UJT Relaxation Oscillator and Sawtooth Generator

Study Notes

A UJT (Unijunction Transistor) is commonly used in a relaxation oscillator (or sawtooth generator) to trigger other switching devices, such as the silicon-controlled rectifier (SCR).
The oscillator circuit consists of a capacitor C, resistor R, and switch S; when S is closed, C charges up through R.
When the capacitor voltage (Vc) reaches the peak voltage (VP), it overcomes the reverse biasing of the junction EB1, causing the junction resistance to drop to a very low value and C to discharge quickly.

Four-layer switching devices are referred to as thyristors, which can be two-, three-, or four-terminal devices.
The silicon-controlled rectifier (SCR) is a three-terminal four-layer diode, also known as a thyristor, and is very useful in many applications, such as power switching and control circuits.

The pnpn diode is the basic structure and simplest member of the thyristor family, consisting of four alternately doped silicon layers and three pn junctions (J1, J2, and J3).
The two outer terminals are the anode and cathode; the anode terminal is connected to the p-region, and the cathode terminal is connected to the n-region.
The device has two operational states: ON (three junctions forward biased) and OFF (infinite resistance between anode and cathode).

In the OFF state, the applied voltage appears across the reverse-biased junction J2, with very small current flowing.
When the anode voltage is increased beyond a certain value (firing or breakover voltage, VBO), the device starts to conduct heavily and the potential difference between the anode and cathode decreases suddenly, resulting in a negative resistance region on the I-V curve.
This is followed by a region of very low resistance, where the device is in the ON state.

The two-transistor model explains the behavior of the four-layer diode, where the device is composed of an npn and a pnp bipolar transistor connected in a specific manner.
When the anode-to-cathode voltage is increased, the current increases due to avalanche multiplication at the reverse-biased junction J2.
The collector current of Q1 is fed to the base of Q2, and vice versa, which drives both Q1 and Q2 into saturation, resulting in a very small voltage across the diode (ON state).

The SCR is the most widely used of the thyristor family, with a similar construction to a four-layer diode, but with an additional terminal, called the gate, attached to one of the base regions.
The gate current can be used to change the value of VBO, allowing more control capability.
Gate control of an SCR is very efficient, as only a small gate current is sufficient to drive the SCR into the ON state, where it can conduct a larger anode current.

A GTO is an SCR whose gate can be used to turn off the device when a negative bias is applied.
When the gate is reverse biased, holes are fed from the base, and the npn transistor is pulled out of saturation.
A specially designed GTO avoids the problem of lateral current flow and nonuniform biasing of the junctions.

A diac is a bidirectional device equivalent to two four-layer diodes in parallel with their polarities reversed.
The diac can be switched ON each half of an AC waveform.
A resistor is usually placed in series with the diac to limit the current to a safe level in either direction.

A triac is considered as two SCRs in parallel with polarities reversed and the gate terminals tied together.
A triac can be triggered on both halves of an AC waveform.
The triac representation is shown as a symbol and as two SCRs connected in parallel in reverse polarity.