Unijunction transistor (UJT),Silicon-controlled rectifier (SCR) and family,Unijunction transistor letter symbols,Field effect transistor letter symbols and Common transistor and diode data.

By Ahmed Farahat - April 19, 2015

Unijunction transistor (UJT)

The unijunction transistor is a three-terminal device consisting of an n-type bar terminated in connections B 1 and B2, shown in (a). About halfway along there is diffused a p-type region called the emitter.

With the emitter open circuit, V 1 is determined by simple voltage division along the slice. This voltage is known as YEo• shown in (b). When VEB 1 is made greater than VEO• holes are injected into the bar, causing increased conductivity, and RB 1 falls. This causes the voltage at the junction of RB 1 and RB2 to fall, initiating a cumulative effect until RB 1 becomes negligibly small.

A relaxation oscillator using a UJT is shown in (c). UJT oscillators are widely used in SCR firing circuits.

Silicon-controlled rectifier (SCR) and family

The SCR is sometimes called a thyristor, although the latter term is also used to describe a whole family of devices. In essence, an SCR is a three-terminal, four-layer device as shown in (a); the correspond ing circuit symbol is shown in (b).

If a positive voltage is connected between anode and cathode, negligible current flows, as the central section is reverse biased. If the voltage is increased, the device suddenly avalanches, passing current limited only by the resistance of the external circuit. Once avalanche has occurred, the current can only be stopped by reducing it to a value below a critical level called the 'holding current'.

Avalanching can also be initiated, however, by applying a positive pulse to the gate. As before, when current flow has started, it can only be stopped by reducing the current below the holding current. A practical way to do this is to momentarily short the anode to cathode by means of a capacitor or a similar technique. The gate current required to tum on an SCR is small. A typical power SCR will control in excess of 50 A with gate currents of 20 rnA.

The regenerative action of the avalanche effect can be considered as the action of the pnp/npn transistor pair in (c). Once a positive pulse is applied to the gate, transistors TR 1, and TR2 will both turn hard on. This device is called a cathode-controlled SCR. With an npnp construction, an anode-controlled SCR is made. This requires a negative gate pulse to turn it on. SCRs can only pass current in one direction. By utilising a combined anode-controlled and cathode controlled device in the one package, a bidirectional device called a triac is produced. The construction and circuit symbol are shown. The triac is widely used to control AC circuits.

A four-layer, two-terminal device can be made, utilising the avalanche effect described above. This device is called a diac. The device presents a high impedance between MT1 and MT2 until the avalanche voltage is reached. Conduction then starts, voltage falls to a low value, and current is limited solely by external resistance. As before, current only ceases when it is taken below the holding value.

Bipolar transistor letter symbols