they
touch, define
VDS,sat
satvne 

6. Avalanche Transit
Time Devices

7. INTRODUCTION
Rely on the effect of voltage breakdown across a
reverse biased p-n junction.
The avalanche diode oscillator uses carrier impact
ionization and drift in the high field region of a
semiconductor junction to produce a negative
resistance at microwave frequencies.

8. INTRODUCTION
Two distinct modes of avalanche oscillator is
observed i) IMPATT(impact ionization avalanche
transit time operation)
Dc-to-RF c.e is 5 to 10%
ii) TRAPATT (Trapped plasma avalanche triggered
transit operation). 20 to 60%
Another type of active microwave device is BARITT
(barrier injected transit time diode)

9. IMPATT DIODE
Form of high power diode used in high frequency
electronics and microwave devices
Typically made from silicon carbides due to their
high breakdown fields.
3 to 100 GHz
High power capability
From low power radar systems to alarms
Generate high level of phase noise – avalanche
process.

10. The IMPATT diode family includes many
different junctions and metal semiconductor
devices.
The first IMPATT oscillation was obtained from a
simple silicon p-n junction diode biased into a
reverse avalanche break down and mounted in a
microwave cavity.

11. The original proposal for a microwave device of the
IMPATT type was made by Read.
The Read diode consists of two regions (i) The
Avalanche region (a region with relatively
high doping and high field) in which avalanche
multiplication occurs and (ii) the drift region (a
region with essentially intrinsic doping and constant
field) in which the generated holes drift towards
the contact.
Read diode is the basic type in the IMPATT diode
family

12. IMPATT DIODE
A wide variety of solid state diodes and transistor
have been developed for microwave use.
• IMPact ionization Avalanche Transit-Time
• Function as microwave oscillator.
• Used to produce carrier signal for microwave
transmission system.
• IMPATT can operate from a few GHz to a few hundred
GHz

14. IMPATT DIODE Operation
Figure 1: Impatt Diode Operation
• The diode is operated in reverse bias near
breakdown, and both the N and N- regions
are completely depleted
• Because of the difference in doping between
the "drift region" and "avalanche region",
the electric field is highly peaked in the
avalanche region and nearly flat in drift
region.
• In operation, avalanche breakdown occurs
at the point of highest electric field, and this
generates a large number of hole-electron
pairs by impact ionization.
• The holes are swept into the cathode, but
the electrons travel across the drift region
toward anode.

15. IMPATT DIODE Operation
Figure 2: The Build Up Of Microwave Oscillation.

16. IMPATT DIODE Operation
• As they drift, they induce image charges on the
anode, giving rise to a displacement current in the
external circuit that is 180° out of phase with the
nearly sinusoidal voltage waveform
• Figure 2 shows the buildup of microwave oscillations
in the diode current and voltage when the diode is
embedded in a resonant cavity and biased at
breakdown

17. Figure 3: Close Up Current And Voltage.
IMPATT DIODE Operation

18. IMPATT DIODE Operation
• Figure 3 shows a close-up of the current and
voltage waveforms after oscillations have
stabilized. It is clear from Fig. 3 that the
current is 180° out of phase with the voltage
• This represents a NEGATIVE AC RESISTANCE

19. TRAPATT
 Trapped Plasma Avalanche Triggered Transit
mode
 High efficiency microwave generator capable of
operating from several hundred MHz to several
GHz
 n+ -p -p+ or (p+ -n –n+)
 The doping of the depletion region is such that
the diodes are well “punched through” at
breakdown; i.e the dc electric field in the
depletion region just prior to breakdown is well
above saturated drift velocity level.

20. Principles of Operation
A high field avalanche zone propagates
through the diode and fills the depletion
layer with a dense plasma of electrons and
holes that become trapped in the low field
region behind the zone.

21. Voltage and Current waveforms

22. At point A the electric field is uniform throughout
the sample and its magnitude is large but less than
the value required for avalanche breakdown.
The current density is
At the instant of time at point A, the diode current
is turned on.

23. The charge carriers present are those due to
thermal generation, hence the diode initially
charges up like a linear capacitor, driving the
magnitude of electric field above the breakdown
voltage.
When a sufficient number of carriers are generated,
the particle current exceeds the external current
and the electric field is depressed throughout the
depletion region, causing the voltage to decrease.
(B to C)

24. (B to C) During this time interval the electric field is
sufficiently large for the avalanche to continue, and
a dense plasma of electrons and holes are created.
Some of the electrons and holes drift out of the
ends of the depletion layer, the field is further
depressed and “traps” the remaining plasma.
The voltage decreases to point D.
A long time is required to remove the plasma
because the total plasma charge is large compared
to the charge per unit time in the external current.

