Semiconductor Electronic Components 1 Semiconduct or Electronic Components Outline from Physics 520: Electronics Fengning Ding A.Junction Diodes 1.Consists of a P-type material and an N-type material joined together. 2.Between the P and N regions, the movement of holes and electrons create depletion region a.The N-side takes on positive charge, and the P-type takes negative charge b.Depletion stops when the barrier voltage repels further movements of electrons and holes i.0.3 volts for germanium, 0.7 volts for silicon 3.When we apply bias voltage, if we make the N-side (Cathode) more negative than P-side (Anode), we getforward bias . a.The extra voltage causes the electrons that acc umulated in anode side of depletion zone move away to the positive terminal. b.Once barrier voltage is exceeded and neutralized, diode can conduct current. c.Normally, diode resistance is very low, so main resistance of circuit is in the resistor. i.To calculate the current following, we must take the battery voltage, subtract the barrier voltage, and then divide by external resistance. 4.If the anode is made more negative than the cathode, we get reverse bias. a.The depletion zone increases b.Diode acts as open circuit. Only small leakage currentor reverse currentcan flow. 5.Differences between silicon and germanium diodes: a.Silicon: higher barrier potential, lower leakage current b.Germanium: lower barrier potential, higher leakage current 6.Voltage-Current graph: a.After barrier voltage exceeded, rapid increase in conduction. b.For reverse bias, after breakdown bias, back conduction occurs. This could damage diode by overheating it.
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1. Consists of a P-type material and an N-type material joined together.2. Between the P and N regions, the movement of holes and electrons create depletion region
a. The N-side takes on positive charge, and the P-type takes negative charge
b. Depletion stops when the barrier voltage repels further movements of electrons and holes
i. 0.3 volts for germanium, 0.7 volts for silicon
3. When we apply bias voltage, if we make the N-side (Cathode) more negative than P-side
(Anode), we get forward bias.
a. The extra voltage causes the electrons that accumulated in anode side of depletion zone move
away to the positive terminal.
b. Once barrier voltage is exceeded and neutralized, diode can conduct current.
c.
Normally, diode resistance is very low, so main resistance of circuit is in the resistor.i. To calculate the current following, we must take the battery voltage, subtract the
barrier voltage, and then divide by external resistance.
4. If the anode is made more negative than the cathode, we get reverse bias.
a. The depletion zone increases
b. Diode acts as open circuit. Only small leakage current or reverse current can flow.
5. Differences between silicon and germanium diodes:
a. Silicon: higher barrier potential, lower leakage current
b. Germanium: lower barrier potential, higher leakage current
6. Voltage-Current graph:
a. After barrier voltage exceeded, rapid increase in conduction.
b. For reverse bias, after breakdown bias, back conduction occurs. This could damage diode by
1. Usual for diodes with zener voltage of more than 5 volts.
ii. Some have negative voltages
iii. Possible to build temperature compensated zener diodes consisting of a forward-
biased junction diode (negative temperature coefficient) in series with reverse biased
zener diode (positive temperature coefficient of same magnitude)
5. Function: Voltage regulation
a. The unstable input voltage create fluctuating current
b. Since zener diode maintains an almost constant voltage drop even with large changes of
current, the output is effectively stable.
C. Bipolar Transistors
1. Two types: PNP and NPN:
a. Three leads: Emitter, base, and collector.
i. Base is much thinner than emitter and collector
b. Voltage is applied at emitter, base, and collector.
i. Biasing: the emitter-base junction must be forward biased, and the collector-base
junction must be reverse biased!
2. Amplification:
a. Current flows into emitter and, with little resistance, into the base. Since the base is so thin,
the electrons over flow into the collector, with very little current coming out of the base.b. When resistor is place in series with the collector lead, since the resistance of collector-base
junction is so large (due to reverse bias), the extra resistor will not significantly lower the
current. We can take voltage drop across that resistor.
c. An external DC voltage is needed to properly bias the components.
