As shown in Figure 18-46, a unijunction transistor consists of a bar of lightly doped (high-resistivity) N material to which a heavily doped, P-type rod is attached on one side. Ohmic contacts are made at opposing ends of the N-type bar, which are called base / (BI) and “me 2 (B2) of the transistor. The Pv type rod is culled the – emitter and forms a conventional PN junction with the bar. Since this device has only one such junction. it behaves quite differently from a conventional bipolar transistor. The single junction accounts for the name unijunction transistor, and it is also called a double-base diode. Notice that the schematic symbol for a unijunction transistor (UJT) has an arrow pointing in the direction of conventional forward current :hrough the junction. UJTs arc also manufactured with N-type emitters and P-type bars.
Figure 18-47 shows external circuit connections to the UJT and its equivalent circuit. Here, the PN junction is represented by an equivalent diode. Note that base I is the common (ground) for both the emitter input circuit and the supply voltage, VI/II. The intcrbasc resistance RIIII is defined to be the total resistance between HI and B! when the emitter circuit is open (h == 0). R/I/I is therefore the sum of the resistances Rill and Rm between each base and the emitter. when Ie
The value of clearly depends on how close the emitter terminal is to base 2. since Rill becomes larger and Rill becomes smaller as the emitter is moved closer to B!. 11 is typically in the range from 0,50 to O.X5. while RI/I/ may range from 4 kH 10 12 kn
Figure 18-48 shows the equivalent circuit with external emitter voltage connected. Notice that Rill in this case is shown as a variable resistance. If Vf’ is less than the positive voltage VI across Rill. then the diode is reverse biased. and RHI has essentially the same resistance as when J, = O. However, if v.. “”increased to the onset of forward bias, a small amount 0 forward tins to flow through the emitter and into the base 1 region, ausing the resistance of that region to decrease. The decrease in Rill is attributable to the presence of additional charge carriers resulting from the forward current flow. If the forward bias is increased slightly. there is a sudden and. dramatic reduction in Rill. This phenomenon occurs because the increase In current reduces Rill. which further increases the current. which further reduces R/II. and so forth. In other words, a regencratioe action occurs. The value of emitter voltage at which regeneration is initiated is called the peakooltagr, As can be seen in Figure lR,·4X(a). the value that the emitter voltage must reach is VI plus the forward drop \/1> across the diode. Since \/1 = 11V/II/. we . have
Figure 18-48(h) shows a characteristic curve for the unijunction transistor. Once the emitter voltage has reached VI” emitter current increases even as V” is made smuitcr, This fact is conveyed by the..’ negative slope of the characteristic in the region’ to the right of the peak point (decreasing voltage accompanied hy increasing current). The region is therefore appropriately referred to as the negative resistance portion of the characteristic. This region is said to be unstable because the UJT cannot actually he operated there. The curve simply shows the combinations (If VI: and h that the U.lT undergoes during the regenerative transition. Beyond the valley point shown in the figure, the resistance R/II has reached its minimum possible value. called the saturation resistance. In this saturation region. emitter current can be increased further only by again increasing Vf:. The characteristic in this region is similar to that of a conventional forward-biased diode. Note that the region to the lett or the peak point IS called the cutoff region.
In applications. the UJT is used like a voltage-controlled switch. When the input voltage is raised to V turns on. allowing a generous flow of current from Villi to ground. To turn the device off. i.e .• to return it to jhe cutoff region. the emitter current must be reduced below the valley current !v she wn in It must be remembered that VI’ is a function of Villi. as shown by equation lX-37. Thus. if Villi is increased. the emitter voltage must be raised to a higher value to switch the UJT on. Figure IX-49 shows a typical family of characteristic curves, corresponding to different values of 1’1//1, (The cutoff regions are not shown.) The intersection of each characteristic curve with the Vf:-uxis is (approximately) the value of VI’ corresponding to a particular Villi. and it can be seen that VI’ increases with increasing
Figure 18-50(a) shows a common application of a UJT in a relaxation oscillator circuit. Capacitor C charges through Rf.· until the capacitor voltage reaches that time. the lJJT switches on, and the capacitor discharges through the emitter. The resulting surge of current through external resistor R develops a sharp voltage “spike,” as shown in Figure IX-50(b). Once the capacitor has discharged, the switches oack to its off state, provided the emitter curre.nt drop below l», The capacitor then begins to recharge and the cycle is repeated. The resulting capacitorvoltage a d output-voltage waveforms are shown in (b), This type of oscillator is used to generate trigger and timing pubes in a wid variety of control, synchronization and nonsinusoidal oscillator circuits.
The approximation gives a value slightly higher than the design value of 1 ms but considering the variation that is possible in the parameter values, the approximation is quite good. For accurate frequency control, RF. should be made adjust blz current is supplied to the SCR to turn it on. In this ac power-control circuit, the SCR switches off during every negative half-cycle, so the UJT must retrigger it during each positive half-cycle. Adjustir g the value of J f. controls the rate at which the capacitor charges and therefore controls the point in time at which the emitter voltage reaches VI” A small value of RE allows the capacitor to charge quickly, \’ ‘l1ich makes the UJT and SCR switch on early in the half-cycle, and allows load current ‘(1 flow for a significant portion of the h ilf-cycle. A large value of Rf: causes a greater dersy in switching and allows load current to flow during a smaller time interval. Diode DI isolates the UJT from negative half-cycles.
