FET Fundamentals Unit 6 – Unijunction Transistors
The bar of semiconductor material has resistive characteristics. It can be compared to two
resistors connected in series. The PN junction is represented as a diode because it has the same
characteristics. The diode (PN junction) connects at a point along the resistance (N type
material).
A variable resistor represents the resistance below the diode connection (RB1) because its
resistance value initially decreases as the emitter current (IE) increases; the UJT has a negative
resistance characteristic. When the saturation current is reached, the variable resistance starts to
increase again.
The connection between B1 and B2 (RBB) behaves like a resistor. The connection between the
emitter (E) and B2 or B1 behaves like a diode (PN) junction.
Values of RB2 and RB1are related by a device specification called the intrinsic standoff ratio,
which is represented by the Greek letter eta (η).