3. Common Base:
a. The base is “common” to both branches of circuit.
c. We can define current gain alpha (less than 1) as the ratio of output current (current from
collector) to input.
4. Common Emitter:
a. The battery from both branches is directly connected to emitter.
b. This circuit magnifies power.
c. The output signal is 180 degrees out-of-phase.
5. Common Collector (or Emitter follower):
a. The batteries are directly connected to the collector.
b. Primarily used to magnify input.
6. Transistor Ratings:
a. Collector breakdown voltage: the break down voltage of collector-base junction.
b. Emitter breakdown voltage: breakdown voltage of emitter-base junction
c. Maximum collector dissipation: Since most power is dissipated as heat in collector junction(it is reverse biased), this number gives the maximum safe value.
7. Schematic:
a. The arrow indicates the emitter, the bar represents the base
b. For NPN, the arrow points out (Never Points iN). For PNP, the arrow points in (Points iN
1. Two types: Junction Field Effect Transistor (JFET), and Insulated Gate Field Effect Transistor
(IGFET) or Metal-oxide Semiconductor Field Effect Transistor (MOSFET).
2. Junction Field-Effect Transistors: operates in depletion mode
3. Construction:
a. Made of a substrate (P or N) within which an oppositely-doped region is formed to form a U-
shaped P-N Junction (Channel)
b. Specified as P-channel or N-channel JFET
c. Three terminals: Source, Drain, Channel. In most JFETS, the source and drain are identical.
4. Two bias voltages: one between source and drain to force current to flow through channel, and
one between gate and source to control amount of current flowing
a. For an N-channel JFET, the drain-to-source voltage is applied so the source would be more
negative than the drain. Current flows since the voltage forces the electrons in the N-channel
to flow from source to the drain. This is called drain current, I_D.
b. The gate-to-source voltage is chosen so the P-type gate is more negative than the N-type
source. This reverse biases the PN Junction of the gate and the channel. As the gate-to-source
voltage is increased, the depletion zone increases, and less drain current flows
c.
The gate-to-source voltage is the input, and the drain current is the outputd. When the gate-to-source voltage is increased past a certain point, the channel is depleted of
majority carriers, so practically no drain current flows. This voltage is called gate-to-source
cutoff voltage
e. As the drain to source voltage increases, the drain current increases, but the depletion zone in
channel also increases. When the drain-source voltage exceeds the pinch-off voltage, drain
current no longer increases. The pinch-off voltage usually equals the gate-source cutoff
voltage.
5. Transconductance: defined as drain current divided by gate-source voltage when drain-source
voltage is held constant. This measures amplification ability.
a. Bias: the anode is made more positive than cathode. If there is no voltage at the gate, no
current conducts since the emitter junction of Q2 is not forward biased. Of course, when the
voltage is big enough (larger than the forward breakover voltage), the element will conduct.
b. If the gate is made more positive than cathode, Q2 starts to conduct since the emitter-junction
of Q2 is forward biased. More technically, the gate voltage will lower the forward breakover
voltage.
c. Key: This forces base current through Q1, allowing Q1 to conduct. But then, current willflow into the base of Q2, allowing it to continue conducting, even if the gate voltage is
removed !! The minimal current flowing through anode to cathode required to maintain this
loop is called the holding current.
d. To turn off the SCR again, we must reduce the anode-cathode voltage to almost zero.
e. The SCR will conduct only a small current if the anode was made more negative than
cathode. The SCR will have a reverse breakdown voltage. When this voltage is exceeded,
damage could occur.
f. For an SCR to be most effective, we must not apply gate voltage constantly, but in pulses.
4. Applications: control the DC and AC power to various loads.
F. Bi-Directional Triode Thyristors
1. Also known as a triac
2. Same switching characteristics as SCR, but can control current in both directions (current can
flow from cathode to anode or from anode to cathode)
3. Gate voltage can be more negative or more positive than Main Terminal (MT) 1
4. Equivalent to two SCR in parallel in opposite directions, with a common gate voltage
5. Disadvantages compared to SCR: less different ratings available