25. At point E the plasma is removed, but a residual
charge of electrons remains in one end of the
depletion layer and a residual charge of holes in the
other end.
As the residual charge is removed, the voltage
increases (E to F).
At F, all the charge that was generated internally
has been removed.

26. From point F to G, the diode charges up again like a
fixed capacitor.
At G, the diode current goes to zero for half a
period and the voltage remains constant at VA until
the current comes back on and the cycle repeats
The electric field expression

27. Thus the time t at which the electric field reaches
Em at a given distance x into the depletion region is
Differentiating w r t time t

28. - nominal transit time of the diode in the
high field.

29. Therefore the TRAPATT mode is still a
transit-time mode
That is the time delay of carriers in transit
(time between injection and collection) is
utilized to obtain a current phase shift
favorable for oscillation.

30. TUNNEL DIODE (Esaki Diode)
• It was introduced by Leo Esaki in 1958.
• Heavily-doped p-n junction
– Impurity concentration is 1 part in 10^8 as compared to 1
part in 10^3 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

31. TUNNELING
• The movement of valence electrons from the
valence energy band to the conduction band
with little or no applied forward voltage is
called tunneling.

34. VI CHARACTERISTICS

35. • As the forward voltage is first increased, the tunnel diode is
increased from zero, electrons from the n region tunnel
through the potential barrier to the potential barrier to the p
region. As the forward voltage increases the diode current also
increases until the peak to peak is reached. Ip = 2.2mA. Peak
point voltage =0.07V
• As the voltage is increased beyond Vp the tunneling action
starts decreasing and the diode current decreases as the
forward voltage is increased until valley point V is reached at
valley point voltage Vv= 0.7V between V and P the diode
exhibits negative resistance i.e., as the forward bias is
increased , the current decreases. When operated in the
negative region used as oscillator.

36. - Ve Resistance Region
VfVp
Ip
Vv
Forward Voltage
Reverse
voltage
Iv
ReverseCurrent
ForwardCurrent
Ip:- Peak Current
Iv :- Valley Current
Vp:- Peak Voltage
Vv:- Valley Voltage
Vf:- Peak Forward
Voltage
CHARACTERISTIC OF TUNNEL DIODE

37. ENERGY BAND DIAGRAM
Energy-band diagram of pn junction in thermal equilibrium in which both the n
and p region are degenerately doped.

38. -Zero current on the I-V diagram;
-All energy states are filled below EF on both sides of the junction;
AT ZERO BIAS
Simplified energy-band diagram and I-V characteristics of the tunnel diode at zero bias.

39. -Electrons in the conduction band of the n region are directly opposite to
the empty states in the valence band of the p region.
-So a finite probability that some electrons tunnel directly into the empty
states resulting in forward-bias tunneling current.
AT SMALL FORWARD VOLTAGE
Simplified energy-band diagram and I-V characteristics of the tunnel diode at a slight forward bias.

40. -The maximum number of electrons in the n region are opposite to the
maximum number of empty states in the p region.
- Hence tunneling current is maximum.
AT MAXIMUM TUNNELING CURENT
Simplified energy-band diagraam and I-V characteristics of the tunnel diode at a forward bias
producing maximum tunneling current.

41. -The forward-bias voltage increases so the number of electrons on the n side,
directly opposite empty states on the p side decreases.
- Hence the tunneling current decreases.
AT DECREASING CURRENT REGION
Simplified energy-band diagram and I-V characteristics of the tunnel diode at a higher forward
bias producing less tunneling current.

42. -No electrons on the n side are directly opposite to the empty
states on the p side.
- The tunneling current is zero.
-The normal ideal drift current exists in the device.
AT HIGHER FORWARD VOLTAGE
Simplified energy-band diagram and I-V characteristics of the tunnel diode at a forward bias
for which the diffusion current dominates.

43. - Electrons in the valence band on the p side are directly opposite to
empty states in the conduction band on the n side.
-Electrons tunnel directly from the p region into the n region.
- The reverse-bias current increases monotonically and rapidly with
reverse-bias voltage.
AT REVERSE BIAS VOLTAGE