#### CHAPTER 14 SEMICONDUCTORS

Transistor as a device

The transistor can be used as a device application depending on the configuration used (namely CB, CC and CE), the biasing of the E-B and B-C junction and the operation region namely cutoff, active region and saturation.

1. Transistor as a switch - base-biased CE configuration

Applying Kirchhoff’s voltage rule to the input and output sides of this circuit, we get,

VBB = IBRB + VBE and VCE = VCC – ICRC.

Let us assume VBB is a dc input voltage Vi, then VCE will also be a dc output voltage VO. Hence,

Vi = IBRB + VBE

and Vo = VCC – ICRC.

For an Si transistor,

• As long as input Vi is less than 0.6 V, the transistor will be in cut off state and current IC will be zero. Hence Vo = VCC

• When Vi becomes greater than 0.6 V the transistor is in active state with some current IC in the output path. The output voltage Vo decreases as the term ICRC increases. Since, IC increases almost linearly as Vi increases, Vo decreases linearly till its value becomes less than about 1.0 V.

• Beyond this, the change becomes non linear and transistor goes into saturation state. With further increase in Vi the output voltage is found to decrease further towards zero though it may never become zero. If we plot the Vo vs Vi curve, [also called the transfer characteristics of the base-biased transistor], we see that there are regions non-linearity between cut off state and active state and also between active state and saturation state. This shows that the transitions from cutoff state to active state and from active state to saturation state are not sharply defined.

• As long as Vi is low and unable to forward-bias the transistor, Vo is high (at VCC). If Vi is high enough to drive the transistor into saturation, then Vo is low, very near to zero. When the transistor is not conducting it is said to be switched off and when it is driven into saturation it is said to be switched on.

• We can define low and high states as below and above certain voltage levels corresponding to cutoff and saturation of the transistor. We see that a low input switches the transistor off and a high input switches it on. Alternatively, we can say that a low input to the transistor gives a high output and a high input gives a low output. The switching circuits are designed in such a way that the transistor does not remain in active state.