Q:What do we use diodes for?
a) Protect circuits by limiting the voltage (clipping and clamping)
b) Turn AC into DC (voltage rectifier)
c) Voltage multipliers (e.g. double input voltage)
d) Non-linear mixing of two voltages (e.g. amplitude modulation)
• Symbol for Diode:
Q: What is Diode made out of?
- Semiconductors: A solid substance that has a conductivity between that of an insulator and that of most metals.Example- Silicon and Germanium.
- We then, dope this semiconductors with trivalent or pentavalent impurity to make ap-type or an n-type semiconductor respectively.
- Then we form a P-N junctions by joining n-type and p-type semiconductor materials.
- And, hence a diode is formed.
- Here’s a image of unbiased diode :
A) Diode is Forward Biased :
When, Vanode > Vcathode.
Characteristics of diode in Forward Biased :
- Diode conducts current strongly
- Voltage drop across diode is (almost) independent of diode current
- Effective resistance (impedance) of diode is small.
B) Diode is Reverse Biased :
When, Vanode < Vcathode.
Characteristics of diode in Reverse Biased :
- Diode conducts current very weakly (typically < mA).
- Diode current is (almost) independent of voltage, until breakdown.
- Effective resistance (impedance) of diode is very large.
C) Current-voltage relationship for a diode :
I = Is*(e^(eV/ kT) -1)
known as: “diode”, “rectifier”, or “Ebers-Moll” equation,
where,
- Is = reverse saturation current (typically < mA),
- k = Boltzmann’s constant,
- e = electron charge,
- T = temperature.
Specification That Must Be Checked For A Diode :
- Reverse Saturation Current – is the current flowing through the diode due to the minority charge carriers across the junction when the diode is reversed biased.
- Peak Current and Voltage
- Transition Capacitance – when the diode is reversed biased, the majority charge carriers move away from the diode resulting in the increase in the width of depletion region.This leads to the increase in the immobile charges, which eventually starts to act like a capacitor.
- Diffusion Capacitance – is the change in the charge ‘ Q’ due to change in applied voltage across the diode in forward biased condition.
- Reverse Recovery Time – is the time taken by the diode to change its state from conducting to non-conducting state, when the voltage across the diode is reversed. It is useful to for we need diode for fast switching action.
- Sensitivity to Temperature
Types Of Diodes :
- Junction Diode (ordinary type)
- Light emitting (LED)
- Photodiodes (absorbs light, gives current)
- Schottky (high speed switch, low turn on voltage, Al. on Silicon)
- Tunnel (I vs. V slightly different than jd’s, negative resistance!)
- Veractor (junction cap. varies with voltage)
- Zener (special junction diode, use reversed biased)
~Jay Mehta
