Page 277 - Mechatronics with Experiments

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ELECTRONIC COMPONENTS FOR MECHATRONIC SYSTEMS 263
charge is positive, such semiconductors are called p-type semiconductors (Figure 5.10d).
Similarly, if the added material is from group V of the periodic table (i.e., phospore (P),
arsenic (As)), there will be an extra electron around the added impurity atom. This provides
the conductor property to the semiconductor material. Since the net added electrical charge
to the crystal structure is a negative charge due to the extra electron, this type is called n-type
semiconductor (Figure 5.10e). All semiconductor materials conduct electricity through the
holes and electrons introduced by the doping material into its crystal structure.
Semiconductor materials are the basis of electronic switches which contain various
junctions of n-type and p-type semiconductor materials. The conductivity property of these
junctions are controlled by a base current under our control or a function of the voltage
bias between the two terminals. The device acts as a conductor (closed switch) and as a
non-conductor (insulator, open switch) under different operating conditions.

5.5.2 Diodes
A diode is a two-terminal device made of a p-n junction (Figure 5.11a). It works like
an electronic switch that allows current flow only in one direction. It functions like a
one-directional check valve. It lets the flow in one direction when the pressure is above
a threshold value and it blocks the flow in the reverse direction. It is a conductor in the
positive direction, and an insulator in the negative direction. Once it starts conducting, the
resistance of it is negligable. It is like a closed switch. The voltage and current relationship
across a diode has three regions: forward bias region, reverse bias region, and reverse break-
down region. When the voltage is larger than the forward bias voltage value (V = 0.3V
F
for germanium diode, V = 0.6V to 0.7 V for silicon diode), V > V , the diode becomes
F
D
F
a conductor and acts as a closed switch. Hence, the current increases very fast indicating
very low resistance at this device. The actual value of the current is determined by the rest
of the circuit. In this region, the diode can be considered as a closed switch with negligable
resistance and a small voltage drop of V . When the diode is in the reverse bias region,
F
V < V < V , then the diode acts as an insulator or open switch. In other words, it acts
Z D F
like a very large resistor. When the V < V , the diode operates in the brea-kdown region,
D Z
and acts as a closed switch. This is referred to as the avalanche effect and a large current
flows in the reverse direction. If this reverse current is larger than a rated value for the
diode, the component will fail. The design parameters of a diode include:
1. V , forward bias voltage.
F
2. V , reverse break-down voltage.
Z
3. f , maximum switching frequency the diode can respond to.
sw
4. i , the forward average (continuous) current the diode can conduct without dam-
FWD,ave
age at a given temperature. As temperature increases, this current rating decreases.
5. i , the forward peak (short period, i.e., 10 ms) current the diode can conduct
FWD,peak
without damage. More specifically, this rating is a function of the repetetion rate (pulse
width), and as the repetetion (pulse rate) increases, this rated current decreases.
6. i , the maximum reverse current the diode will conduct while the reverse bias
REV,max
is voltage below the break-down voltage.
7. P nom nominal power dissipation at the diode, which is approximately the voltage drop
across it times the current it is conducting. Generally this is a small amount since at
all operating modes of a diode, either the voltage drop across it or the current through
it is very small.
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8. Temperature range the diode is rated to operate, i.e. −65 C to 200 C.