Semiconductors Intro
P-N
[2] Ch3
- P stands for positive (+): Carries holes (misses \(e^-\))
- N stands for negative (-): Carries \(e^-\)
Silicon has 4 valence \(e^-\), and is the typical semiconductor base. It is doped with:
- Boron, owns 3 valence \(e^-\), lacks a \(e^-\): P type (anode)
- Phosphorous, owns 5 valence \(e^-\), donates one: N type (cathode)
Put P and N together: Diode (aka PN junction)
Diodes, simplified
Let the diode have a fixed voltage drop \(v_D\) (also known as cut-in voltage \(v_{\gamma}\)).
If \((V_{in} > v_D) :\ V_{out} = V_{in} - v_D\) -> Behaves like a resistor
Else: \(i_D = 0\) -> Open circuit
Standard values: \(v_{\gamma} = 0.7\) for simple diodes, \(v_{\gamma} = 1.8V\) for red LEDs.
Zener Diodes
Normal diodes get damaged if a high reverse voltage is applied. A zener diode is designed to allow reverse current by operating at a special voltage \(V_Z\):
If \((V_{in} > v_D) :\ V_{out} = V_{in} - v_D\)
If \((V_{in} < -V_Z):\ V_{out} = V_Z\)
Else: \(i_D = 0\) -> Open circuit
\(V_Z\) is usually set to be much higher than \(v_{\gamma}\), over 6V.
AC to DC
Rectification:
- Diode rectifier
- Filter
- Voltage regulator
Diode rectifier
Type 1: Four diodes in diamond
- The current flows through 2 diodes
Type 2: Two diodes at each end
- The current flows through 1 diode
- The receiving winding is center-tapped to ground, in parallel to load
Filter
A capacitor between \(V_{out}\) and ground, parallel to the load.
It creates ripple voltage:
\[V_r = \frac{V_M}{2fRC}\]Where \(V_M\) is the maximum voltage at \(V_{out}\).
Voltage Regulator
An ideal diode has a constant voltage. So we put one in parallel to our load to get DC.
The voltage drop for a normal diode \(v_{\gamma}\) is usually low. So we use a Zener diode in reverse bias to use the high voltage \(V_Z\).
A voltage regulator has a safe operating range (min and max \(V_Z\)). \(R_i\) helps protect the diode.
Let \(I_Z\) be the current through the diode.
\[I_Z(min) = \frac{V_{in}(min) - V_Z}{R_i} - I_L (max)\] \[I_Z(max) = \frac{V_{in}(max) - V_Z}{R_i} - I_L (min)\]
Example: let \(V_Z = 9\ V,\ V_{in} = (11, 13.6)\ V,\ I_{RL} (max) = 100\ mA.\) If \(I_Z (min) = 0.1 I_Z (max)\), what should be \(R_i\)?
Transformer overview
Other diode uses
Clipper circuit
Clip the input voltage to a limit
Let \(V_B\) be a constant DC volt source, with polarity against the diode.
The voltage between the diode and ground remains the same, since \(v_{\gamma}\) and \(V_B\) do not change.
Clamper circuit
Shift a DC voltage without changing its “shape”