Semiconductor Microwave Devices
Like conventional ordinary vacuum tubes cannot be used at high frequency, because
some parameters generate complicated situations and these parameters are
1. The interelectrode capacitance effect 2. The Lead inductance effect 3.Transit time
To overcome the above problems one should use either a high frequency
transistor or some other special type of semiconductor devices. Like negative
resistance and non-linearity in the operation make these special devices (i.e. like
Varactor diode, PIN diode, IMPATT diode, TRAPATT diode, Tunnel diode and Gunn
diode along with the high frequency transistors) suitable for their operations in the
microwave region. Some observations we conclude that
Bulk semiconductor device- Gunn diode
Ordinary p-n junction diodes- Varactor and Tunnel diodes
Modified p-n junction diodes- IMPATT, TRAPATT, PIN diodes such as p+-n or p-i-n type
Microwave semiconductor devices have been developed for various applications like,
detection, mixing, frequency multiplications, attenuation, switching, limiting,
amplification or oscillation etc. Advantages of these devices are low cost, small size,
less weight, low noise, greater bandwidth, lesser switching time, also employed in
microwave integrated circuit and other improvements in performance characteristics
for achieving the above functions.
Classification:Microwave Semiconductor Devices
Transferred Electron
Devices (TEDs)
Gunn diode,
LSA diode
FET
Microwave Transistors
time devices
JFET, MOSFET
NMOS, CMOS
BJT, HBT,
Tunnel devices
Avalanche Transit
READ, IMPATT
TRAPATT, BARRITT
Frequency of operation up to
C,X and Ku bands
5 GHz
LSA- Limited Space Charge Accumulation
IMPATT- Impact Ionization Avalanche Transit Time Device
TRAPATT- Trapped Plasma Avalanche Triggered Transit Device
BARITT- Barrier Injected Triggered Transit Device
multimeter wave
frequency
Transferred Electron Devices
TEDs are bulk devices having no junction or gates as compared to microwave
transistors. These devices exhibit negative resistance, in which the voltage and current
are 1800 out of phase. The voltage drop across negative resistance is negative and a
power of –I2R is generated by the power supply associated with the negative
resistance. We can say positive resistance absorbs power (i.e. passive device) and
negative resistance generates power (i.e. active device).
TEDs are fabricated from compound semiconductors such as GaAs (Gallium
arsenide), InP (Indium phosphite) or CdTe (Cadmium Telluride). TEDs are operate with
hot electrons whose energy is very much greater than the thermal energy.
Since Gunn diode is fabricated from compound semiconductors and exhibits
negative resistance criteria or we can say it follow the characteristics of TED devices
with RWH theory, so it is also a TED device.
Gunn Effect
J.B. Gunn (1963) observed the Gunn effect in n type GaAs bulk diode. He proved, when
the applied voltage exceeded a certain critical value 2000-4000 volts/cm, the periodic
fluctuations of current passing through the n-type GaAs bulk diode are established.
This fluctuation took the form of a periodic oscillation superimposed upon the pulse
current. The period of oscillation was inversely proportional to the specimen length
and closely equal to the transit time of electrons between the electrodes. The
calculated velocity is equal to the 107 cm/sec. Thus
The period of oscillation  (specimen length)-1
The frequency of oscillation  Specimen length
The frequency of oscillation does not depend on external parameter of circuit.
“Drift velocity increases linearly from 0 to maximum value corresponds to
