Bootstrap modal in ASP.NET C#

bootstrap modal sending a null value from textbox in asp.net c# .Do anybody know  the reason behind this ?

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myLargeModalLabel"
                aria-hidden="true">
                <div class="modal-dialog modal-lg">
                    <div class="modal-content">
                        <div class="modal-header">
                            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">
                                ×</button>
                            <h4 class="modal-title" id="myModalLabel">Login/Sign Up </h4>
                        </div>
                        <div class="modal-body">
                            <div class="row">
                                <div class="col-md-8" style="border-right: 1px dotted #C2C2C2; padding-right: 30px;">
                                    <!-- Nav tabs -->
                                    <ul class="nav nav-tabs">
                                        <li class="active"><a href="#Login" data-toggle="tab">Login</a></li>
                                        <li><a href="#Registration" data-toggle="tab">Sign Up</a></li>
                                    </ul>
                                    <!-- Tab panes -->
                                    <div class="tab-content">
                                        <div class="tab-pane active" id="Login">
                                            <form role="form" class="form-horizontal">
                                                <div class="form-group">
                                                    <div class="col-sm-12">
                                                        <asp:TextBox ID="txtLoginEmail" runat="server" CssClass="form-control" placeholder="Email/Phone"></asp:TextBox>
                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-12">
                                                        <asp:TextBox ID="txtLoginPassword" runat="server" CssClass="form-control" placeholder="Password"></asp:TextBox>
                                                    </div>
                                                </div>
                                                <div class="row">

                                                    <div class="col-sm-12">

                                                        <asp:Button ID="btnLogin" runat="server" Text="Login" CssClass="btn btn-primary btn-sm" />

                                                        <a href="">Forgot your password?</a>
                                                    </div>
                                                </div>
                                            </form>
                                        </div>
                                        <div class="tab-pane" id="Registration">
                                            <form id="signup" role="form" class="form-horizontal">
                                                <div class="form-group">
                                                    <div class="col-sm-10">
                                                        <div class="row">
                                                            <div class="col-md-6">
                                                                <asp:TextBox ID="txtFirstName" runat="server" CssClass="form-control" placeholder="First Name"></asp:TextBox>

                                                            </div>
                                                            <div class="col-md-6">
                                                                <asp:TextBox ID="txtLastName" runat="server" CssClass="form-control" placeholder="Last Name"></asp:TextBox>
                                                            </div>
                                                        </div>
                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-12">
                                                        <asp:TextBox ID="txtEmail" runat="server" CssClass="form-control" placeholder="Email"></asp:TextBox>
                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-12">
                                                        <asp:TextBox ID="txtMobile" runat="server" CssClass="form-control" placeholder="Mobile No."></asp:TextBox>
                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-6">
                                                        <asp:TextBox ID="txtPassword" runat="server" CssClass="form-control" placeholder="Password" TextMode="Password"></asp:TextBox>

                                                    </div>
                                                    <div class="col-sm-6">
                                                        <asp:TextBox ID="txtConfirmPassword" runat="server" CssClass="form-control" placeholder="Confirm Password" TextMode="Password"></asp:TextBox>
                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-12">
                                                        <asp:TextBox ID="txtBirthDay" runat="server" CssClass="form-control" placeholder="Birth Date" TextMode="Date"></asp:TextBox>

                                                    </div>
                                                </div>
                                                <div class="form-group">
                                                    <div class="col-sm-2">
                                                        <asp:Label ID="lblGender" runat="server" Text="Gender" CssClass="form-control"></asp:Label>
                                                    </div>
                                                    <div class="col-sm-6">
                                                        <div class="col-sm-5">
                                                            <asp:RadioButton ID="RadiobtnMale" runat="server" Text="Male" GroupName="gender" />
                                                        </div>
                                                        <div class="col-sm-5">
                                                            <asp:RadioButton ID="RadiobtnFemale" runat="server" Text="Female" GroupName="gender" />
                                                        </div>
                                                    </div>
                                                </div>
                                                <div class="row">

                                                    <div class="col-sm-10">
                                                        <asp:Button ID="btnSignUp" runat="server" Text="Sign Up" CssClass="btn btn-primary btn-signup" OnClick="btnSignUp_Click" UseSubmitBehavior="false" data-dismiss="modal" />

                                                    </div>
                                                </div>
                                            </form>
                                        </div>
                                    </div>
                                    <div id="OR" class="hidden-xs">
                                        OR
                                    </div>
                                </div>
                                <div class="col-md-4">
                                    <div class="row text-center sign-with">
                                        <div class="col-md-12">
                                            <h3>Sign in with</h3>
                                        </div>
                                        <div class="col-md-12">
                                            <div class="btn-group btn-group-justified">
                                                <a href="#" class="btn btn-primary">Facebook</a> <a href="#" class="btn btn-danger">Google</a>
                                            </div>
                                        </div>
                                    </div>
                                </div>
                            </div>
                        </div>
                    </div>
                </div>
            </div>