电子技术基础笔记
PREFACE
Copyright: WRITTEN BY RYKER ZHU in Shanghai under CC BY-NC-SA
Digital Integrated Circuits
Analog Integrated Circuits
Basic Physics of Semiconductors
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The sharing of valence electrons, i.e. electrons in the outermost shell, produces strong covalent bonds that hold the atoms together.
A piece of intrinsic (pure) silicon at room temperature has, at any instant, a number of conduction-band (free) electrons that are unattached to any atom and are essentially drifting randomly throughout the material while an equal number of holes are created, which is called intrinsic excitation.
P-N Junction
The conductivity of silicon can be drastically increased by the controlled addition of impurities to the intrinsic semiconductor. This process, called doping, increases the number of current carriers (electrons or holes), thus increasing the conductivity and decreasing the resistivity. The two categories of doped intrinsic silicon are N-type and P-type.
When a piece of intrinsic silicon is doped so that half is N-type and the other half is P-type, a PN junction is formed between the two regions where the area on both sides of the junction becomes essentially depleted of any conduction electrons or holes and is known as the depletion region.
Reverse Breakdown
Bias
The term bias in electronics refers to a fixed DC voltage that sets the operating conditions for a semiconductor device. Forward bias is the condition that permits current across a PN junction while the reverse bias prevents current across it.
Avalanche Breakdown
| If the external reverse-bias voltage is increased to a large enough value, avalanche breakdown occurs. Just like nuclear fission, when a free electron acquires enough energy, i.e. higher voltage than the \(V_{\mathrm{BR}}\) (see the figure below), from the external source to accelerate it toward the positive end of the diode and then it would collide with atoms so there will be more valence electrons turned into free electrons, resulting in a rapid buildup of reverse current. |
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Diode Models and Circuits
| There is nearly no forward current for forward voltages below the barrier potential (0.7V on the figure, which is the typical for silicon and the germanium's is 0.3V) but as the voltage reaches it, the current increases drastically and must be limited. For a forward-biased diode, the voltage is also called the diode drop. |
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Special-Purpose Diodes
Zener Diode
For the general purpose diodes, it should not operate in the reverse-breakdown region but the zener diode does. From the figure above, when the diode is reverse biased and reaches \(V_{\mathrm{BR}}\), the current would increase drastically but the voltage remains almost unchanged.
Bipolar Junction Transistors
The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions called emitter, base, and collector, where each of two are separated by two PN junctions.