electric field varies from 0 to Ethreshold. When electric field is greater than 2000
V/cm for n-type GaAs and approach to Ethreshold, the drift velocity is decreased
and GaAs bulk diode exhibit negative resistance.”
Gun effect can be explained on the basis of Ridley Watkins Hilsum (RWH) theory
Ridley Watkins Hilsum (RWH) theory:- The basic concept of RWH theory is that, When
either a voltage or a current is applied to the terminals of the sample, the III-IV group
elements show the differential negative resistance. There are two modes of negative
resistance devices and which are
1. Voltage controlled Mode (VCM)
2. Current controlled mode (CCM)
i) The current density can be multivalued. (i) Voltage can be multivalued.
ii) In which high field domains are formed, (ii) The major effect of the appearance of a
separating two low field regions. The
differential current controlled mode is to
interface separating low and high field
render the sample electrically unstable.
domains lie in a plane perpendicular to
As a result, initially homogeneous sample
current direction.
becomes electrically heterogeneous in an
attempt to reach stability.
Iii) Voltage controlled mode is widely
(iii) In CCM the splitting results in high the
Preferable.
current filaments running along field
direction.
Now we study Two Valley Model Theory
Two Valley Model Theory:- Before the Gunn effect, a negative mass microwave
amplifier, which depended on energy band theory of the n-type GaAs, was discovered.
In the energy band theory of n-type GaAs, a high mobility lower valley is separated by
energy of 0.36 eV from a low-mobility upper valley in the conduction band as shown in
below fig.
Fig. Two Valley Model of electron energy versus wave number for n-type GaAs
Electron Density in the Lower and Upper valley:Density of status in Upper valey
Density of status in Lower valey
= 60
In equilibrium condition, electron densities in the lower and upper valleys remain the
same. But
(i) When the applied electric field is lower than the electric field of the lower valley
(E<EL), no electrons will transfer to the upper valley as shown in below fig(a).
(ii) When EL<E<EU then electrons will begin to transfer to the upper valley as shown in
below fig(b).
(iii) When EU<E, all electrons will transfer to the upper valley as shown in above fig(c)
Three main Criteria:- On the basis of RWH theory, the semiconductor material is
supposed to satisfy 3 main criteria as given below
1. E > KT( 0.026 eV) at room temperature. The separation energy between the
lower valley and upper valley must be several times larger than the thermal
energy (i.e. about 0.026 eV).
2. E<ER. The E must be smaller than the gap energy between the conduction
band and valance band.
3. Electrons in the lower valley must have high mobility, small effective mass and a
low density of state, whereas those in upper valley must have low mobility,
large effective mass and a high density of state.
Phenomenon of controlled –ve resistance:1. Negative resistance profile for VCM:- When the increase of biasing voltage the
electrons gets accelerated under electric field and as a result their kinetic energy
increases. Thus more electrons from the lower valley will be transferred to the upper
valley, cause a redistribution of electrons between two valleys of the conduction band.
Initially the lower valley electrons had the high value of mobility but due to transfer to
the upper valley, mobility of the lower valley electrons decreases. Thus –ve resistance
characteristic is generated.
dJ
Negative resistance condition→
< 0 which implies negative resistance -Rn
dE

