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
piecewise linear model
Shockley’s equation and the equation for junction capacitance give a very precise model of a diode, but his takes a lot of math.
For this reason, diode are typically modeled as piecewise linear models. The more precision needed, the more complicated the model
ideal diode
the model of an ideal diode has
- zero resistance in the forward bias
- infinite resistance in the reverse bias direction
piecewise linear model
diodes are usually operated so the nonlinear characteristics can be ignored
tunnel diode (Esaki diode)
- very high doping concentration
- very narrow unbiased depletion region
- electrons need only very small forward bias to cross narrow barrier (tunneling)
- as voltage increases, holes in p-type material fill, barrier widens, and current decreases (negative resistance)
- as voltage increases further, behaves as forward biased diode
Zener diode
- designed to operate within breakdown
- high doping concentration
- behaves as normal diode in forward bias direction
- behaves as voltage source when reverse bias exceeds breakdown voltage (zener voltage)
avalanche
when the anode-to-cathode voltage in a zener diode exceeds breakdown voltage,
- the electrons in the depletion region are accelerated to high velocities
- these electrons collide with atoms
- this releases more free electrons which get accelerated and collide and so on (called the avalanche)
- the junction conducts (suffers breakdown)
the diode is not destroyed by the breakdown (unless too much high current is applied), but it now behaves as a voltage source
Zener diode (continued)
The forward bias resistance rf, and the forward bias voltage, V0, are the same as for ordinary diode
The avalanche resistance, ra, is very small, even compared with rf, and can be ignored
For an ideal zener diode, V0, rf, and ra are all equal to zero
transistor
- a three-terminal (two-port) device
- made from three semiconductor layers to give two pn junctions
- layers may be npn or pnp; middle layer is thin
- two major types: bipolar junction transistor (BJT’s) & field-effect transistors (FETs)
bipolar junction transistor (BJT)
- three terminals are
- -> base (thin middle layer)
- -> emitter
- -> collector
- current flowing into the base controls amount of current flowing between collector and emitter
gain, gm
gm = IcQ/Vt
function of quiescent current, ICQ, and thermal voltage VT
small-signal collector-to-emitter gain,
typical values could be > 100
small-signal base-to-emitter gain
typically 0.99
active region
the collector current is linearly controlled by the base current
saturation region
the collector-to-emitter and base-to-emitter voltages are fixed. Further increases in base current do not decrease those voltages
cutoff region
the transistor is reverse biased and effectively open
bias circuit
a bias circuit places a voltage or current on one terminal of a semiconductor component to put it into a state where small changes in the input at that terminal will cause a large, linearly related response between the other two terminals
Capacitors are included to isolate the DC bias from the AC input and output. The capacitors are effectively
- open circuits to the DC bias j
- short circuits to the AC
bias circuit (continued)
- the DC biases the transistor into a linear region of operation
- the AC passes the capacitor and shifts the operating point of the transistor up and down in the linear region, effectively moving up and down among the family curves
- this moves the output up and down amplified by the gain of the transistor
- the output passes the capacitors on the output and the result is an amplified version of the input on the output
field-effect transistor (FET)
- bidirectional device using electric field to control conductivity of a channel of one type of charge carrier in a semiconductor material
two main types:
- junction gate field-effect transistor (JFET)
- metal oxide semiconductor field-effect (MOSFET)
The connections are
- gate
- drain
- source
The voltage on the gate affects the electric field within the transistor, changing the conductance from the source to drain
n-channel junction FET (JFET)
an increase in gate-source voltage causes a decrease in drain-source current and vice versa
The pinch-off voltage, Vp
- depends on the construction of the FET
- is analogous reverse vias in a BJT
zero-biased n-channel JFET
- JFET characteristics are measured with source shorted to gate
- as VDS, increases, carriers are depleted
small-signal JFET model
- Once amplifier circuit has been biased, transistor can be replaced by small-signal approximation
- Circuit can be analyzed with small signal approximation, ignoring capacitance impedance
- Circuit analysis is simple with the small-signal approximation model
- The small-signal low-frequency circuit model is the same for depletion and enhancement FETs
depletion mode MOSFET
- similar to JFET:
- -> Can be used in enhancement mode
