Download Series 5- Thyristors and Compound Semiconductors

Series 5- Thyristors and Compound Semiconductors
Solutions:
5.1
U sinusoidal; R=100 Ω or R=10 kΩ
• 0 < t < T/4 : PR=0
• T/4 < t < T/2: PR =U2/R
• T/2 < t < T : PR =0
U =100 V; R=100 Ω
• 0 < t < T/4 : PR=0
• T > T/4: PR =100 W
U =100 V; R=10 kΩ
• 0 < t < T/4 : PR=0
• T/4 < t < 3T/4: PR =1 W
• 3T/4 <t <T: PR=0
5.2
•
•
•
•
•
0 < t < t1 : UA=200U/201 ; I=UA/RT with RT=200 kΩ
t1 < t < t2 : UA=2,5 V ; I=(U-2,5)/R
t2< t < t3 : UA=200U/201 ; I=UA/RT with RT=200 kΩ
t3< t < t4 : UA=-220 V; I=(U-220)/R
t4< t < T : UA=200U/201 ; I=UA/RT with RT=200 kΩ
ωt1=arcsin(201/300); ωt2 it is close to π; ωt3=arcsin(-221,1/300) < 3T/4;
ωt4=arcsin(-221,1/300) > 3T/4;
5.3
a)
b)
c)
d)
W=1,243/λ with W in eV and λ in µm.
x = 0,5y + 0,532
In0,61Ga0,39P0,16As0,84.
θL=66,5º
Goal: Analysis and understanding of the operation of the thyristor as an electronic
switch.
Conclusions (thyristors):
1.
Thyristors are power devices. They can have two terminals (anode and cathode) or
three terminals (anode, gate and cathode). They are structures with 4 or more
alternate layers p and n. In the structures p-n-p-n there are three junctions, named J1,
J2 e J3.
2. Thyristors are basically electronic switches that command the energy flux from the
source to the load. In the 1st quadrant they have two distinct zones. In the first zone,
the thyristor presents a high resistance (ideally, it can be considered as an open circuit):
the thyristor is blocked (OFF state). In the second zone, the thyristor presents small
voltages (ideally, it can be considered as a short circuit): the thyristor conducts (ON
state). In the 3rd quadrant the characteristic of a thyristor is similar to the one for a
diode.: blocked till the disruption. The latter situation is attained when both junctions
J1 and J3 breakdown. As this situation is thermally unstable it should be avoided .
Therefore, at least one of the junctions J1 or J3 should be associated to a high
modulus of the breakdown voltage.
3. In the blocked zone of the 1st quadrant, junctions J1 and J3 are forward biased and J2 is
reversed-biased. The transition OFF/ON corresponds to the situation for which J2 is
also forward-biased. It is necessary that the anode-cathode voltage attains the
breakdown voltage(UB), which in thyristors has typical values of hundreds of Volt.
After the commutation, this voltage is negligible, since it corresponds to the voltage of
three forward biased Si junctions in series. In the commutation, there is a discontinuity
in the voltage and current of the thyristor. In order to reach the commutation, the
thyristor current should attain a minimum value that is called the latch current (IL). The
commutation ON/OFF is possible when the current is smaller than the holding current
(IH). The system presents hysteresis, since the paths OFF/ON and ON/OFF are different
4. The 4 layer diode has two terminals. The voltage UB is fixed. The Silicon Controlled
Switch (SCS) or Silicon Controlled Rectifier (SCR) is a four layer structure with three
terminals. The 3rd terminal is called gate ainda it allows the control of the voltage UB.
With positive gate pulses of adequate duration, the voltage UB decreases, making the
commutation OFF/ON easier. For amplitude pulses higher than a given value, generally
specified in catalogue, the voltage UB vanishes, meaning that the commutation is
possible at the right moment the pulse is applied, as long as the input voltage is
positive.
5. In alternate sinusoidal regime, the commutation ON/OFF is possible when the input
voltage vanishes. For constant amplitude voltages, the commutaion should be forced.
In effect, negative gate pulses are not used to remove thyristor from conduction, since
the required values would be too high. Instead, special structures are used, such as the
GTO (Gate-turn-off) thyristors.
