Microwave Semiconductor Devices
It is a non linear device and its principle of operation is similar to that of low frequency device.
Heterojunction Bipolar Transistor:
When transistor junctions is formed with two dissimilar metals such as Ge and GaAs, the junction is called Heterojunction.
Microwave Field Effect Transistors: It is constructed with metal and semiconductor.
GaAs MESFET in comparison to Si MESFET:
1. Higher Electric field
2. Higher output power
3. Lower Noise Figure
1. Front end low noise amplifiers of microwave receivers.
2. As a driver amplifier for high power transmitters.
High electron Mobility Transistor (HEMT): Improvement over MESFET:
1. Shorter gate length
2. Reduced gate to source contact resistance
3. Optimized doping profile
4. Higher speed
Advantages of HEMT
1. Low Noise Figure
2. Higher gain at microwave frequencies
Varactor diode: Semiconductor diode in which junction capacitacne can be varied as function of reverse bias voltage of the diode i.e.
n = (1/2)For abrupt or step graded junction
n = (1/3) Linearly graded junction
n = (1/2.5) Diffused junctions
n = 2 Hyper abrupt junctions
Equivalent circuit of varactor diode:
Applications of varactor diode:
1. For direct generation of FM
2. Harmonic generation
3. Microwave frequency mulitplication
4. Low noise amplification
5. Pulse generation and pulse shaping
Parametric Amplifier :
1. fo > fs + fp
f0 - output frequency,,
fp - pump frequency..
fs - signal frequency
2. Power gain
g q- Figure of merit for non-linear capacitanc.
Rd - Series resistane of p-n junction diode Noise Figure :
Td - Diode temperatuer, TTo - 300º K (room temperature)
Parametric Down Converter:
fi = fp – fs fi = Idler frequency
Power gain =
Rg - Output resistance of signal generator Ri - Output resistance of idler generator RTS - Total series resistance at fs RTi - Total series resistance at fi
Tunnel Diode (Esaki diode): Symbol :
Resistive cut-off frequency:
Rs - Resistance of packing circuit of tunnel diode
Rn - Negative resistance of tunnel diode
C - Junction capacitance of tunnel diode
Self Resonance frequency:
Ls - Inductance of pa ckaging circuit of tunnel diode Gain of tunnel diode:
For parallel load gain:
Rl - Series load
Transferred Electron devices (TEDs): Difference between transistors and TED
1. TEDs have no p-n junctions like transistors.
2. Transistors a re fabricated from Ge or Si, where as TEDs are from GaAs, InP, CdTe.
3. TRansistor s operate with warm electrons where as TEDs operates with hot electrons.
Microwave amplification and oscillations are derived from the bulk negative resistance property of uniform semiconductors rather than from the junction negative resistance property between two different semiconductors as in tunnel diode.
Gunn diodeL: modes of operation 1 .
Gunn Oscillatory Mode:
For this mode f L 107 cm/s to 2×107 cm/s, 1012/ cm2 < n0L<1014/cm2
Frequency of oscillation
Vdom - Domain Velocity Leff - Effective length that the domain travels from the time it is formed until the time that now domain begins to form
2 . Limit ed space charge accumulation mode (LSA mode): fL < 2 × 107 cm/sec, n0L 1012/cm2,
Efficiency : 20%
When the frequency is very high the domains do not have sufficient time to form while the field is above threshold. As a result, most of domains are maintained in negative conductance state during a large fraction of the voltage cycle.
3. Stable amplification mode: (n0L < 1012/cm2) In this mode device exhibits amplification at transit time frequency rather than spontaneous oscillation.
Avalanche transit time device:
1 . Read diode
vd - Avalanche breakdown voltage
V - Applied voltage Vb - Avalanche breakdown voltage n - 3 to 6 for Si
L - Drift region Length
2. MPATT diodes (Impact Ionization Avalanche Transit time diodes):
Vd - Carrier drift velocity L - Drift region length.
Transit time t
Its efficiency is around 30%.
So, it is not as good as Klystron or Gunn diodes.
TRAPATT diodes (Trapped Plasma Avalanche Triggered Transit diodes):
Avalanche zone velocity:
J - Current density
N - Doping concentration of n – region
BARITT diode (Barrier Injected Transit Time diode):
N - Dopint concentration L - semiconductor thickness
Breakdown electric field =
Breakdown voltage Vbd = 2Vc