Electronics Flashcards
semiconductor
- has conductivity less than conductor but greater than insulator
- current conducted through movement of charge carriers
- -> free electrons (negative charge)
- -> holes (positive charge)
- electrons and holes flow in opposite directions
The electrical conductivity of a semiconductor is
- poor in tis natural state
- easily modified by adding impurities (doping)
- easily controlled by applying an electric field
semiconductor device
- a device constructed so that is conductivity can be controlled by an electric field
- essentially, a controllable resistor
Two basic and common semiconductor devices are:
- diodes
- transistors
Diode
- passes current in one direction
- conductivity between terminals controlled by voltage between terminals
the conductivity of a diode is either
- very high (diode is forward biased)
- very low (diode is reverse biased)
transistor
- voltage or current applied to one pair of terminals controls conductivity between another pair of terminals
- can be used to switch current on/off
- small signal can control much larger signal, so can be used as amplifier
- can be modeled as a two-port (three - terminal) device with three variables – current, voltage, and impedance for each port
performance model
- the performance of all semiconductor devices is inherently nonlinear over the full range of response
- in small-signal analysis, performance is evaluated over small variations in input (signal) voltage
- within each of these smaller performance regions, a straight line can approximate the device’s behavior
- for this reason, a linear model is used within each performance region, and the overall model is called piecewise linear model
doping
adding impurities to a semiconductor material to increase number of free electrons or holes
- increasing number of free electrons produces n-type material
- increasing number of holes produces p-type material
pn junction
- layer of p-type material adjacent to layer of n-type material
- basic building block of semiconductor devices
n-type material
the conduction band contains a high density of electrons (negative charge), which are the majority charge carriers
p-type material
the valence band contains a high density of holes (positive charge), which are the majority charge carriers
depletion region
- region immediately around junction
- few holes, few free electrons
- also called space charge region
diffusion current
- free electrons flow from n-type material to holes in p-type material
- dope atoms retain charge, resulting in potential difference across junction, called built-in potential, Vbi
drift current
- free electrons pushed by electric field from p-type to n-type material
- direction depend on polarity of field
equilibrium
- when diffusion current and drift current exactly balance
- occurs only with forward bias voltage
bias
the DC voltage applied to a semiconductor junction
zero bias
no externally applied voltage
equilibrium condition: diffusion current creates built- in potential, which creates electric field that opposes diffusion process
forward bias
- positive voltage applied to p-type material
- strong force pushes holes in p-type material and electrons in n-type material toward junction
- static charge on depletion region must be overcome
- results in current from p-type material to n-type material
reverse bias
- positive voltage applied to n-type material
- strong force pushes holes in p-type material and electrons in n-type material away from junction
- depletion region widens
- results in strong barrier to current flow
built in potential of a pn junction, Vo
Vo= (kT/q) ln (Na Nd/ni^2)
- in natural state, there is no significant current across junction; free electrons and holes in n-type and p-type materials are in equilibrium
- to obtain significant current, free electrons and holes must overcome built-in potential, Vo
- Vo is due to equilibrium between diffusion current and drift current, which depend on doping concentrations, Na and Nd, and intrinsic concentration of carriers, Ni
Thermal voltage, Vt
Vt= kT/q
- useful quantity for establishing the behavior are a pn junction
- affects many important semiconductor parameters
Characteristic Curve
a real diode exhibits leakage current, Is, when reverse biased
in reverse bias region, leakage current is small, consisting mainly of drift current
When diode is reverse biased beyond its breakage voltage, Vr (also called Zener voltage, Vz or avalanche voltage), diode renter its breakdown region (also called avalanche region)
Shockley equation
- give approximate current through diode, iD, as function of voltage across diode, VD
- not valid in breakdown region (that is valid for VD, > VZ)
leakage current, Is
- Depends on construction of diode
- also called saturation current or reverse saturation current
emission coefficient, N
- property of semiconductor
- usually
- 2 for silicon ( discrete components)
- 1 for silicon ( integrated circuits)
- 1 for germanium
half-wave rectifier
- half of symmetric AC signal gets through
- used in AC-to-DC converters
full-wave (bridge) rectifier
- current always in same direction
- used in AC-to-DC converters
- more efficient than half-wave rectifier
clamping circuit
- shifts DC component of signal
- output voltage is
Vout= Vin + Vp - Vm
Vin= input voltage
Vp= clamping voltage
Vm= maximum voltage of input
for a clamping circuit output with a sinusoidal input:
average voltage
Vave= Vp - Vm
Rms voltage
Vrms= square root (Vp^2 + Vm^2)/2
clipping circuit
clips waveforms
- base clipper
- peak clipper
- valley clipper
- combined clipper
junction capacitance, C
C(V) = Co/ square root ( 1- V/Vbi)
- the electric field in depletion region of a pn junction and the buildup of static charge create capacitance
- the capacitance of pn junctions is usually not significant to the operation in practical circuits
junction contact potential, Vbi
same voltage as the built-in potential of a pn junction, Vo