d(E)
dE
<0
d(E)
d(E)
 [+ (E)
]< 0  [1+ (E/)
]< 0 ……(i)
dE
dE
If equation(i) satisfy, it means we find the region between Em and Ev, where current
density decreases with increasing electric field. As the electrons transfer to the upper
valley, their mobility start decreasing and their effective mass is increasing, resulting in
decreasing the current density and hence the –ve differential conductivity. This is also
known as population inversion of charges from lower valley to upper valley. It will
occur at a certain threshold field (i.e. approximately equals to 3.3 kV/cm and above).
2. Negative resistance profile for CCM:- We can easily understood step by step
a) If an electric field E0 is applied to the sample, the current density J0 is generated.
As shown in below fig.
b) As the applied electric field is increased to E2, the current density is decreased to J2.
c) As the applied electric field is decreased to E1, the current density is increased to J1.
Formulae
Let E = Applied Electric field, Eth = Threshold field, Device length = L, J = Current density
n= -ve electron velocity, n = doping concentration, 0 = Transit time and
vd = Electron drift velocity
1. Drift velocity vd = L  f = L/0
2. Current density J = envd
3. –ve electron density n = -vd/E
4. Current density J = E
Numerical
Nu. A typical n-type GaAs Gunn diode has the following parameters : threshold
field(Eth) = 2800V/cm, applied field E = 3200 V/cm, device length L = 10 µm,
doping concentration n0 = 2 x1014 cm-3, operating frequency f = 10 GHz.
(a) compute the electron drift velocity
(b) calculate the current density
(c) estimate the negative electron mobility.
Sol. (a) electron Drift velocity vd = L  f = L/0
= (10  10-6)  (10  109) = 105 m/sec = 107 cm/sec.
(b) Current density J = envd = (1.6  10-19)(2  1020)(105) = 3.2  106 A/m2
= 320 A/cm2
(c) negative electron mobility = n = -vd/E = -107/3200 = -3100 cm2/V.sec
Nu. In an n type GaAs Gunn diode has the following parameters:
applied field = 3 kV/cm, device length = 15 m and operating at a frequency of
10 GHz. Calculate the negative electron mobility.
Sol. negative electron mobility = n = -vd/E = - L x f/E = - 15 x 10-4x 10 x 109 /(3 x 103)
= - 5000 cm2/V.sec
Tunnel Diode
The Tunnel diodes are heavily doped p-n junction and its impurity concentrations of
1019 to 1020 atoms/cm3 are used. The depletion layer barrier at the junction is very thin
100 Ȧ or 10-6 cm. The tunnel diode is a negative resistance semiconductor p-n junction
diode because of the tunnel effect of electrons in the p-n junction. These diodes are
used as a microwave amplifier or oscillator.
Due to heavy doping the width of the depletion
region becomes very thin and an overlap occurs between the conduction band level on
the n-side and the valence band level on the p-side.
Operation- Note- In ordinary diodes the Fermi level exists in the forbidden band. Since
the tunnel diode is heavily doped, the Fermi level exists in the valance band in P-type
and in conduction band in N-type semiconductor.
Fig. Under open circuit condition
We can understood the operation by some steps
Fig.x.
I-step:- Under open circuit condition or at zero bias equilibrium condition- in this
condition , the upper levels of electron energy of both the P-type and N-type are lined
up at the same Fermi level.
Since there are no filled states on one side of the junction that are at the same
energy level as empty allowed states on the other side. There is no flow of charge in
either direction across the junction and the current is zero, as shown at point ‘a’ in
characteristic fig.x.
II-step:- As forward bias is applied the energy level on N-side are raised as shown in
below fig.
The electrons in conduction band on N-side see empty state just across the barrier and
tunneling takes place. This tunneling will read a maximum value Ip at a forward bias Vp
as shown at point ‘b’ in characteristic fig.x. At forward bias Vp a maximum number of
electrons can tunnel through the barrier from filled state to empty state.
III-step:- If the bias voltage is further increased, conduction band electron energy
levels are raised above the available energy levels in the valance band and becomes
equal to levels in forbidden band. No direct tunneling occurs and current decreases
with increases in forward voltage till valley voltage Vv and this corresponds to the
negative resistance region of the diode characteristics Point ‘b’ to ‘c’.
IV-step:- As the forward voltage is increased beyond Vv, current increases with same
manner as a semiconductor diode.
Equivalent Circuit of Tunnel Diode
The input impedance of the equivalent circuit is
Zin = RS + jLs + [equivalent resistance of parallel combination of (-Rn) and C]

