1. Cut-off Region

Here the operating conditions of the transistor are zero input base current ( I_B ), zero output collector current ( I_C ) and maximum collector voltage ( V_CE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully-OFF”.

Cut-off Characteristics

 

 

 

 

• The input and Base are grounded (0v)
• Base-Emitter voltage V_BE < 0.7v
• Base-Emitter junction is reverse biased
• Base-Collector junction is reverse biased
• Transistor is “fully-OFF” (Cut-off region)
• No Collector current flows ( I_C = 0 )
• V_OUT = V_CE = V_CC = “1”
• Transistor operates as an “open switch”

Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, V_B < 0.7v and I_C = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.