PN Junction Devices. Electromagnetic Waves. Induction A changing magnetic field causes an induced electric field A changing electric field causes an induced magnetic field Resonant RLC circuits cause charges to flow back and forth along the antenna
A radio wave can be transmitted long distances. To get our audio signal to travel long distances we piggyback it onto a radio wave. This process is called MODULATION.
The radio wave is called the CARRIER.
The audio signal is called the MODULATION.
Amplitude is periodically changed. Information can be sent at a relatively slow rate, using the high frequency as carrier.
Detecting the variation is done by the speaker. It has a high inductance so that it does not respond to high frequency, but only to the average.
The lattice affects the structure of the energy levels of each atom – we now have joint levels for the entire structure
A hole is effectively a positive charge
More negative than rest of PThe PN Junction Diode
Start with a P and N type material. Note that there are excess negatives in the n-type and excess positives in the p-type
Merge the two – some of the negatives migrate over to the p-type, filling in the holes. The yellow region is called the depletion zone.
Apply a voltage as indicated. The free charge carriers (negative charges in the N material and positive charges in the P material) are attracted to the ends of the crystal. No charge flows across the junction and the depletion zone grows. This is called reverse bias.
Switch polarity. Now the negative charges are driven toward the junction in the N material and the positive charges also are driven toward the junction in the P material. The depletion zone shrinks and will disappear if the voltage exceeds a threshold. This is called forward bias.
N material (cathode)
Forward BiasDiode Circuit Symbols
Recall Ohm’s Law (V=IR) Put it into slope-intercept form
to get I = V/R. The slope of the graph is 1/R. Large slopes
mean small R.
Low r indicates better filtering
For ½-wave rectifier r = 1.21
For full-wave rectifier r = 0.48
When f is small, denominator is large and the voltage ratio is small.
When f is large, denominator is almost one
R = 0.1 MW C = 0.16 mF
Why is it called a high pass filter?
Low frequency means denominator is small and Vo / Vi 1
High frequency means denominator is large and Vo / Vi is small
Note the half power frequency again.
Full wave rectifier, using ground loop on each half cycle
Full wave rectifier, using ground loop on each half cycle
Combines the effects of a capacitive and L-section filter. Regulation is poor when load varies, however.
For a large change of current, voltage remains at VZ
Zener acts like an automatically varying resistor. Can be obtained with VZ from 2.4 – 200 V.
Since the load is in parallel with the diode, the voltage drop across RL is always the same as across VR1 and is VZ = constant Zener voltage
The input voltage V must be greater than VZ.
Zener MUST be operated under load. If not, the zener is still delivering power (more than usual) and may melt. Recall that the zener can draw large currents all at the same voltage.
R protects D from surges
When D is conducting, C charges to Vpeak
During reverse bias half cycle, C discharges through load at the peak voltage
Vout = Vpeak = 2 Vrms
First Half Cycle – CCW – D1 conducting, C1 charged. C2 remains uncharged because of the low resistance in the forward biased D1
Second Half Cycle – CW – D2 conducting, C2 charged by (source + C1) – about twice the source voltage
Next Half Cycle – Repeat of first stage with the addition that C2 can now discharge through RL. Notice that C2 is charged once/cycle, so output has the same frequency as input.
Notice that the dc sources V1 and V2 are reverse biasing the diodes in each branch.
Clockwise half cycle - VS flows to the output until it becomes equal to V1. At that point diode 1 can conduct, another path is found, and any voltage above V1 goes through the first diode branch.
Counter Clockwise half cycle - VS flows to the output until it becomes equal to V2. Excess voltage flows then through diode 2.
Output is “clipped” at V1 and V2.
VClamping at any voltage
Incoming photons collide with electrons in the valence band of the p-type material.
These electrons are promoted to the conduction band, increasing the number of minority carriers.
This increases the reverse bias. Watch the Animation
When curve crosses the horizontal axis (I = 0), we note the open circuit voltage, V0C
When curve crosses the vertical axis (V = 0), we note the short circuit current, ISC
V0C is logarithmic
ISC is linear
Si is more sensitive in the IR. Se matches the human eye better.
Since the diode provides light, no external or internal source of light is needed to see the display.
If the diodes are visible when not lit, the result can be hard to read
With no light we have a small “dark current” (Idark) due to thermal electron-hole pairs in the depletion zone.
With light the reverse bias current increases (Idet)
Dark Vr = 3 V, I = 25 mA
R = Vr/I = 120 kW
For Intensity 25000 lumens/m2
Vr = 3 V, I = 375 mA
R = 8 kW
So we can use it as a variable resistor controlled by light
Response is very linear so photodiodes are used to measure light intensity
Typically, more sensitive to longer wavelengths.
N materialLaser Diodes
Used in forward bias.
Electrons move into depletion zone and recombine with holes, producing light (like an LED).
More electron-hole recombinations can be stimulated by this photon, producing more photons at the same wavelength.
The mirrors reflect the photons back and forth through the depletion zone, stimulating more photon at each pass.
Eventually, the beam passes out of the right hand mirror.
Highly Reflective Mirror
Partially Reflective Mirror
PhotodiodeLaser Diode Application
Also used as bar code readers, laser pointers, fiber optics, etc.