Zin = RS + jLs + [(-Rn)II(1/jC)]

Zin = RS + jLs + Rn(j/C)/{-Rn-(j/C)}

Zin = RS + jLs + Rn(j/C){-Rn+(j/C)} /{-Rn-(j/C)}{-Rn+(j/C)}

Zin = RS + jLs + [{-Rn2j/C – Rn/2C2} / {Rn2- (j/C)2}]

Zin = RS + jLs + [{-Rn2j/C – Rn/2C2} / {Rn2+ (1/C)2}]

Zin = RS + jLs + [{-Rn2j/C – Rn/2C2} / Rn2{1+ (1/RnC)2}]

Zin = RS + jLs + {-Rn2j/CRn2{1+ (1/RnC)2} – Rn/2C2Rn2{1+ (1/RnC)2}

Zin = RS + jLs + {-j/C{1+ (1/RnC)2} – 1/2C2Rn{1+ (1/RnC)2}

Zin = RS + jLs - jCRn2/{1+ (RnC)2} – Rn/{1+ (RnC)2}

Zin = RS – Rn/{1+ (RnC)2} + j[Ls - CRn2/{1+ (RnC)2}] ………………(1)
Case I:- For the resistive cut-off frequency, the real part of the input
impedance Zin must be zero and  becomes c
 RS – Rn/{1+ (RncC)2} = 0
 RS = Rn/{1+ (RncC)2}  {1+ (RncC)2} = Rn/Rs
 fc = (1/2RnC)[(Rn/Rs)-1] ………………(2)
Case II:- For the reactive or self resonance cut-off frequency, the imaginary
part of the input impedance Zin must be zero and  becomes r
 Ls - CRn2/{1+ (RnrC)2} = 0
 Ls = CRn2/{1+ (RnrC)2}  {1+ (RnrC)2} = CRn2/Ls
 fr = (1/2RnC)[(Rn2C/Ls)-1] …………..(3)
Nu. A microwave tunnel diode has a negative resistance Rn and the resonant
circuit has a circuit resistance RL. Derive the equation for the gain of a
microwave Tunnel diode amplifier.
Sol. The tunnel diode can be connected either in parallel or in series with a
resistive load as an amplifier; its equivalent circuits are shown below
Parallel Loading
The power output in the load resistance
Pout = V2/RL
One part of this output power is
generated by the small input power through
the tunnel diode amplifier with a gain of A,
and this part is
Pin = V2/ARL
Another part of the output power is
generated by the –ve resistance (Rn) and it
is
Pn = V2/Rn
since, Pout = Pin + Pn
 V2/RL = V2/ARL + V2/Rn
 1/RL = 1/ARL + 1/Rn
 Gain A = Rn/(Rn-RL) ……….(2)
When Rn →RL , then A → and system goes
into oscillation.
Series Loading
Similarly as equation (2) we can
derive the equation of gain for
series loading
Gain A = RL/(RL-Rn) ……….(3)
The device can remains stable in
negative resistance region,
if and only if RL < Rn.
Nu. A Tunnel diode has, negative resistance Rn = 20 Ω, series resistance = 5 Ω,
resistive cut-off frequency = 8 GHz. (i) Calculate the junction capacitance
(ii) When diode is used as an amplifier with load of 10 Ω in parallel, calculate
the gain.
Sol. (i) Since, resistive cut-off frequency, fc = (1/2RnC)[(Rn/Rs)-1]
 Junction capacitance, C = (1/2Rnfc)[(Rn/Rs)-1]
C = (1/23.14208109)[(20/5)-1]
C=
(ii) Since, gain A = Rn/(Rn-RL) = 20/(20-10) = 2
Nu. A tunnel diode has Rn = 26 Ω, Rs = 1Ω and C = 5 pF, then (i) Find resistive cutoff frequency (ii) if series inductance 10 mH, calculate self resonance
frequency (iii) Compute gain at 24 Ω when tunnel diode use as an amplifier in
parallel load.
Nu. A tunnel diode has negative resistance 26 Ω, series resistance 1 Ω and
junction capacitance 5 pF. Calculate
(i) the resistive cut-off frequency (ii) the gain, when the load is parallel.
Ans- (i) 6.12 MHz (ii) 13
Avalanche Transit Time Devices
It is proposed by the W.T.Read in 1958, that the delay between voltage and current
in an avalanche together with transit time through the material. By which it is
possible to make a microwave diode which exhibits negative resistance. These types
of devices are known as Avalanche transit time devices.
Difference between Transferred Electron Devices and Avalanche Transit Time
Devices:-
Types of Avalanche Transit Time Devices:There are three types1. IMPATT (Impact Ionization Avalanche Transit Time) device
2. TRAPATT (Trapped Plasma Avalanche Triggered Transit Time) device
3. BARITT (Barrier Injected Transit time) device
1. IMPATT (Impact Ionization Avalanche Transit Time) device
An IMPATT diode (Impact ionization Avalanche Transit-Time) is a form of highpower diode used in high-frequency electronics and microwave devices. They operate
at frequencies between about 3 and 100 GHz or more.
It has many forms like n +pip+, p+nin+ read device, p+nn+ abrupt junction and p+in+
diode. Here positive (+) sign indicates a high level of doping and i stands for intrinsic
or pure silicon.
Types IMPATT diodes and Doping Profile
The two common types of IMPATT structures are SDR (Single Drift Region)
and DDR (Double Drift Region). The doping and field profile of both SDR and
DDR are shown in above figure.
IMPATT diodes can be manufactured from Ga, Si, GaAs or InP. However GaAs
provides the highest priority, because of its high efficiency, the highest operating
frequency and least noise figure. But the fabrication process is more difficult and
costly than Si.
A main advantage is high-power capability. These diodes are used in a variety of
applications from low-power radar systems to alarms.
A major drawback of using IMPATT diodes is the generation of phase noise
which result from the statistical nature of the avalanche process. Nevertheless
these diodes make excellent microwave generators for many applications.
Operation:- A very high voltage 400 kV/cm is applied to the IMPATT diode,
resulting in a very high current. A normal diode would easily break down under this
