Unijunction transistor

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Unijunction transistors

A unijunction transistor (UJT) is a three-lead electronic semiconductor device with only one junction that acts exclusively as an electrically controlled switch. The UJT is not used as an amplifier control. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). If no potential difference exists between its emitter and either of its base leads, an extremely small current flows from B2 to B1. 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 will flow from its emitter and join the current flowing from B2 to B1, which would create a larger B1 output current.

The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2 when the emitter is open-circuit is called interbase resistance.

There are three types of unijunction transistors:

  • 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, defining the device parameter \eta. The 2N2646 is the most commonly used version of the UJT.
  • The complementary unijunction transistor, or CUJT, that is a bar of P type semiconductor material into which N type material has been diffused somewhere along its length, defining the device parameter \eta. The 2N6114 is one version of the CUJT.
  • The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consists of four P-N layers and has an anode and a cathode connected to the first and the last layer, and a gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function. In a proper circuit configuration with two "programming" resistors for setting the parameter \eta, they behave like a conventional UJT. The 2N6027 is an example of such a device.
Graph of UJT characteristic curve, emitter-base1 voltage as a function of emitter current, showing current controlled negative resistance (downward-sloping region)

The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes conductivity modulation which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.

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. For example, they were used for relaxation oscillators in variable-rate strobe lights.[1] Later, as integrated circuits became more popular, oscillators such as the 555 timer IC became more commonly used.

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 (SCR, TRIAC, etc.). In fact, 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. This application is important for large AC current control.

UJTs can also be used to measure magnetic flux. The hall effect modulates the voltage at the PN junction. This affects the frequency of UJT relaxation oscillators.[2] This only works with UJTs. PUTs do not exhibit this phenomenon.

See also[edit]

Unijunction transistor (abbreviated as UJT), also called the double-base diode is a 2-layer, 3-terminal solid-state (silicon) switching device. The device has-a unique characteristic that when it is triggered, its emitter current increases regeneratively (due to negative resistance characteristic) until it is restricted by emitter power supply. The low cost per unit, combined with its unique characteristic, have 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. The device is in general, a low-power-absorbing device under normal operating conditions and provides tremendous aid in the continual effort to design relatively efficient systems.

Construction of a UJT[edit]

Structure of a UJT
Circuit symbol for a UJT

The basic structure of a unijunction transistor is shown in figure. It essentially consists of a lightly doped N-type silicon bar with a small piece of heavily doped P-type material alloyed to its one side to produce single P-N junction. The single P-N junction accounts for the terminology unijunction. The silicon bar, at its ends, has two ohmic contacts designated as base-1 (B1) and base-2 (B2), as shown and the P-type region is termed the emitter (E). The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is not symmetrical, because symmetrical unit does not provide optimum electrical characteristics for most of the applications.

The symbol for unijunction transistor is shown in the accompanying figure. The emitter leg is drawn at an angle to the vertical line representing the N-type material slab and the arrowhead points in the direction of conventional current when the device is forward-biased, active or in the conducting state.

A complementary UJT is formed by diffusing an N-type emitter terminal on a P-type base. Except for the polarities of voltage and current, the characteristics of a complementary UJT are exactly the same as those of a conventional UJT.

The worth noting points about UJT are given below:

  • The device has only one junction, so it is called the unijunction device.
  • The device, because of one P-N junction, is quite similar to a diode but it differs from an ordinary diode as it has three terminals.
  • The structure of a UJT is quite similar to that of an N-channel JFET. The main difference is that P-type (gate) material surrounds the N-type (channel) material in case of JFET and the gate surface of the JFET is much larger than emitter junction of UJT.
  • In a unijunction transistor the emitter is heavily doped while the N-region is lightly doped, so the resistance between the base terminals is relatively high, typically 4 to 10 kilo Ohm when the emitter is open.
  • The N-type silicon bar has a high resistance and the resistance between emitter and base-1 is larger than that between emitter and base-2. It is because emitter is closer to base-2 than base-1.
  • UJT is operated with emitter junction forward- biased while the JFET is normally operated with the gate junction reverse-biased.
  • UJT does not have ability to amplify but it has the ability to control a large ac power with a small signal. It exhibits a negative resistance characteristic and so it can be employed as an oscillator.

UJT Operation-Equivalent Circuit Operation of a UJT

Imagine that the emitter supply voltage is turned down to zero. Then the intrinsic stand-off voltage reverse-biases the emitter diode, as mentioned above. If VB is the barrier voltage of the emitter diode, then the total reverse bias voltage is VA + VB = Ƞ VBB + VB. For silicon VB = 0.7 V.

Now let the emitter supply voltage VE be slowly increased. When VE becomes equal to Ƞ VBB, IEo will be reduced to zero. With equal voltage levels on each side of the diode, neither reverse nor forward current will flow. When emitter supply voltage is further increased, the diode becomes forward-biased as soon as it exceeds the total reverse bias voltage (Ƞ VBB + VB). This value of emitter voltage VE is called the peak-point voltage and is denoted by VP. When VE = VP, emitter current IE starts to flow through RB1 to ground, that is B1. This is the minimum current that is required to trigger the UJT. This is called the peak-point emitter current and denoted by IP. Ip is inversely proportional to the interbase voltage, VBB. Now when the emitter diode starts conducting, charge carriers are injected into the RB region of the bar. Since the resistance of a semiconductor material depends upon doping, the resistance of region RB decreases rapidly due to additional charge carriers (holes). With this decrease in resistance, the voltage drop across RB also decrease, cause the emitter diode to be more heavily forward biased. This, in turn, results in larger forward current, and consequently more charge carriers are injected causing still further reduction in the resistance of the RB region. Thus the emitter current goes on increasing until it is limited by the emitter power supply. Since VA decreases with the increase in emitter current, the UJT is said to have negative resistance characteristic. It is seen that the base-2 (B2) is used only for applying external voltage VBB across it. Terminals E and B1 are the active terminals. UJT is usually triggered into conduction by applying a suitable positive pulse to the emitter. It can be turned off by applying a negative trigger pulse.

References[edit]

  1. ^ Ronald M. Benrey (Oct 1964). "A Repeating Flash You Can Build". Popular Science 185 (4): 132–136. 
  2. ^ Agrawal, S. L.; D. P. Saha, R. Swami, R. P. Singh (23 April 1987). "Digital magnetic fluxmeter using unijunction transistor probe". International Journal of Electronics 63 (6). doi:10.1080/00207218708939196. Retrieved 24 September 2011. 

External links[edit]