The graph of fig. 4 shows the relationship between the emitter voltage and current. vE is plotted on the vertical axis and IE is plotted on the horizontal axis. The region from vE = 0 to vE = VP is called cut off region because no emitter current flows (except for leakage). Once vE exceeds the peak point voltage, IE increases, but v E decreases. up to certain point called valley point (VV and IV). This is called negative resistance region. Beyond this, IE increases with vE this is the saturation region, which exhibits a positive resistance characteristic.
The physical process responsible for the negative resistance characteristic is called conductivity modulation. When the vE exceeds VP voltage, holes from P emitter are injected into N base. Since the P region is heavily doped compared with the N-region, holes are injected to the lower half of the UJT.
Fig. 4
The lightly doped N region gives these holes a long lifetime. These holes move towards B1 to complete their path by re-entering at the negative terminal of VEE. The large holes create a conducting path between the emitter and the lower base. These increased charge carriers represent a decrease in resistance RB1, therefore can be considered as variable resistance. It decreases up to 50 ohm.
Since h is a function of RB1 it follows that the reduction of RB1 causes a corresponding reduction in intrinsic standoff ratio. Thus as IE increases, RB1 decreases, h decreases, and Va decreases. The decrease in V a causes more emitter current to flow which causes further reduction in RB1, h, and Va. This process is regenerative and therefore Va as well as vE quickly drops while IE increases. Although RB decreases in value, but it is always positive resistance. It is only the dynamic resistance between VV and VP. At point B, the entire base1 region will saturate with carriers and resistance RB1 will not decrease any more. A further increase in Ie will be followed by a voltage rise.
The diode threshold voltage decreases with temperature and RBB resistance increases with temperature because Si has positive temperature coefficient.
