Difference between PUT and UJT: (i) The intrinsic stand-off ratio of a UJT is fixed hence operating characteristics cannot be alterd. The PUT, on the other hand has operating characteristics that can be altered. These include base-base resistance, intrinsic stand-off voltage, valley current and peak current and all these can be altered by setting the values of two external resistors. (ii) The UJT is made up of a lightly doped n-region known as the base region onto which is joined a small heavily doped p-region called the emitter. The PUT on the other hand is a four layer device similar to an SCR except that the gate terminal of the PUT is connected to the n-region adjacent to the anode. (iii) PUT has high forward conductance, so it can provide high peak current pulses, even with low value capacitor in relaxation oscillator circuit. (iv) PUT has fast rise time, which allows faster rise in current and provides healthy output pulses as compared to UJT.