N-type N-type N-type P-type Terminal Terminal Gate P-type N-type N-type material E P-type material I junction B1 B2 +10V +8V +6V +4V +2V +0V E B1 B2 +10V 11-16 Figure 11-35.  The voltage gradient in a UJT.  Figure 11-34.  A unijunction transistor (UJT). Figure 11-33.  The semiconductor layering in a triac. A positive or  negative gate pulse with respect to the upper terminal allows current  to flow through the devise in either direction.  collector material. One electrode is attached to the emitter and  two electrodes are attached to the base material at opposite  ends. These are known as base 1 (B1) and base 2 (B2). The  electrode configuration makes the UJT appear physically  the same as a bipolar junction transistor. However, there is  only one PN junction in the UJT and it behaves differently.  The base material of a UJT behaves like a resistor between  the electrodes. With B2 positive with respect to B1, voltage  gradually drops as it flows through the base.  [Figure 11-35] By placing the emitter at a precise location along the base  material gradient, the amount of voltage needed to be applied  to the emitter electrode to forward bias the UJT base-emitter  junction is determined. When the applied emitter voltage  exceeds the voltage at the gradient point where the emitter  is attached, the junction is forward biased and current flows  freely from the B1 electrode to the E electrode. Otherwise, the  junction is reversed biased and no significant current flows  although there is some leakage. By selecting a UJT with the  correct bias level for a particular circuit, the applied emitter  voltage can control current flow through the device. UJTs of a wide variety of designs and characteristics  exist. A description of all of them is beyond the scope of  this discussion. In general, UJTs have some advantages  over bipolar transistors. They are stable in a wide range of  temperatures. In some circuits, use of UJTs can reduce the  overall number of components used, which saves money  and potentially increases reliability. They can be found in  switching circuits, oscillators, and wave shaping circuits.  However, four-layered semiconductor thyristors that function  the same as the UJT just described are less expensive and  most often used.  The behavior of semiconductor materials is exploited through  the construction of numerous transistor devices containing  various configurations of N-type and P-type materials. The  physical arrangement of the materials in relation to each  other yields devices with unique behaviors and applications.  The transistors described above having two junctions of  P-type and N-type materials (PN) are known as bipolar  junction transistors. Other more simple transistors can be  fashioned with only one junction of the PN semiconductor  materials. These are known as unijunction transistors (UJT).  [ Figure 11-34] The UJT contains one base semiconductor material and a  different type of emitter semiconductor material. There is no  Unijunction Transistors (UJT)