The UJT is biased with a positive voltage between the two bases. It exhibits a negative resistance characteristic and so it can be employed as an oscillator. The device has a unique characteristic that when it is triggered, its emitter current increases regeneratively until it is restricted by emitter power supply. It is a current-controlled negative resistance device. A UJT is operated with the emitter junction forward-biased while the JFET is normally operated with the gate junction reverse-biased. The structure of a UJT is similar to that of an N-channel JFET, but p-type (gate) material surrounds the N-type (channel) material in a JFET, and the gate surface is larger than the emitter junction of UJT. A complementary UJT uses a p-type base and an n-type emitter, and operates the same as the n-type base device but with all voltage polarities reversed. The schematic diagram symbol for a unijunction transistor represents the emitter lead with an arrow, showing the direction of conventional current when the emitter-base junction is conducting a current. On the other hand, if an adequately large voltage relative to its base leads, known as the trigger voltage, is applied to its emitter, then a very large current from its emitter joins the current from B 1 to B 2, which creates a larger B 2 output current. If no potential difference exists between its emitter and either of its base leads, there is an extremely small current from B 1 to B 2. The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is not symmetrical, because a symmetrical unit does not provide optimum electrical characteristics for most of the applications. The resistance between B1 and B2 when the emitter is open-circuit is called interbase resistance. The emitter is of p-type is heavily doped this single PN junction gives the device its name. Two ohmic contacts B 1 and B 2 are attached at its ends. The base is formed by a lightly doped n-type bar of silicon. The UJT has three terminals: an emitter (E) and two bases (B 1 and B 2) and so is sometimes known a "double-base diode".
UJT die: the larger contact in the centre of the crystal is the emitter, the smaller one is B 1 B 2 is at the bottom of the crystal This affects the frequency of UJT relaxation oscillators. The hall effect modulates the voltage at the PN junction. UJTs can also be used to measure magnetic flux. This application is important for large AC current control. A DC voltage can be used to control a UJT or PUT circuit such that the "on-period" increases with an increase in the DC control voltage. In addition to its use as the active device in relaxation oscillators, one of the most important applications of UJTs or PUTs is to trigger thyristors ( silicon controlled rectifiers (SCR), TRIAC, etc.). Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used. For example, they were used for relaxation oscillators in variable-rate strobe lights. Unijunction transistor circuits were popular in hobbyist electronics circuits in the 1960s and 1970s because they allowed simple oscillators to be built using just one active device. The 2N6027, 2N6028 and BRY39 models are examples of such devices. The original unijunction transistor, or UJT, is a simple device that is essentially a bar of n-type semiconductor material into which p-type material has been diffused somewhere along its length, fixing the device parameter \displaystyle, they behave like a conventional UJT.There are three types of unijunction transistor: Graph of UJT characteristic curve, emitter-base1 voltage as a function of emitter current, showing current-controlled negative resistance (downward-sloping region) The original unijunction transistor types are now considered obsolete, but a later multi-layer device, the programmable unijunction transistor, is still widely available. In the 1960s, the low cost per unit, combined with its unique characteristic, warranted its use in a wide variety of applications like oscillators, pulse generators, saw-tooth generators, triggering circuits, phase control, timing circuits, and voltage- or current-regulated supplies. It is widely used in the triggering circuits for silicon controlled rectifiers. It is used in free-running oscillators, synchronized or triggered oscillators, and pulse generation circuits at low to moderate frequencies (hundreds of kilohertz). The UJT is not used as a linear amplifier. A unijunction transistor ( UJT) is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch.
0 kommentar(er)
