I am trying to drive an LED to rapidly (~10 ns) turn on, remain constant and turn off.

I have tested the LED with a pulse generator (1 ns switching and 5 V, 50 ohm output). A high speed detector is used to monitor the LED output. The resultant signal is shown in Figure 1. The layout is the top diagram in Figure 2.

Figure 1. Signals recorded, sources as per legend.

Figure 2. Schematic diagrams of systems used.

However I would like the LED to be brighter so have used a MOSFET to drive the LED, see bottom Figure 2. This does make the LED brighter but the rise time seems to be vastly increased. As well as the LED output I have also measured the voltage across the MOSFET (again Figure 1) it's fall time seems fast so the slow rise time does not seem to be attributable so the MOSFET switching time.

Any suggestions on why this occurs and how to resolve it? I have tried changing MOSFETs, with little effect.

I am trying to drive an LED to rapidly (~10 ns) turn on, remain constant and turn off.

I have tested the LED with a pulse generator (1 ns switching and 5 V, 50 ohm output). A high speed detector is used to monitor the LED output. The resultant signal is shown in Figure 1. The layout is the top diagram in Figure 2.

Figure 1. Signals recorded, sources as per legend.

Figure 2. Schematic diagrams of systems used.

However I would like the LED to be brighter so have used a MOSFET to drive the LED, see bottom Figure 2. This does make the LED brighter but the rise time seems to be vastly increased. As well as the LED output I have also measured the voltage across the MOSFET (again Figure 1) it's fall time seems fast so the slow rise time does not seem to be attributable so the MOSFET switching time.

Any suggestions on why this occurs and how to resolve it? I have tried changing MOSFETs, with little effect.

EDIT: Lots to questions about the MOSFET and drive. I used the 50 ohm signal generator to drive the MOSFET gate. The MOSFET is a NDS351AN. The data sheet quotes a 1.3 nC gate charge. For a 5 V, 50 ohm drive I calculate this takes 13 ns to 'fill'. That is also why the MOSFET drain voltage is plotted - surely if the MOSFET switching time was an issue then the voltage would still be dropping?