Applets illustrating the operation of diodes and transistors
Falstad is a simple Java simulator with the basic components, and some demos about diodes, like this, and transistors, like this. It may not be the most beautiful or most complete, but it's quite intuitive to use.

Take a look, but I would suggest you to try matching the examples with the theory, to understand better what is the transconductance \$ \beta _F\$ and so on.