- -> susceptible to catastrophic breakdown
n-channel:
VG=VGS
VD=VDS
p-channel:
VG=VSG
VD=VSD
operational amplifier (op amp)
electronic device used to perform mathematical operations on analog signals
- has 2 inputs:
- -> V1 ( + or noninverting)
- -> V2 (-, or inverting)
- has one output: Vo
Vo= Av (V1-V2)
-current flowing in or out of + or - is very small - gain (in amps) is large (>10^4 A)
ideal operational amplifier
theoretical device in which
- the current flowing in or out of + or - is zero
- gain (in amps) is infinite
- V1 - V2 is zero (when operating linearly)
ideal and real
most op amp circuits that might be on the exam can be analyzed as special bases of the general op amp circuits
inverting and noninverting are special cases of the same circuit
ideal and real (continued)
the current produced by the output of the op amp is whatever value is needed to keep the voltage at the inverting (-) input equal to Vb (called negative feedback )
input impedance
usually high in operational amplifier circuits so that they do not affect input signal circuits
important in op amp applications (might be on the exam)
For a simple inverting amplifier (Vb=0) input impedance will be R1, because the voltage at the input of the op amp will be maintained at zero volts
For a more complicated circuit, usually best to calculate input impedance as input voltage divided by input circuit
noninverting op amp
- ideal op amp where va is zero
- output produces voltage that keeps the input voltages the same (V1=V2)
summing amplifier
a variation of the inverting amplifier
- can treat currents as independent forces trying to push electrons into (or out of) node A (superposition theorem)
- feedback current must be equal and opposite to sum of other currents
noninverting summing amplifier
- can be made with voltages summed into + input
- for noninverting amplifier,
V0 = ( 1 + R1/R2) V1 - to find summing amplification, Vo, must first find + input, V1
- best way: superposition theorem
- then formula at left give Vo
differential amplifier
- first stage of an operational amplifier
- amplifies the difference of the two input signal voltages
- differential voltage (preferred mode) is defined as
Vid= V1-V2 - the common-mode voltage (undesired mode) is defined as
Vicm =( V1 + V2)/2
measuring instrument
commonly used to describe a measurement system, whether it contains only one element or many elements
measuring system
exists to provide information about the physical value of some variable being measured
transducer
- any device that converts one form of energy into another form of energy
- for example, a loudspeaker converting electrical signals into sound
- a key element of a measuring systems
sensitivity
the ratio of the change in magnitude of the electrical signal to the change in magnitude of the parameter (the physical phenomenon) being measured
linearity
the degree to which the output of a transducer is in direct proportion to the parameter being measured
measurement accuracy
a measurement is said to be accurate if it is substantially unaffected by (i.e. is insensitive to) all variation outside the measurer’s control
measurement precision
A measurement is said to be precise if an experiment is repeated with identical results
- the two terms are not synonymous
Measurements could be precise and yet be inaccurate due to measurement bias
stable measurement
A measurement is said to be stable if it is insensitive to minor changes in the measurement process
Stability and insensitivity are synonyms (As are sensitivity and instability)
uncertainty
a measurement:
- tell us about a property of something
(how heavy an object is, or how hot or how long) - gives a number to that property
- is always made using an instrument of some kind (ruler, stopwatch, weighing scale, thermometer)
resistance temperature detectors (RTDs)
- also known as resistance thermometers
- change resistance in a predictable way in response to changes in temperature
- first-order approximation is sufficient in many practical applications
strain gauge
metallic resistance device whose electrical resistance varies in proportion to the amount of strain in the device
gage factor, GF
- strain gauge sensitivity factor
- ratio of the fractional change in resistance to the fractional change in length (strain) along the detecting axis of the gauge
- from a practical standpoint, however, the gage factor and gauge resistance are provided by the gauge manufacturer. Only the change in resistance is measured
Wheatstone bridge
- one type of resistance bridge
- can be used to determine unknown resistance of a resistance transducer
- typically one resistor is adjustable and is adjusted with the voltage Vo is zero
- bridge circuit is referred to as balanced when Vo is zero
quarter bridge circuit
- special case of the Wheatstone bridge circuit
- has 3 identical resistors and a 4th resistor that differs slightly in resistance from the other 3
- the difference, delta R, can be positive or negative