condition, but IMPATT diode is constructed such that it will withstand these
conditions repeatedly. Such a high potential gradient back biasing the diode causes a
flow of minority carriers across the junction.
If it is now assumed that oscillations exist, we may consider the effect of a positive
swing of the RF voltage superimposed on top of the high dc voltage. Electron and hole
velocity has now become so high that these carriers form additional holes and electrons
by knocking them out of the crystal structure, by so called impact ionization. We have
two steps to understand the operationStep I-These additional carriers continue the process at the junction and the voltage
will be exceeded during the whole of the +ve RF cycle. The avalanche current
multiplication will be taking place during this entire time. Since avalanche is a
multiplication process, it is not instantaneous or we can say it is a cumulative process.
This process takes time such that the current pulse maximum, at the junction, occurs at
the instant when the RF voltage across the diode is zero and going negative. A 90 0
phase difference between voltage and current has been obtained.
Step II- The current pulse in the IMPATT diode is situated at the junction.
However it does not stay there because of the reverse bias, the current pulse flows to the
cathode, at a drift velocity depending on the presence of the high dc field. The time
taken by the pulse to reach the cathode depends on this velocity and on the thickness of
the highly doped n+ layer. The thickness of the drift space is adjusted such that time
taken for current pulse to arrive at the cathode corresponds to further 90 0 phase
difference.
Thus voltage and current are 1800 out of phase and a dynamic RF negative
resistance has been proved to exist. In summary, negative resistance phenomenon is
taken into account by using
1) The impact multiplication avalanche effect, which causes the minority
current to lag the microwave output voltage by 900. phase shift
2) The effect of transit time through the drift region, this results in the external
current lagging the microwave voltage by a further 900 phase shift.
TRAPATT Diode
An TRAPATT diode (Trapped Plasma Avalanche Triggered Transit-Time), is a very
high efficiency microwave generator, capable of operating from several hundred MHz
to several GHz. It is derived from the IMPATT diode and is closely related to it.The
basic operation of the TRAPATT oscillator is a semiconductor p-n junction diode
reverse biased to current densities well in excess of these encountered in normal
avalanche operation.
High peak power diodes are typically silicon n+-p-p+ or p+-n-n+ structures with the n
type depletion region varying from 2.5 to 12.5 µm.
The doping of depletion region is generally such that the diodes are well punched through
at breakdown.
The device P+ region is kept as thin as possible at 2.5 to 7.5 µm.
The Trapatt diodes diameter ranges from as small as 50 µm for µw operation to 750 µm at
lower frequency for high peak power device.
Good result from TRAPATT diodes below 10 GHz.
Principle of operation:- A high field avalanche zone propagates through the diode and fills
the depletion region with a dense plasma of electrons and holes that become trapped in the low
field region behind the zone.
Operation:-
To know the operation, we follow some given steps
1. Curve from A to B:- At the instant of time at point A, the diode current is turned ON. Since
only the charge carriers present, are those caused by the thermal generation, the diode initially
charges up like a linear capacitor. Curve AB shows the magnitude of the electric field above
the break down voltage.
2. Curve from B to C:- When a sufficient number of carriers is generated, the particle current
exceeds the external current and the electric field is depressed throughout the depletion
region, causing the voltage to decrease from point B to C. A dense plasma formation is
started.
3. Curve from C to D:- During the time instant the electric field is sufficiently large for the
avalanche to continue and the dense plasma of electrons and holes is increased. In this plasma
of electrons and holes, some of the electrons and holes drift out of the ends of the depletion
layer. Thus the field is further depressed and traps the remaining plasma.
4. Curve from D to E:- 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. At point E the
plasma is removed then voltage increases from point D to E.
5. Curve from E to F:- A residual charge of electrons remains in one end of the depletion
region and a residual charge of holes in the other end. As the residual charge is removed, the
voltage increased from point E to point F. at point F all the charge that was generated initially
has been removed.
6. Curve from F to G:- The diode charges up again like a fixed capacitor. At point G the diode
current goes to zero for half a period and the voltage remain constant at voltage V A until the
current comes back on the cycle repeats.