  Using test particle simulations we study electron acceleration at
collisionless shocks with a two-component model turbulent magnetic field with
slab component including dissipation range. We investigate the importance of
shock normal angle $\theta_{Bn}$, magnetic turbulence level
$\left(b/B_0\right)^2$, and shock thickness on the acceleration efficiency of
electrons. It is shown that at perpendicular shocks the electron acceleration
efficiency is enhanced with the decreasing of $\left(b/B_0\right)^2$, and at
$\left(b/B_0\right)^2=0.01$ the acceleration becomes significant due to strong
drift electric field with long time particles staying near the shock front for
shock drift acceleration (SDA). In addition, at parallel shocks the electron
acceleration efficiency is increasing with the increasing of
$\left(b/B_0\right)^2$, and at $\left(b/B_0\right)^2=10.0$ the acceleration is
very strong due to sufficient pitch-angle scattering for first-order Fermi
acceleration, as well as due to large local component of magnetic field
perpendicular to shock normal angle for SDA. On the other hand, the high
perpendicular shock acceleration with $\left(b/B_0\right)^2=0.01$ is stronger
than the high parallel shock acceleration with ($\left(b/B_0\right)^2=10.0$),
the reason might be the assumption that SDA is more efficient than first-order
Fermi acceleration. Furthermore, for oblique shocks, the acceleration
efficiency is small no matter the turbulence level is low or high. Moreover,
for the effect of shock thickness on electron acceleration at perpendicular
shocks, we show that there exists the bend-over thickness, $L_{\text{diff,b}}$.
The acceleration efficiency does not change evidently if the shock thickness is
much smaller than $L_{\text{diff,b}}$. However, if the shock thickness is much
larger than $L_{\text{diff,b}}$, the acceleration efficiency starts to drop
abruptly.
