How to add a separate Primary Key to a Join Table in Hibernate

I have a question about Hibernate ManyToMany mappings. I have two classes A and B and the mapping between them is a ManyToMany mapping resolved by Hibernate:

@Entity
@Table(name="A")
public class A {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToMany
    @JoinTable(name="C", joinColumns=@JoinColumn(name="a_id"), inverseJoinColumns=@JoinColumn(name="b_id"))
    private Set bs;
}

@Entity
@Table(name="B")
public class B {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToMany(mappedBy="bs")
    private Set bs;
}

As you can see, the Join Table I use is C. The foreign keys to A and B are "a_id" and "b_id". My understanding is, that Hibernate creates a composed Primary Key with a_id and b_id for table C.

I don't want to have an entity C in my model. But instead of a composed primary key on table C, I would like to have a generated ID and a unique constraint on the fields a_id and b_id.

Is it possible to tell Hibernate to use a separate primary key? Without adding an entity C?

I would appreciate any help.

Thanks a lot!