6. There are 5 layer structures that basically correspond to two SCS in parallel and
opposite polarities. They allow the conduction in both quadrants and the energy flux
control in either half cycles of the input voltage. The conduction period may be
doubled. These structures are called TRIAC (Triode for alternating current).
7. Thyristors are structures that are intrinsically non one-dimensional.
8. There are basic models that explain the thyristor operation using two BJT, one p-n-p
and the other n-p-n, each one having its base zone connected to the collector zone of
the other. In the structure p-n-p-n, the two interior layers has characteristics of base
regions: the 2nd layer (n) represents the base of the p-n-p BJT; the 3rd layer (p) is the
base region of the n-p-n BJT. The exterior layers has characterists of emitter zones. The
1st layer (p) corresponds to the emitter region of the p-n-p BJT; the 4th layer (n)
corresponds to the emitter region of the n-p-n BJT. It should be referred that these
two “transistors” have long bases, that is, they are associated to small values for the
current gain αF, which is not typical in conventional BJTs. Only this way it is possible to
ensure the blocked zone of the 1st quadrant, which corresponds to having both
transistors in the active zone. The thyristor conduction zone corresponds to having
both transistors saturated (the situation that would be found in this circuit using
typical BJTs). The voltage UB corresponds to the commutation OFF/ON, which is
attained when the sum of the current gains for each “transistor” is 1.
9. It should be enhanced that the previous considerations are obtained using the
stationary characteristics of the thyristor. They are valid as far as the dynamical
aspects relative to the transitions between states are respected. These aspects
correspond to the periods of time needed for the junction J2, depleted of carriers, be
enriched in the transition OF/ON, or in order that junctions J1 e J3, full of carriers, be
depleted in the ON/OFF transition. For instance, in the transition OFF/ON, the pulse in
the SCR should last at least the turn-on time, which depends either on the thyristor
features or in the load of the circuit.
Goal: compound semiconductors: analysis of their properties taking into account their
applications.
Conclusions (Compound Semiconductors):
1. They can be binary, ternary or quaternary.
2. The ternary compound are generally represented as AxB1-xC, in which elements A and B
are of one type, for instance, of valence 3, and C is from other type, for instance, of
valence 5. Indexes of A and B represent the weights of these elements in their
combination with the element C. Therefore, these indexes must sum 1 and must fall
into the range 0 < x <1. For x =0, we will have the binary compound BC; for x =1, we
will have the binary compound AB. Therefore, AxB1-xC represents not a compound but
a family of ternary compounds.
3. In general, the properties of a ternary compound are not a linear combination of the
same properties related to the associated binary compounds. It is the case for the gap
and for the carrier mobility, for instance. However, other properties can be assumed
as a linear combination of their values for the binary compounds, such as the lattice
constant a or the electric ε
aABC = x aAC +(1-x) aBC
εABC= x εAC +(1-x) εBC
4. In general, ternary compound are obtained in lab by epitaxial techniques using special
binary compound substrates. The most common used substrates are InP and GaAs.
Therefore, in order that the device properties are not commanded by the interface
properties, the materials in the hetero-junctions InP/AxB1-xC or GaAs/AxB1-xC should be
matched, that is, their lattice constants must be equal:
aGaAs = x aAC +(1-x) aBC para o GaAs
aInP = x aAC +(1-x) aBC para o InP
5. In quaternary compounds, two elements are of a given type and the other two are of
the other type, being commonly represented as AxB1-xCyD1-y. The use of quaternary
compounds instead of ternary compounds allows the inclusion of an additional
freedom degree. This gives the possibility of taking into account some other important
property of the material for a specific application.
6. For instance, in the important area of Optical Communication it is particularly
important the emission of light in the vicinity of λ=1,55 µm, due to the best
performances presented by fiber optics in this range of wavelengths. This fact imposes
a condition on the semiconductor band gap (WG=hc/λ); the other condition has been
referred in the previous section, which ensures a perfect matching between the
quaternary compound and the binary used for the substrate (aABCD = aInP). Both
conditions lead to the calculation of weights x and y that will define the quaternary
compound for the specific application.