Digital Circuits/Transistor Basics
What is a Transistor? 
A transistor is an electronic control device, where an electrical signal input can control another electrical signal. The very name derives from the fact that this control action was seen as an input current flowing into one resistor (base-emitter) causing a larger current in an other resistor (collector-emitter). The early named for this device was the transfer-resistor and hence transistor.
There are many different variations of transistors that are appropriate for a wide variety of applications. The transistors that are used in modern digital electronics are generally pairs of MOSFETs, but bipolar transistors are occasionally used.
Transistor Functions 
In digital circuits, transistors are always used as a switch.
Like a common lightswitch, the ideal digital transistor is, at any one instant, either "off" with zero current flowing through it, or "on" with nearly zero voltage across it. There are occasional transitions between the two states, but in this book we ignore the details of what happens during the transition.
Transistor Circuits 
Transistors may be put together in configurations that allow more flexibility than simply diodes. With two diodes and a resistor, you can construct an AND gate and an OR gate. If we add transistors into the mix, we can construct the three basics, the AND, OR, and NOT gates. The NOT gate allows us to expand further into NAND and NOR gates.
Propagation Delay 
In all transistors today, there is a certain amount of latency between the time that a control signal is applied and the time that the output is affected. This delay is called propagation delay. These delays can be broken up into two parts.
• tPLH This is the delay time resulting from a LOW to HIGH transition
• tPHL This is the delay time resulting from a HIGH to LOW transition
Keep in mind: These two values may not be the same and are usually defined on the device's datasheet.
Propagation delay can be influenced by capacitance in the circuit. Propagation delay is also a rough indicator of effective speed of the device. For example, a gate with a 5 nanosecond propagation delay will respond much faster than a gate with a 120 nanosecond propagation delay.
Propagation delay is measured at the 50% mark and measures the time elapsed between the input and output signal changes.
Further Reading 
For details of what transistors are really made of, see Semiconductors.
In digital circuits, we are mainly focusing on transistors that are in cutoff or saturation. For details of behavior of transistors when they are forced into an "in-between" state between fully off and fully on, see Analogue Electronics.