Semiconductor Components: Diodes, Transistors and SCRs

The Diode and Semiconductor Fundamentals

Conductors: These are unstable atoms that easily lose electrons from their outer orbits.

Insulators: These are materials that do not allow the peripheral exchange of electrons. Their atoms are normally stable.

Semiconductors: These have an intermediate resistance to the passage of electrons. An important feature is having 4 electrons in their outermost orbit.

Voltage Threshold

The voltage threshold is the diode strength needed to start conducting.

The Transistor

Polarization of a transistor: Emitter-d, base-d, and collector-i.

Work Zones

There are three primary work zones:

1. Zone of Saturation: The transistor behaves like a closed switch. Ic = Vcc / Rc and Vce = 0V.
2. Active Area: Located between the cutting zone and the zone of saturation. Vce = Vcc / 2 and Ic = Vcc / 2Rc.
3. Cutting Zone: In this zone, the transistor behaves like an open switch. Ic is almost 0 and Vce is almost Vcc.

Beta Factor

Beta: This is the amplification factor of a transistor.

The Thyristor (SCR)

Internally, a thyristor consists of two transistors. Without applying any voltage to the thyristor gate, it is non-conductive and behaves like an open switch. For the thyristor to start conducting, it is necessary to apply a positive voltage at the gate of about 5V. This impulse saturates the two transistors, and the device behaves like a closed switch.

Vak is currently very small and is called the sustaining voltage because it maintains the diode in saturation. To stop conducting, Vak should be lowered or canceled.

Operation in AC (SCR)

During negative half-cycles, the device will not conduct because when the AC signal passes through zero, Vka is less than the sustaining voltage and the thyristor is turned off. This is why it is considered a unidirectional element. Depending on the moment we apply the trigger pulse at the gate, we can remove more or less of the input signal. This is why it is called a thyristor controlled rectifier.

The Triac

Internal Constitution

The triac consists of two thyristors in parallel and opposition. This is a bidirectional device, meaning it supports different polarizations.

Operation

When we apply a fixed polarity to the circuit, if there is sufficient voltage at the gate, the triac starts conducting because one of the two internal thyristors is polarized for that specific polarity.

The Diac

It comprises two Shockley diodes. When we apply a voltage of fixed polarity, one of the diodes conducts; if we apply the opposite polarity, the other will conduct. It is a bidirectional element.

The FET (Field Effect Transistor)

• The input resistance of the FET is much higher than that of a bipolar transistor, since the only current flowing through the gate is the reverse leakage current, which is in the order of nanoamperes.
• The voltage gain of a FET is much smaller than that of a bipolar transistor, since the variations of Vgs (for maximum variation of Id and thus Vds) must be in the order of several volts.
• There are three areas: Saturation (VDS), Active (the horizontal portion of the characteristic curve), and Cutting (Vgs <= V(p)gs).