LTSpice ideal models

The simplest idealised diode can be modeled in the following way

This is very ideal! LTSpice will work on a three region model whose transitions are not smooth. You could encounter integration or convergence issues during simulation, especially if your circuit is complex. To avoid this, let's write a slightly more ideal model.

With this model, you can see how LTSpice applies smooth transition between diode operating regions.

Well, MOSFETs are very intricated as there are several parameters and many of them are related to physical properties of the device. However, I found useful the following description

Here I specify the threshold voltage, the transconductance value, drain and source resistance, Miller capacitance minimum and maximum values, gate-source and zero-bias diode capacitances, diode resistance. This should be enough to run a simulation avoiding all those parameters that increase complexity and distract you from the circuit functionality.

When you are sure your circuit behaves as expected, then you will substitute the ideal model with the one of the chosen component.

Sometimes, ideal switches are better than transistor: they remove any. However they generate abrupt state changes that, in some cases, should be made smoother to improve simulation and ensure convergence.

R_on and R_off are pure resistive elements, placed dynamically in the circuit according to the switch state. V_t is the center threshold level, making the switch changing its state; V_h is the input hysteresis: switch turns on at V_t+V_h and turns off at V_t-V_h.

Again, since switches turn on and off very quickly, high current spikes could appear in your circuit, partially invalidating it. To make this components more similar to a silicon-based device, you can add some extra parameters, as shown below:

L_ser is a series inductance, useful to limit the current peaks during state transitions. I_limit is the maximum current allowed in the component; if this limit is reached, current is clamped.