________ diode is not used as a microwave mixer or detector.
PIN
Mixing and detection both require the diode to behave non-linearly at the signal frequency, and the PIN diode is the one device in the list that cannot — option 2.
The structure explains it. A PIN diode has a thick intrinsic layer between p and n regions. Forward bias floods that layer with stored charge, and the charge takes the carrier lifetime — hundreds of nanoseconds to microseconds — to recombine. Above roughly
\(f\gt\dfrac{1}{2\pi\tau}\)
the stored charge cannot follow the RF cycle at all. The diode stops rectifying and instead presents a constant resistance set by its DC bias: a fraction of an ohm when forward biased, kilohms when reverse biased.
The property that disqualifies it is the property that makes it valuable. Being linear to the RF signal, a PIN diode introduces no distortion and generates no unwanted mixing products — which is exactly what is wanted in a microwave switch, a variable attenuator, a phase shifter or a limiter. The same linearity makes it useless for the two jobs this question names, since a mixer must multiply two signals and a detector must rectify one.
| Diode | Mixer / detector? | Why |
|---|---|---|
| Schottky | ✓ | Metal-semiconductor, majority carrier, no stored charge |
| PIN | ✗ | Stored charge makes it linear at RF |
| Crystal (point contact) | ✓ | The original microwave detector |
| Backward | ✓ | Strong curvature at zero bias — ideal for weak signals |
The three that do work. A Schottky barrier diode is a majority-carrier device with essentially no minority storage, so it switches in picoseconds and stays non-linear into the millimetre-wave region — the standard mixer element. A crystal or point-contact diode is its historic predecessor, used in wartime radar receivers for the same reason. A backward diode is a tunnel diode doped just short of showing negative resistance; its forward characteristic is very sharply curved near the origin, giving good rectification of signals too small to turn on an ordinary junction, and it is used as a square-law detector.
Hence, the diode not used as a microwave mixer or detector is the PIN.
Similar Questions
A PIN diode can not be used as
i. Microwave switch
ii. Microwave mixer
iii. Microwave detectorWhich is correct ?
Match the given lists :
List – I (Name of the Device) List – II (Application) a. Diode i. Rectification b. Tunnel diode ii. Microwave switching c. Zener diode iii. An oscillator d. PIN diode iv. Voltage regulation Codes :
An important figure of merit for microwave application of Schottky diode is forward bias cut off frequency fco and is given by :
Assertion (A) : A tunnel diode consists of simple p-n junction mode of degenerate semiconductor. The current voltage characteristics consists of three components and shows the negative differential region over part of the forward characteristics.
Reason (R) : In tunnel diode the p-side is highly doped and n-side is highly doped and also used as detector.
Varacter diodes have the following properties :
A. Junction capacitance varies with bias voltage
B. Junction capacitance varies with time
C. Non linearity of varacter diodes makes them suitable for frequency multipliers
D. Linearity of varactor diode makes them suitable for frequency multipliers.
Choose the correct answer from the options given below:
In a tunnel diode, the degeneracy on the n-side can be expressed as
The static current voltage characteristics of a tunnel diode consist of
(a) Tunneling current and thermal current
(b) Thermal current and excess current
(c) Tunneling current only
(d) Tunneling current, excess current and thermal current
Out of these which are correct ?
Match the following :
List – I List – II a. Gunn diode i. degenerate silicon b. Tunnel diode ii. n-type Ga As c. BJT iii. Silicon d. Diode (P-N junction) iv. \(I_o\left[\exp\left(\dfrac{qV}{kT}\right)-1\right]\) Choose the correct answer from the codes given below:
A TRAPATT diode has the doping concentration NA = 2 x 1015/cm3 and a current density of 20 kA/cm2. The value of avalanche zone velocity is given by :
Assertion (A) : The Gunn diode characteristics shows the region of negative resistance.
Reason (R) : The Gunn diode is made of n-type and p-type degenerate semiconductor.
Select your answer using the codes given below.
Important Questions from Types of Diode
For which of the following diodes when the voltage is increased the current flowing through it decreases?
Which of the following is the main application of Zener diode?
A tunnel diode is
When we tune in any desired station in a radio receiver by rotating the tuning control we vary
Which diode is used as a voltage regulator?
