  As the superior improvement on wireless network coverage, spectrum efficiency
and energy efficiency, Intelligent reflecting surface (IRS) has received more
and more attention. In this work, we consider a large-scale IRS-assisted
heterogeneous cellular network (HCN) consisting of $K$ ($K \geq 2$) tiers of
base stations (BSs) and one tier of passive IRSs. With tools from stochastic
geometry, we analyze the coverage probability and network spatial throughput of
the downlink IRS-assisted $K$-tier HCN. Compared with the conventional HCN, we
observe the significant gain achieved by IRSs in coverage probability and
network spatial throughput. The proposed analytical framework can be used to
understand the limit of gain achieved by IRSs in HCN.
