  We calculate the transition form factors that occur in heavy $\Lambda$-type
baryon semileptonic decays as e.g. in $\Lambda_{b} \to \Lambda_{c}^{+} + l^{-}
+ \bar{\nu}_{l} $. We use Bauer-Stech-Wirbel type infinite momentum frame wave
functions for the heavy $\Lambda$-type baryons which we assume to consist of a
heavy quark and a light spin-isospin zero diquark system. The form factors at $
q^{2} = 0 $ are calculated from the overlap integrals of the initial and final
$\Lambda$-type baryon states. To leading order in the heavy mass scale the
structure of the form factors agrees with the HQET predictions including the
normalization at zero recoil. The leading order $\omega$-dependence of the form
factors is extracted by scaling arguments. By comparing the model form factors
with the HQET predictions at ${\cal O}(1/m_{Q})$ we obtain a consistent set of
model form factors up to ${\cal O}(1/m_{Q})$. With our preferred choice of
parameter values we find that the contribution of the non-leading form factor
is practically negligible. We use our form factor predictions to compute rates,
spectra and various asymmetry parameters for the semi-leptonic decay
$\Lambda_{b} \to \Lambda_{c}^{+} + l^{-} + \bar{\nu}_{l} $.
