  A theory of the transient spectroscopy of quantum well (QW) structures under
a large applied bias is presented. An analytical model of the initial part of
the transient current is proposed. The time constant of the transient current
depends not only on the emission rate from the QWs, as is usually assumed, but
also on the subsequent carrier transport across QWs. Numerical simulation was
used to confirm the validity of the proposed model, and to study the transient
current on a larger time scale. It is shown that the transient current is
influenced by the nonuniform distribution of the electric field and related
effects, which results in a step-like behavior of the current. A procedure of
extraction of the QW emission time from the transient spectroscopy experiments
is suggested.
