The effect of impact ionization on pseudomorphic high electron mobility transistors is studied using Monte Carlo simulations when these devices are scaled into deep decanano dimensions. The scaling of devices with gate lengths of 120, 90, 70, 50 and 30 nm has been performed in both lateral and vertical directions. The impact ionization is treated as an additional scattering mechanism in the Monte Carlo module. The critical drain voltage, at which device characteristics begin to indicate breakdown, decreases as the gate voltage is lowered. Similarly, the breakdown drain voltage is also found to decrease during the scaling process.